Buick LeSabre 1991 - 1991 ENGINE PERFORMANCE Self-Diagnostics - 3.8L H Body PFI - ECM/PCM

Buick LeSabre 1991 - INTRODUCTION

Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, perform checks and inspections covered in BASIC TESTING article. Failure to do so may result in lost diagnostic time.

If no faults were found while performing BASIC TESTING , proceed with DIAGNOSTIC PROCEDURE. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.) in this section. If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article in this section.

Buick LeSabre 1991 - SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM) or Powertrain Control Module (PCM). Unless specifically stated, references to ECM also apply to PCM equipped vehicles. Control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in control module memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN) in this article. Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

Buick LeSabre 1991 - HARD FAILURES

Hard failures cause SERVICE ENGINE SOON light to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic (code) charts. If a sensor fails, control module will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but most likely degraded driveability will be encountered.

Buick LeSabre 1991 - INTERMITTENT FAILURES

Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in control module memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from control module memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article in this section.

Buick LeSabre 1991 - DIAGNOSTIC PROCEDURE

Diagnosis of the computerized engine control system should be performed in the following order:

1. Make sure all engine systems not related to the computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. DIAGNOSTIC CIRCUIT CHECK must be performed prior to utilizing trouble code charts. See BASIC TESTING article in this section.
2. If trouble codes were displayed (other than Code 12), decide whether codes are hard or intermittent trouble codes. Hard codes will cause the SERVICE ENGINE SOON light to illuminate continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article in this section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.
3. If no trouble codes were displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article in this section. The comments there will send you to the proper system or component to check in SYSTEM/COMPONENT TESTS article in this section.
4. After any repairs are made, clear any trouble codes and perform FIELD SERVICE MODE check in BASIC TESTING article in this section.

Buick LeSabre 1991 - RETRIEVING CODES (NON-SCAN)

NOTE:

For information on retrieving codes using a "Scan" tester, refer to owners manual supplied with "Scan" tester.

1. Turn ignition on. DO NOT start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector attached to control module wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For exact location of ALDL, refer to appropriate COMPONENT LOCATIONS illustration in article SYSTEM/COMPONENT TESTS in this section. Turn ignition on with engine not running. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. See Fig. 1.

   NOTE:

   Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of the SERVICE ENGINE SOON light will not indicate codes if this is done. See FIELD SERVICE MODE in BASIC TESTING article.

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   Fig. 1: Buick LeSabre 1991 - Component Locations - ALDL Connector Terminal Identification
   

2. SERVICE ENGINE SOON light should begin to flash codes. Each code will be repeated 3 times. If codes are not flashed or SERVICE ENGINE SOON light does not illuminate, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

Buick LeSabre 1991 - READING TROUBLE CODES

The control module stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of the SERVICE ENGINE SOON light, or by reading the output of a diagnostic "Scan" tester connected to the ALDL connector. The tester is faster to use, more accurate, and capable of reading information which otherwise would necessitate testing individual control module and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER USAGE and tables under SCAN DATA in this article.

NOTE:

When using a "Scan" tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.

If "Scan" tester is not available, it is possible to read flashes of the SERVICE ENGINE SOON light by grounding the diagnostic test terminal "B" of the ALDL with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, identifies Code 21. The first series of flashes are the first digit of trouble code. The second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL test terminal is grounded.

NOTE:

Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others may require operation for 5 minutes or longer at normal operating temperature, road speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate condition under which code will set.

Buick LeSabre 1991 - TROUBLE CODE DEFINITION

Buick LeSabre 1991 ECM/PCM TROUBLE CODE DEFINITION ⁽¹⁾

Code No.	Circuit Affected
31	Park/Neutral Switch
33	MAP Sensor Signal Voltage High
34	MAP Sensor Signal Voltage Low
35	IAC Idle Speed Error
38	Brake Switch
39	TCC
41	Cam Sensor Loss
42	EST Circuit Open Or Grounded
43	ESC Error
44	Lean Exhaust Indication
45	Rich Exhaust Indication
46	Power Steering Pressure Switch
48	Misfire Diagnosis
51 (2)	Faulty PROM, MEM-CAL or ECM/PCM
52 (2)	Faulty/Missing CALPAC or MEM-CAL
58	Anti-Theft System Fault
63, 64 or 65	EGR Flow Check Error (H Body)
66	Low A/C Refrigerant Charge
69	A/C Head Pressure Switch
(1) Display of a Code 12 is normal when no reference pulses are received by control module (engine not running). (2) PCM equipped models.

NOTE:

Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime control module-related Codes 51, 52 or 55 are displayed with another code, start with 50-series code first, then proceed to low profile numbered codes.

Buick LeSabre 1991 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION

During any diagnostic procedure, it must be determined if codes are hard failure codes or intermittent failure codes. Diagnostic charts will not usually help analyze intermittent codes. To determine hard codes and intermittent codes, proceed as follows:

1. MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See CLEARING TROUBLE CODES.
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. SERVICE ENGINE SOON light should go out. Run warm engine at specified curb idle for 2 minutes and note SERVICE ENGINE SOON light.
3. If SERVICE ENGINE SOON light comes on, MANUALLY enter diagnostic mode. Read and record trouble codes. This will reveal hard failure codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset hard failure after trouble codes were cleared.
4. If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.

Buick LeSabre 1991 - CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at ALDL connector. Turn ignition switch to OFF position and remove control module fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. If fuse cannot be located, pigtail at battery can be disconnected. When power to ECM is removed, degraded driveability may be exhibited until control module "relearns" optimum operational parameters.

Buick LeSabre 1991 - ECM/PCM LOCATION

On most vehicles the engine control module is located behind the right or left side of the dash, or behind the right or left kick panel. For illustration of engine control module locations see COMPONENT LOCATIONS in SYSTEM/COMPONENT TESTS article in this section.

Buick LeSabre 1991 - DIAGNOSTIC MATERIALS Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are additional tips used to help diagnose trouble codes when inspected circuit checks out okay. Diagnostic aids may help lead to a definitive solution to that trouble code problem.

Buick LeSabre 1991 - Field Service Mode Check

If ALDL test terminal "B" is grounded with engine running, SERVICE ENGINE SOON light will indicate operational mode of engine. This test confirms proper operation of fuel system and verifies closed loop operation. Clear codes and perform this test after any repair is completed. Field service mode check can be found by proceeding to FIELD SERVICE MODE CHECK in BASIC TESTING article in this section.

Buick LeSabre 1991 - SPECIAL TOOLS (DIAGNOSTIC)

NOTE:

Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line (terminal "E", or terminal "M" on P-4 systems). These testers can save a great deal of time. For additional information, see SCAN TESTER USAGE and tables under SCAN DATA in this article.

