Buick LeSabre 1990 - 1990 ENGINE PERFORMANCE 3.8L PFI Model Self-Diagnostics

Buick LeSabre 1990 - 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 H - TESTING W/O CODES article for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.). If only intermittent codes are present, see INTERMITTENTS in H - TESTING W/O CODES article in this section. The ECM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, ECM 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 ECM 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 1990 - 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 unit will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is functional, but loss of good driveability will most likely be encountered.

Buick LeSabre 1990 - 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 ECM memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in H - TESTING W/O