The computerized engine control system is most easily diagnosed using a "Scan" tester; however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These tools are a tachometer, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (for both TBI and PFI) and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

Buick LeSabre 1991 - SCAN TESTER USAGE

NOTE:

Prior to connection of "Scan" tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received by "Scan" tester will be accurate. This is done by performing DIAGNOSTIC CIRCUIT CHECK located in the BASIC TESTING article in this section. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.

The "Scan" tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control systems by providing instant access to circuit voltage information without need to crawl under dash or hood to backprobe sensors and connectors. "Scan" testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See tables under SCAN DATA in this article. They may also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication that output signals have been sent to devices by the control module. It does not indicate if devices have responded properly to that signal. This will need to be verified at output device using a voltmeter or test light.

NOTE:

Code 12 should always exist when ALDL is grounded with key on and engine not running, but may not be indicated by all makes of "Scan" testers.

If trouble codes are not present, this is not an indication that there is not a problem. Driveability related problems with codes displayed occur about 20 percent of the time, while driveability problems without codes occur about 80 percent of the time. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Using a "Scan" tester is the easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See tables under SCAN DATA in this article.

NOTE:

Information obtained by "Scan" tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, it will be necessary to verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off, remove tester, turn ignition on and ground ALDL test terminal "B". If same codes are not flashed by SERVICE ENGINE SOON light that were indicated by "Scan" tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.

Buick LeSabre 1991 - SCAN DATA

NOTE:

Information contained in the following tables is typical of readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/4 seconds. On systems using P-4 computers, parameter updates are more often. Not all devices and systems are used on all models and the following lists represent only the most commonly used parameters. For additional information, refer to owners manual furnished with tester.

Buick LeSabre 1991 PORT FUEL INJECTION

Tester Position	Units Measured	Nominal Data Value
A/C Clutch	On/Off	Off (On With A/C)
A/C Request	Yes/No	No/Yes (With Request)
AIR Divert Sol.	On/Off	On (Air To Switching Sol.); Off (Air To Atmosphere)
AIR Switching Sol.	On/Off	On (To Exhaust Manifold); Off (To Catalytic Converter)
BARO	Volts	3-4.5
Battery Voltage	Volts	13.5-14.5
Block Learn	Counts	118-138 (128 Normal)
Canister Purge Sol.	On/Off	On/Engine Cold (Idle Some)
Clear Flood	On/Off	See Tester Manual
Coolant Fan	On/Off	Off below 216?F (102?C)
Coolant Temp.	?C	85-105? (Norm. Temperature)
Crank RPM	RPM	100-900
Cross Counts	Counts	0-255EGR
Solenoid	On/Off	On When Energized
EGR Duty Cycle	0-100%	0/Closed-100/fully Open
Fan Relay	On/Off	On When Energized
Fan Request	On/Off	On With Request
Fuel Back-Up	Yes/No	Yes When Engaged
IAC	Counts	0-50
Ignition/Crank	On/Off	On With Ignition/Crank
Injector Pulse Width	Mil./Sec	.8-3.0
INT (Integrator)	Counts	110-145 (128 Normal)
Knock Retard (ESC)	Counts	0-255
Knock Signal	Yes/No	Yes When Knock Exists
MAT	?C	10-90?
MAP	Volts	1 (Idle) to 4.5 (WOT)
Open/Closed Loop Status	Ol/Cl	Closed/Open During Extended Idle
O2 Sensor	Millivolts	100 (Lean) To 999 (Rich)
P/N Switch	P/N/RDL	Park/Neutral
P/S Switch	Norm/Hi	Normal
PROM I.D.	PROM #	Original Factory Number
RPM	RPM	Spec. +/-25 RPM Drive (Auto.); Spec. +/-50 RPM Neut. (Man.)
Spark Advance	No. Deg.	Varies
TCC	On/Off	Off (On With Command)
TPS	Volts	1.25 (Idle) to 5.0 (WOT)
Throttle Angle	0-100%	0 (Idle) To 100 (WOT)
Trouble Codes	Code #	No Codes
Upshift Light (Man. Trans.)	On/Off	Off
VSS or MPH	MPH	0-Actual
Water Injection	On/Off	On When Injecting
1st Gear Switch	On/Off	On/1st Gear Only
3rd Gear Switch	On/Off	On/3rd & 4th Gear
4th Gear Switch	On/Off	On/4th Gear

Buick LeSabre 1991 - TEST PROCEDURE SUMMARY

If no hard fault codes are present, driveability symptoms exist or intermittent codes exist, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures in this section.

Buick LeSabre 1991 - DIAGNOSTIC FLOW CHARTS

NOTE:

Schematics are supplied courtesy of General Motors Corp.

Buick LeSabre 1991 - CODE 13, OPEN OXYGEN SENSOR CIRCUIT

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This tests if problem still exists. Vehicle cannot enter closed loop mode if oxygen sensor circuit is open. Code 13 indicates an open in the O2 sensor circuit. Code will set if:
   • Engine is at normal operating temperature.
   • Neither Code 21 nor Code 22 are stored.
   • O2 sensor voltage is constant within a specified range (.34-.55 volt).
   • Throttle angle is greater than idle.
   • A precalibrated amount of time has elapsed since start-up.
   • All conditions have existed for a precalibrated amount of time.
2. Determines if O2 sensor, wiring or control module is at fault. If wiring is good, grounding oxygen sensor wire will cause .45 volt reference supplied by the control module to pull low.
3. This tests O2 sensor circuit wiring. Use only a high impedance (10-megohm minimum) digital voltmeter.

Buick LeSabre 1991 - Diagnostic Aids

Control module will not go into closed loop if Code 13 is set. Code 13 may set if vehicle runs out of fuel or stalls while vehicle is in motion. If oxygen sensor ground becomes loose, a false oxygen sensor reading will occur. This can result in a Code 13 being set.

Fig. 2: Buick LeSabre 1991 - Component Locations - Code 13, Schematic, Open Oxygen Sensor Circuit

Fig. 3: Buick LeSabre 1991 - Component Locations - Code 13, Open Oxygen Sensor Circuit

Buick LeSabre 1991 CODE 13 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
O2 Signal	YD2	Purple
O2 Ground	YD3	Tan

Buick LeSabre 1991 - CODE 14, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE LOW

NOTE:

This chart assumes engine cooling system is functioning properly (not overheating). Test numbers refer to test numbers on diagnostic chart.

1. Code 14 indicates the control module has seen low coolant sensor voltage signal (high temperature) at control module terminal for a precalibrated period of time. This checks if conditions for Code 14 still exist.
2. This tests for grounded sensor signal line between control module and coolant sensor.

Buick LeSabre 1991 - Diagnostic Aids

After the engine is started, temperature should rise steadily to about 190?F (88?C), then stabilize when thermostat opens. At normal operating temperature, signal voltage at control module terminal should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table. When Code 14 is set, control module will turn on electric cooling fan(s), if equipped.

Fig. 4: Buick LeSabre 1991 - Component Locations - Code 14, Schematic, Coolant Temp Sensor Signal Voltage Low

NOTE:

For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 5: Buick LeSabre 1991 - Component Locations - Code 14, Coolant Temp Sensor Signal Voltage Low

Buick LeSabre 1991 CODE 14 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
CTS Signal	YD4	Yellow
CTS Ground	YD5	Black

Buick LeSabre 1991 TEMPERATURE-TO-RESISTANCE VALUES ⁽¹⁾

Temperature ?F (?C)	Ohms
210 (100)	185
160 (70)	450
100 (38)	1800
70 (20)	3400
20 (-7)	13,500
0 (-18)	25,000
-40 (-40)	100,700
(1) Measure resistance across sensor terminals.

Buick LeSabre 1991 - CODE 15, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 15 indicates control module has seen high resistance in coolant sensor circuit. This could be due to high resistance (cold temperature) or high voltage at coolant sensor terminal at control module for a precalibrated period of time. This checks if conditions for Code 15 still exist.
2. This test simulates conditions for a Code 14. If control module recognizes the low voltage signal, "Scan" tester will display greater than 130?C. This indicates the control module and wiring are not at fault.
3. This test determines if coolant sensor ground or signal circuit is open.

Buick LeSabre 1991 - Diagnostic Aids

After the engine is started, temperature should rise steadily to about 190?F (88?C), then stabilize when thermostat opens. At normal operating temperature, voltage at control module sensor signal line should be 1.5-2.0 volts. Check sensor for shifted calibration by using sensor temperature-to-resistance table. When Code 14 is set, control module will turn on electric cooling fan(s), if equipped.

Fig. 6: Buick LeSabre 1991 - Component Locations - Code 15, Schematic, Coolant Temp Sensor Signal Voltage High

NOTE:

For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 7: Buick LeSabre 1991 - Component Locations - Code 15, Coolant Temp Sensor Signal Voltage High

Buick LeSabre 1991 CODE 15 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
CTS Signal	YD4	Yellow
CTS Ground	YD5	Black

Buick LeSabre 1991 TEMPERATURE-TO-RESISTANCE VALUES ⁽¹⁾

Temperature ?F (?C)	Ohms
210 (100)	185
160 (70)	450
100 (38)	1800
70 (20)	3400
20 (-7)	13,500
0 (-18)	25,000
-40 (-40)	100,700
(1) Measure resistance across sensor terminals.

Buick LeSabre 1991 - CODE 16, SYSTEM VOLTAGE HIGH

The control module monitors battery voltage on the battery feed circuit. If the control module detects battery voltage greater than 16 volts for more than 10 seconds, it will set a Code 16 in memory.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Test alternator output to determine proper operation of voltage regulator. Increase engine speed to a moderate level and measure voltage across battery terminals. If reading is greater than 16 volts, service alternator.

Buick LeSabre 1991 - Diagnostic Aids

Starting engine with battery charger connected may set Code 16. Check for poor connections or damaged harness. Also, check for an intermittent condition by starting engine and wiggling connection while monitoring battery voltage on the "Scan" tester. If voltage status changes abruptly or engine stalls, check for loose connections.

Fig. 8: Buick LeSabre 1991 - Component Locations - Code 16, Schematic, System Voltage High

Fig. 9: Buick LeSabre 1991 - Component Locations - Code 16, System Voltage High

Buick LeSabre 1991 - CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks if code is the result of a hard failure or an intermittent condition.
2. This test simulates conditions for a Code 22. If control module recognizes the change of state, the control module and wiring are okay.
3. This step isolates a faulty sensor, control module or open sensor ground circuit. If sensor ground is shared by another sensor, there may be an accompanying code related to that sensor.

Buick LeSabre 1991 - Diagnostic Aids

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate, as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 10: Buick LeSabre 1991 - Component Locations - Code 21, Schematic, Throttle Position Sensor Signal Voltage High

NOTE:

For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 11: Buick LeSabre 1991 - Component Locations - Code 21, Throttle Position Sensor Signal Voltage High

Buick LeSabre 1991 CODE 21 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
TPS Signal	YD13	Dark Blue
TPS Ground	YD5	Black
TPS Reference	BC14	Gray

Buick LeSabre 1991 - CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test checks if code is the result of a hard failure or an intermittent condition.
2. This test simulates conditions for a Code 21. If control module recognizes the change of state, the control module and wiring are okay.
3. This simulates a high signal voltage to check for an open in the TPS signal line to control module. "Scan" tester should recognize this signal and display high TPS voltage.

Buick LeSabre 1991 - Diagnostic Aids

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be low. Voltage should increase gradually to about 4.5 volts at a steady rate, as throttle angle is increased. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 12: Buick LeSabre 1991 - Component Locations - Code 22, Schematic, Throttle Position Sensor Signal Voltage Low

NOTE:

For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 13: Buick LeSabre 1991 - Component Locations - Code 22, Throttle Position Sensor Signal Voltage Low

Buick LeSabre 1991 CODE 22 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
TPS Signal	YD13	Dark Blue
TPS Ground	YD5	Black
TPS Reference	BC14	Gray

Buick LeSabre 1991 - CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This checks if code is the result of a hard failure or an intermittent condition. Code 23 will set if engine has been running for a precalibrated period of time, has reached operating temperature and signal voltage indicates a MAT temperature less than -22?F (-30?C).
2. This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the control module and wiring are not at fault.
3. This checks for continuity of sensor signal and ground circuits. If ground circuit is shared by other sensors and ground circuit is open, accompanying codes related to those sensors may be present.

Buick LeSabre 1991 - Diagnostic Aids

If the engine is allowed to cool overnight, the coolant and MAT sensors should read close to each other, when measured with a "Scan" tester. A Code 23 will result if signal and ground circuits become open. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

Fig. 14: Buick LeSabre 1991 - Component Locations - Code 23, Schematic, MAT Sensor Signal Voltage High

NOTE:

For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 15: Buick LeSabre 1991 - Component Locations - Code 23, MAT Sensor Signal Voltage High

Buick LeSabre 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
MAT Signal	YD11	Tan
MAT Ground	YD5	Black

Buick LeSabre 1991 TEMPERATURE-TO-RESISTANCE VALUES ⁽¹⁾

Temperature ?F (?C)	Ohms
210 (100)	185
160 (70)	450
100 (38)	1800
70 (20)	3400
20 (-7)	13,500
0 (-18)	25,000
-40 (-40)	100,700
(1) Measure resistance across sensor terminals.

Buick LeSabre 1991 - CODE 24, VEHICLE SPEED SENSOR (VSS)

The speed sensor, which is a Permanent Magnet (PM) generator, provides the control module with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is more than 3 MPH. The voltage level and pulses increase with vehicle speed. The control module converts the pulsing voltage to MPH, which is used by the control module in calculations to determine vehicle adjustments.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 24 will set if vehicle speed is less than 3 MPH when engine is running, Code 29 and 31 are not set, vehicle is in 4th gear and all conditions have been met for 2-40 seconds.
2. Before replacing control unit, check MEM-CAL for correct application.

Fig. 16: Buick LeSabre 1991 - Component Locations - Code 24, Schematic, Vehicle Speed Sensor (VSS)

NOTE:

Disregard Code 24 that sets while wheel are not turning.

Fig. 17: Buick LeSabre 1991 - Component Locations - Code 24, Vehicle Speed Sensor (VSS)

Buick LeSabre 1991 - CODE 25, MAT SENSOR SIGNAL VOLTAGE LOW

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 266?F (130?C) is sensed for more than a precalibrated period.
2. This simulates condition for Code 23. if control module recognizes the open circuit and "Scan" tester displays a temperature of less than -30? C, control module and wiring are okay.

Buick LeSabre 1991 - Diagnostic Aids

If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor should read close to each other, when measured with a "Scan" tester. A Code 25 will result if sensor signal circuit is shorted to ground. Check sensor for shifted calibration by using sensor TEMPERATURE-TO-RESISTANCE VALUES table.

Fig. 18: Buick LeSabre 1991 - Component Locations - Code 25, Schematic, MAT Sensor Signal Voltage Low

NOTE:

For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 19: Buick LeSabre 1991 - Component Locations - Code 25, MAT Sensor Signal Voltage Low

Buick LeSabre 1991 CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
MAT Signal	YD11	Tan
MAT Ground	YD5	Black

Buick LeSabre 1991 TEMPERATURE-TO-RESISTANCE VALUES ⁽¹⁾

Temperature ?F (?C)	Ohms
210 (100)	185
160 (70)	450
100 (38)	1800
70 (20)	3400
20 (-7)	13,500
0 (-18)	25,000
-40 (-40)	100,700
(1) Measure resistance across sensor terminals.

Buick LeSabre 1991 - CODE 26, (1 OF 3) QUAD-DRIVER CIRCUIT

Each ECM Quad-Driver has a fault line which is monitored by the ECM. The ECM compares voltage values of the fault line with acceptable values in ECM memory. If the ECM senses other than accepted values, a Code 26 will set.

Some QDM circuits will normally cycle between high and low, such as depressing the brake pedal, cooling fans cycling on and off. QDM 2 will not set a Code 26. Some "Scan" testers may set a false Code 26 if engine is running, tester is installed and brake pedal is depressed for more than 30 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. The ECM does not know which controlled circuit set the Code 26 so this chart will go through each circuit to determine which is at fault. On vehicles equipped with an ECM controlled hot light, this test checks hot light driver and hot light circuit.
2. QDM SYMPTOMS
   • TCC inoperative, Code 39
   • EGR inoperative, Codes 63, 64 and 65.
   • Hot light on all the time, off during bulb check.
   • Cooling fan on low speed all of the time or will not come on at all.
   • Poor driveability due to 100 percent canister purge.

Buick LeSabre 1991 - Diagnostic Aids

The coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light. Check coolant sensor temperature-to-resistance values in SENSOR OPERATING RANGES article.

Fig. 20: Buick LeSabre 1991 - Component Locations - Code 26, Schematic, Quad-Driver Circuit

Fig. 21: Buick LeSabre 1991 - Component Locations - Code 26, Quad-Driver Circuit

Buick LeSabre 1991 - Hot Light Diagnosis

NOTE:

These checks assume vehicle is not overheating. Verify proper operation of cooling system prior to diagnosing hot light.

Hot light is powered by the 10-amp INDIC fuse. Light will turn on when ECM provides a ground for the circuit. If circuit grounds between light and ECM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3). If hot light does not illuminate, check the following:
   1. 10-amp INDIC fuse.
   2. Faulty instrument cluster bulb
   3. Open circuit between fuse and hot light.
2. Backprobe terminal YC9 at ECM with a test light to battery voltage. Turn ignition on. If test light does not illuminate, ECM terminal connection is bad or ECM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper terminal YC9 to ground. Turn ignition on. If hot light does not illuminate, check for open circuit between hot light and ECM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. See DIAGNOSTIC AIDS. If test light is on, turn ignition off. Disconnect ECM connector. Probe ECM harness terminal YC9 with a test light to battery voltage. If light is off, replace ECM. If light is on, repair short to ground in circuit No. 35. If no short is present, replace instrument cluster.

Buick LeSabre 1991 - CODE 26, (2 OF 3) QUAD-DRIVER CIRCUIT

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. 3) This determines which circuit is out of specification. All circuits except SERVICE ENGINE SOON light and hot light should have battery voltage with engine running and ALDL test terminal "B" not grounded.

Buick LeSabre 1991 - Diagnostic Aids

Monitor the voltage of each terminal while moving related harness connectors, including control module harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace control module.

Fig. 22: Buick LeSabre 1991 - Component Locations - Code 26, (2 Of 3) Quad-Driver Circuit

Buick LeSabre 1991 - CODE 26, (3 OF 3) QUAD-DRIVER CIRCUIT

1. 4) This determines if the problem is the circuit or the component. Factory-installed ECM has an internal fuse and it's unlikely ECM needs replacing.

Fig. 23: Buick LeSabre 1991 - Component Locations - Code 26, (3 Of 3) Quad-Driver Circuit

Buick LeSabre 1991 - CODE 27, 28 OR 29, GEAR SWITCH CIRCUITS

The gear switches are located inside the transaxle. Switches are normally closed. As road speed increases, hydraulic pressure applies the specific gear clutches and the gear switch opens. The ECM uses the gear switches to help determine control of fuel delivery and TCC operation.

Code 27 will set if no Code 29 is present, circuit No. 581 indicates ground or closed switch for 10 seconds when vehicle is in 4th gear, or circuits No 581 or No. 446 indicate an open when the engine is first started.

Code 28 will set if circuit No. 108 indicates ground or closed switch for 10 seconds when the vehicle is in 4th gear, or circuits No. 108 or No. 446 indicate an open when engine is first started.

Code 29 will set if circuit No. 446 indicates ground or closed switch for 10 seconds when the vehicle is in 4th gear (TCC locked and brake not applied) or if circuit No. 446 indicates an open when engine is first started.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. A digital volt-ohmmeter must be used in this test. A test light will not work due to the low voltage supplied by the ECM.
2. Checks if the circuit is grounded through the switch.
3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.

Buick LeSabre 1991 - Diagnostic Aids

Check for poor connections at ECM pins. Inspect harness for incorrect routing (too close to high voltage wiring) or chafing. Monitor the voltage of each terminal while moving related harness connectors. If the failure is induced, the voltage reading will change.

Fig. 24: Buick LeSabre 1991 - Component Locations - Code 27, 28 or 29, Schematic, Gear Switch Circuits

Fig. 25: Buick LeSabre 1991 - Component Locations - Code 27, 28 or 29, Gear Switch Circuits

Buick LeSabre 1991 - CODE 31, PARK/NEUTRAL SWITCH CIRCUIT

The park/neutral switch contacts are part of the neutral start switch. The contacts close to ground in Park or Neutral and open in Drive. Code 31 will set if park/neutral signal circuit indicates an open for 3-4 consecutive starts or if conditions occur as follows:

• No Code 29 exists.
• Circuit No. 434 indicates ground.
• Transmission is in high gear.
• TCC is locked on 4T60 trans. only.
• Vehicle speed is greater than 45 MPH and TPS is less than 15 percent (.94 volt) on 3T40 trans. only.
• All above conditions have been met for at least 12 seconds.

  NOTE:

  Test numbers refer to test numbers on diagnostic chart.

1. This tests for a closed switch to ground in Park.
2. This tests for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive, even when wiggling shifter.

Fig. 26: Buick LeSabre 1991 - Component Locations - Code 31, Schematic, Park/Neutral Switch Circuit

Fig. 27: Buick LeSabre 1991 - Component Locations - Code 31, Park/Neutral Switch Circuit

Buick LeSabre 1991 - CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 33 and determines if it is the result of a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high for greater than a precalibrated period of time, TPS voltage indicates throttle is closed and neither Code 21 or 22 is present.
2. This step simulates conditions for a Code 34. If the control module recognizes and sets Code 34, low MAP signal, the control module and 5-volt reference and MAP signal circuits are not at fault. If ground circuit is shared with other sensors and ground circuit becomes open, additional codes related to these sensors may be set.

Buick LeSabre 1991 - Diagnostic Aids

With the ignition switch in the ON position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. Code 33 will result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

Fig. 28: Buick LeSabre 1991 - Component Locations - Code 33, Schematic, MAP Sensor Signal Voltage High

NOTE:

For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 29: Buick LeSabre 1991 - Component Locations - Code 33, MAP Sensor Signal Voltage High

Buick LeSabre 1991 - CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This confirms Code 34 and determines if code was caused by a hard failure or an intermittent fault. Code 34 will set when ignition is on and MAP signal voltage is low. On some systems, engine must be running to set code.
2. Jumpering MAP signal to 5-volt reference at MAP harness connector will determine if sensor is at fault or if there is a problem with the control module or wiring.
3. "Scan" tester may not display 12 volts. The important thing is the control module recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the control module and MAP signal circuit are not at fault.

Buick LeSabre 1991 - Diagnostic Aids

With the ignition switch in the ON position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Compare BARO readings with a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. A Code 34 will also result if 5-volt reference and MAP signal circuits are open or shorted to ground.

Fig. 30: Buick LeSabre 1991 - Component Locations - Code 34, Schematic, MAP Sensor Signal Voltage Low

• For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 31: Buick LeSabre 1991 - Component Locations - Code 34, MAP Sensor Signal Voltage Low Flow Chart

Buick LeSabre 1991 - CODE 34, MAF SENSOR SIGNAL VOLTAGE LOW

Code 34 is set when engine is running with no MAF sensor signal for greater than 4 seconds. If Code 34 is set, control module will substitute a value for MAF based upon RPM, TPS and IAC monitored parameters.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This test determines if code is the result of an intermittent or hard failure.
2. A voltage reading at sensor harness terminal A of less than 4 volts or greater than 6 volts indicates a fault in circuit No. 492 or poor connections.
3. Verifies both ignition voltage and a good ground are available.

Buick LeSabre 1991 - Diagnostic Aids

The MAF sensor produces a frequency signal, which cannot be easily measured. Check for the following:

Buick LeSabre 1991 - Poor Connections

Inspect control module MAF signal pins and harness connectors for backed out terminals, improper connector mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection.

Buick LeSabre 1991 - Harness

Inspect MAF sensor harness to ensure it is not too close to high voltage wires, such as spark plug wires.

Buick LeSabre 1991 - Intermittents

If harness appears okay, use "Scan" tester to check MAF while moving related connectors and wiring harness. A change in display would indicate the intermittent fault location.

• For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.

Fig. 32: Buick LeSabre 1991 - Component Locations - Code 34, Schematic, MAF Sensor Signal Voltage Low

Fig. 33: Buick LeSabre 1991 - Component Locations - Code 34, MAF Sensor Signal Voltage Low (Models Equipped With MAF)

Buick LeSabre 1991 - CODE 35, IAC IDLE SPEED ERROR

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. IAC driver is used to extend and retract IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body.
2. Checks IAC movement quality from step 1). Between 700-1500 RPM, engine speed should change smoothly with each flash of the tester light in both extend and retract. If IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed begins to drop. This is normal on certain engines. Fully extending the IAC may cause engine to stall. This may be normal.
3. Steps 1) and 2) verified proper IAC valve operation while this step checks the IAC circuits. Each light on the node light should flash Red and Green while the IAC valve is cycled. While the sequence of color is not important if either light is off or does not flash Red and Green, check the circuits for faults beginning with poor terminal contacts.

Buick LeSabre 1991 - Diagnostic Aids

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 60 if too low, and zero counts, if too high. If idle is too high, stop engine. Fully extend IAC with driver. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.

Buick LeSabre 1991 - System Too Lean

If air/fuel ratio is too lean, the idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or digital voltmeter (10-megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an oxygen sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.

Buick LeSabre 1991 - System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.

Buick LeSabre 1991 - Throttle Body

Remove IAC and inspect bore for evidence of IAC valve dragging.

Buick LeSabre 1991 - IAC Valve Connections

Inspect carefully for loose or corroded connections.

Buick LeSabre 1991 - PCV Valve

An incorrect PCV valve may cause incorrect idle speed.

Fig. 34: Buick LeSabre 1991 - Component Locations - Code 35, Schematic, IAC Idle Speed Error

Fig. 35: Buick LeSabre 1991 - Component Locations - Code 35, Flow Chart

Buick LeSabre 1991 - CODE 35, IAC IDLE SPEED ERROR USING TECH I

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. The Tech 1 RPM control mode is used to extend and retract the IAC valve. Movement is verified by an engine speed change. If no change in speed occurs, valve can be retested when removed from throttle body. If IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed begins to drop. This is normal on certain engines. Fully extending the IAC may cause engine to stall. This may be normal.
2. This test uses the Tech 1 to command the IAC controlled idle speed. The control module issues commands to obtain the requested idle speed. Each light on the node light should flash Red and Green while the IAC valve is cycled. While the sequence of color is not important if either light is off or does not flash Red and Green, check the circuits for faults beginning with poor terminal contacts.

Buick LeSabre 1991 - Diagnostic Aids

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 60 if too low, and zero counts, if too high. If idle is too high, stop engine. Fully extend IAC with driver. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.

Buick LeSabre 1991 - System Too Lean

If air/fuel ratio is too lean, the idle speed may be either too high (check for vacuum leaks) or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or digital voltmeter (10-megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an oxygen sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.

Buick LeSabre 1991 - System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.

Buick LeSabre 1991 - Throttle Body

Remove IAC and inspect bore for evidence of IAC valve dragging.

Buick LeSabre 1991 - IAC Valve Connections

Inspect carefully for loose or corroded connections.

Buick LeSabre 1991 - PCV Valve

An incorrect PCV valve may cause incorrect idle speed.

Fig. 36: Buick LeSabre 1991 - Component Locations - Code 35, Schematic, IAC Idle Speed Error Using Tech I

Fig. 37: Buick LeSabre 1991 - Component Locations - Code 35, Flow Chart Using Tech 1

Buick LeSabre 1991 - CODE 38, BRAKE SWITCH CIRCUIT

Code 38 will set if no Code 24 is present, status at brake input terminal of control module has not changed from high to low and vehicle speed has been greater than 35 MPH and back to zero MPH a precalibrated number of times.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Jumpering the brake switch determines if the ECM and wiring for the brake switch are okay.
2. Determines if brake switch is out of adjustment or is faulty.
3. Verifies the voltage to the brake switch.

Buick LeSabre 1991 - Diagnostic Aids

A Code 38 in conjunction with a Code 39 or 26 would mean a problem with one or more of the following components.

• Fuse or power supply circuit, brake switch or wire before the splice.
• Code 38 alone is the result of a wire or circuit problem between the splice and the control module, poor connection to the control module, or possibly the control module, itself. If brake switch has failed in an open state, TCC will not engage. Code 38 does not turn on SERVICE ENGINE SOON light on all models.

Fig. 38: Buick LeSabre 1991 - Component Locations - Code 38, Schematic, Brake Switch Circuit

NOTE:

Scan testers display brake status in different manners depending upon manufacturer. What is important is that status changes when brake pedal is depressed.

Buick LeSabre 1991 - If Code 26 is set with Code 38, see Code 26 chart.

Fig. 39: Buick LeSabre 1991 - Component Locations - Code 38, Flow Chart

Buick LeSabre 1991 - CODE 39, WITH 4T60 (440-T4) TRANS. TCC CIRCUIT

Code 39 will set when no Code 28 or 29 is set, brake is not applied, TCC is commanded by ECM, transmission is in high gear and engine speed-to-vehicle speed ratio does not indicate TCC has engaged. All these conditions must have been met for more than 15 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Tests fuse, brake switch and battery power circuit to the TCC solenoid.
2. Tests for ECM driver operation at TCC harness connector.
3. Tests for ECM driver operation at ECM terminal.

Buick LeSabre 1991 - Diagnostic Aids

A Code 39 in conjunction with a Code 38 would mean a problem with one or more of the following components.

• Fuse or power circuit, brake switch or wire before the splice. Code 39 alone indicates a problem at:
• Circuit No. 420 between splice and TCC solenoid.
• Circuit No. 422 between TCC solenoid and ECM.
• Poor connection to the ECM or ECM, itself.

Fig. 40: Buick LeSabre 1991 - Component Locations - Code 39, Schematic, W/ 4T60 (440-T4) Trans. TCC Ckt

Fig. 41: Buick LeSabre 1991 - Component Locations - Code 39, Flow Chart

Buick LeSabre 1991 - CODE 39, WITH 4T60, TCC CIRCUIT USING TECH 1

Code 39 will set when no Code 28 or 29 is set, brake is not applied, TCC is commanded by ECM, transmission is in high gear and engine speed-to-vehicle speed ratio does not indicate TCC has engaged. All these conditions must have been met for more than 15 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Tests fuse, brake switch and battery power circuit to the TCC solenoid.
2. Tests for ECM driver operation at TCC harness connector.
3. Tests for ECM driver operation at ECM terminal.

Buick LeSabre 1991 - Diagnostic Aids

A Code 39 in conjunction with a Code 38 would mean a problem with one or more of the following components.

• Fuse or power circuit, brake switch or wire before the splice. Code 39 alone indicates a problem at:
• Circuit No. 420 between splice and TCC solenoid.
• Circuit No. 422 between TCC solenoid and ECM.
• Poor connection to the ECM or ECM, itself.

Fig. 42: Buick LeSabre 1991 - Component Locations - Code 39, Schematic, W/4T60, TCC Ckt Using Tech 1

Fig. 43: Buick LeSabre 1991 - Component Locations - Code 39, Flow Chart Using Tech 1

Buick LeSabre 1991 - CODE 41, CAM SENSOR CIRCUIT

Quick reference:

• See Fig. 45 for Code 41 flow chart.
• See Fig. 44 for Code 41 supplemental wiring.

Buick LeSabre 1991 - All Updated Information

All the Code 41 information here (including the flow chart) has been updated as per the General Motors Technical Service Bulletins (TSBs) listed below. All outdated information has been discarded.

• Buick 91-6E-14
• Oldsmobile 91-T-45
• Pontiac 91-6-18

Buick LeSabre 1991 - Circuit Description

The 3800 engine uses the simultaneous mode of fuel injection during start-up. As the engine speed attains the 400 RPM level and a cam signal has been received by the ECM from C3I module, the fuel injection switches modes to sequential injection.

This is accomplished by use of a cam interrupter magnet and a cam sensor "Hall Effect" switch. The cam sensor sends a signal (sync-pulse) to the ignition module when cylinder #1 is 25? after top dead center on the compression stroke. This signal is used to start sequential fuel injection with the proper cylinder.

If cam signal is lost to the ECM, the fuel delivery will switch back to the simultaneous mode of injection. THE ENGINE WILL CONTINUE TO RUN. IT WILL RESTART AFTER SHUT DOWN.

Code 41 is set when the following conditions are met:

• Engine is running.
• Cam sensor signal not received by the ECM in last 2 seconds.

Buick LeSabre 1991 - Test Description

The numbers below refer to the circled numbers in the diagnostic flow chart.

1. This step verifies proper operation of circuits 633, 644, and 645. See Fig. 44 for circuit identification.
2. Step validates the integrity of Circuit 630 from the ignition module to the ECM. See Fig. 45 for circuit identification.
3. If the camshaft gear magnet is interfacing with the cam sensor the voltage reading will be zero, bumping engine will cause the condition to go away.
4. If the voltage reading of terminal "BC5" is constantly varying and connection to terminal "BC5" are good, the ECM is faulty.

Buick LeSabre 1991 - Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed-through wire insulation, or a wire broken inside the insulation.

Check for the following:

• Poor Connection or Damaged Harness: Inspect ECM harness connectors for backed out terminal "BC5", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.
• Intermittent Test: If connections and harness check okay, monitor a digital voltmeter connected from ECM terminal "BC5" to ground while moving related connectors and wiring harness. If failure is induced, voltage reading will change. This may help to isolate the location of the malfunction.

Buick LeSabre 1991 - Schematic & Flow Chart

Fig. 44: Buick LeSabre 1991 - Component Locations - Code 41 Supplemental Wiring Schematic, Cam Sensor Signal

Fig. 45: Buick LeSabre 1991 - Component Locations - Code 41 Flow Chart, Cam Sensor Signal

Buick LeSabre 1991 - CODE 42, EST CIRCUIT OPEN OR GROUNDED

Code 42 will set if EST or by-pass circuit is open or grounded at time of engine start-up.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Tests if ECM recognizes a problem. If it doesn't set Code 42 at this point, it is an intermittent problem. Check for a loose connection.
2. With the ECM disconnected, the digital volt-ohmmeter should read less than 200 ohms. This is normal EST circuit resistance through the ignition module. A higher resistance would indicate a fault in circuit No. 423, a poor ignition module connection or a faulty ignition module.
3. If test light was on when connected from 12 volts to ECM harness by-pass circuit, either circuit No. 424 is shorted to ground or the ignition module is faulty.
4. Tests if ignition module switches when by-pass circuit is energized by 12 volts through test light. If ignition module switches, the resistance reading should switch from less than 200 ohms to more than 6000 ohms.
5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise, circuit No. 423 is shorted to ground.

Buick LeSabre 1991 - Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

Buick LeSabre 1991 - Poor Connection

Inspect ECM harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection or damaged harness.

Buick LeSabre 1991 - Intermittents

If connections and harness are okay, connect a digital volt-ohmmeter between affected terminal to ground and monitor meter while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.

Fig. 46: Buick LeSabre 1991 - Component Locations - Code 42, Schematic, EST Circuit Open/Grounded

Fig. 47: Buick LeSabre 1991 - Component Locations - Code 42, Flow Chart

Buick LeSabre 1991 - CODE 43, ESC ERROR WITHOUT ESC MODULE

1. If tapping on engine does not produce a knock signal on "Scan" tester, try tapping the engine closer to the sensor before proceeding.
2. The ECM supplies a 5-volt reference signal on the knock sensor line. With knock sensor connected this signal is pulled low to about 2.5 volts. Knock signal, which is an AC signal, rides on this 2.5-volt DC signal.
3. This test determines if the knock sensor is faulty or if the ESC portion of MEM-CAL is faulty.

Buick LeSabre 1991 - Diagnostic Aids

Check knock sensor signal circuit for a potential open or short to ground. An open knock signal circuit would cause the 5-volt reference to remain high at ECM knock signal terminal. A short to ground in the knock signal circuit would cause the 5-volt reference signal to pull low to near zero volts. Resistance to ground through the knock sensor should be 3300-4500 ohms. Also check for proper installation of MEM-CAL.

Fig. 48: Buick LeSabre 1991 - Component Locations - Code 43, Schematic, ESC Error Without ESC Module

Fig. 49: Buick LeSabre 1991 - Component Locations - Code 43, Flow Chart

Buick LeSabre 1991 - CODE 44, LEAN EXHAUST INDICATION

Oxygen sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Checks to see if O2 sensor is registering a lean condition. Code 44 is set when O2 sensor voltage signal at control module is low (less than .3 volt) for a precalibrated period and system is operating in "closed loop".

Buick LeSabre 1991 - Diagnostic Aids

Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If conditions for a Code 44 exist, the block learn value will be around 150.

Buick LeSabre 1991 - Oxygen Sensor (O2) Wire

O2 sensor wire may be mispositioned and laying against the exhaust manifold. Check for ground between sensor and wire connector.

Buick LeSabre 1991 - Fuel Contamination

Water, even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water may cause a lean exhaust and set Code 44.

Buick LeSabre 1991 - Fuel Pressure

System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING article in this section.

Buick LeSabre 1991 - Exhaust Leaks

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

Buick LeSabre 1991 - Misfire Or Stall

If engine misfires or stalls while vehicle is moving, including running out of fuel, a Code 44 may set. If Code 44 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 50: Buick LeSabre 1991 - Component Locations - Code 44, Schematic, Lean Exhaust Indication

Fig. 51: Buick LeSabre 1991 - Component Locations - Code 44, Flow Chart

Buick LeSabre 1991 CODE 44 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECMTerminal	Wire Color
O2 Signal	YD2	Purple
O2 Ground	YD3	Tan

Buick LeSabre 1991 - CODE 45, RICH EXHAUST INDICATION

Oxygen sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.

Code 45 indicates a rich exhaust and diagnosis should begin with these items: fuel pressure, leaking injector, HEI shielding (ground), vapor canister fuel saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Test checks to see if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than idle, O2 sensor signal at control module is greater than .7 volt for a precalibrated period and time since engine start is one minute or more.

Buick LeSabre 1991 - Diagnostic Aids

If other codes of lower number are set with Code 45, use those charts first. Malfunction in the MAP or TPS sensor circuits can cause a Code 45 to set. If other codes are not set, Code 45, rich exhaust, is most likely caused by one of the following:

Buick LeSabre 1991 - Fuel Pressure High

If fuel pressure is too high, air/fuel ratios will be rich. For fuel pressure checking procedure, see BASIC TESTING article in this section. The control module can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

Buick LeSabre 1991 - Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by control module on reference line of EST harness. Additional pulses result in a higher than actual engine speed signal. The control module will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help in diagnosing this problem.

Buick LeSabre 1991 - Evaporative Fuel Canister

Fuel saturation of the charcoal canister will cause a rich air/fuel ratio. If full of fuel, check canister control valves and hoses.

Buick LeSabre 1991 - MAP Sensor

An output that causes the control module to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the control module to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.

Buick LeSabre 1991 - TPS

An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.

Buick LeSabre 1991 - O2 Sensor Contamination

O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White powdery coating to cover the exterior of the O2 sensor. The false high signal voltage (low oxygen content sensed) produced is interpreted by the control module as a rich mixture, causing the control module to set Code 45.

Buick LeSabre 1991 - EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. Also check for shorted or leaking injector, or fuel contaminated oil. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.

Fig. 52: Buick LeSabre 1991 - Component Locations - Code 45, Schematic, Rich Exhaust Indication

Fig. 53: Buick LeSabre 1991 - Component Locations - Code 45, Flow Chart

Buick LeSabre 1991 CODE 45 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION

Application	ECM Terminal	Wire Color
O2 Signal	YD2	Purple
O2 Ground	YD3	Tan

Buick LeSabre 1991 - CODE 46, POWER STEERING PRESSURE SWITCH

The power steering pressure switch is normally open. Turning the steering wheel increases pressure and closes the switch, pulling voltage on the switch circuit low. Code 46 will set when power steering circuit voltage is low (indicating high pressure) and vehicle speed is greater than 40 MPH. Both of these conditions must be met for more than 15-25 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic charts.

1. Tests for normal operation of power steering switch. "Scan" testers will display this information differently. - the manufacturer's operating manual for additional information.
2. If display does not change to normal (no pressure) when switch connector is removed, check for power steering switch signal circuit shorted to ground. If circuit is not shorted, ECM is faulty. If display does change to normal when switch connector is disconnected, switch is defective.
3. Determines if fault is switch or circuit. A jumpered connector should cause high pressure to be displayed, indicating wiring and ECM are okay. If high pressure is indicated and Code 46 is present, switch or switch connector is faulty.

Buick LeSabre 1991 - Diagnostic Aids

Check harness for poor connections at ECM and switch. If connections and harness are okay, monitor switch status on a "Scan" tester while wiggling harness and connectors. An abrupt change in status indicates a problem.

Fig. 54: Buick LeSabre 1991 - Component Locations - Code 46, Schematic, Power Steering Psi. Switch

Fig. 55: Buick LeSabre 1991 - Component Locations - Code 46, Flow Chart

Buick LeSabre 1991 - CODE 48, MISFIRE DIAGNOSIS

NOTE:

If multiple codes are present, go to the lowest code first. Repairing Code 13, 44 or 45 may correct Code 48.

Code 48 will set if the TPS is .48-1.30 volts, RPM is 1300-2100, MPH is 50-60, O2 sensor cross counts are greater than 21 and all of these conditions are met for 30 seconds.

Buick LeSabre 1991 - Diagnostic Aids Ignition System Checks

Remove and inspect each spark plug. If plug(s) are fouled, check ignition wires, ignition coil and ignition module operation. If plugs are cracked or worn, replace plugs. If no fault is found, perform basic engine checks.

Buick LeSabre 1991 - Fuel System Checks

Check for restricted fuel system (injectors, fuel pump, lines and filter). Perform injector balance test. Verify proper injector circuit operation using Injector Tester (J-34730-3). Check fuel pump pressure and volume.

Buick LeSabre 1991 - Basic Engine Checks

Perform engine compression check. Unless spark plug condition or compression check identifies a specific cylinder, road test vehicle under test conditions to verify Code 48 prior to engine disassembly. Upon disassembly, inspect pistons, rings, valves, valve springs and valve guides. Check for worn or damaged camshaft lobes or lifters.

Fig. 56: Buick LeSabre 1991 - Component Locations - Code 48, Flow Chart Misfire Diagnosis

Fig. 57: Buick LeSabre 1991 - Component Locations - Code 48, Flow Chart

Buick LeSabre 1991 - CODE 51, FAULTY PROM/MEM-CAL

Check that all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace control module.

Buick LeSabre 1991 - CODE 52, FAULTY CALPAK

Check that all pins are fully inserted in socket. If okay, replace CALPAK, clear memory and recheck. If Code 51 reappears, replace control module.

Buick LeSabre 1991 - CODE EO51, FAULTY PROM/MEM-CAL

Check that all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code EO51 reappears, replace control module.

Buick LeSabre 1991 - CODE 63, 64 OR 65, EGR FLOW CHECK ERROR

On a closed throttle coastdown, ECM energizes solenoids inside of EGR valve and monitor changes in RPM and O2 sensor activity. If expected changes do not occur, appropriate code will be set. Code 26 in conjunction with Code 63, 64 or 65 indicates a possible quad-driver problem. Before using this chart, use Code 26 chart, clear codes and test drive to see if EGR codes reset.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. With ALDL not grounded and engine not running, test light should not illuminate.
2. With ALDL test terminal B grounded, ECM should energize the EGR solenoids by providing a ground at the ECM/ This will cause the test light to illuminate when touched to EGR harness terminals A, B and C. If test light does not illuminate, check for open circuit or faulty ECM.

NOTE:

If digital EGR valve shows signs of excessive heat (i.e. a melted condition), check exhaust system for blockage (possible plugged converter).

Fig. 58: Buick LeSabre 1991 - Component Locations - Code 63, 64 or 65 Schematic, EGR Flow Check Error

NOTE:

Use schematic to identify terminal referenced in the following flow chart.

Fig. 59: Buick LeSabre 1991 - Component Locations - Code 63, 64 Or 65, EGR Flow Check Error Flow Chart (Without Tech 1)

Buick LeSabre 1991 - CODE 63, 64 OR 65, EGR FLOW CHECK ERROR USING TECH 1

On a closed throttle coastdown, ECM energizes solenoids inside of EGR valve and monitor changes in RPM and O2 sensor activity. If expected changes do not occur, appropriate code will be set. Code 26 in conjunction with Code 63, 64 or 65 indicates a possible quad-driver problem. Before using this chart, use Code 26 chart, clear codes and test drive to see if EGR codes reset.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. A noticeable change in RPM should occur as each solenoid is cycled on. Solenoid No. 1 should cause the smallest change in RPM and solenoid No. 3 should cause the largest change and may even stall the engine.
2. Checks for an open circuit in harness between EGR valve and ECM, faulty ECM connection or faulty ECM.

NOTE:

If digital EGR valve shows signs of excessive heat (i.e. a melted condition), check exhaust system for blockage (possible plugged converter).

Fig. 60: Buick LeSabre 1991 - Component Locations - Code 63, 64 or 65, Schematic, EGR Flow Check Error Using Tech 1

NOTE:

Use schematic to identify terminal referenced in this flow chart.

Fig. 61: Buick LeSabre 1991 - Component Locations - Code 63, 64 Or 65, EGR Flow Check Error Flow Chart (With Tech 1)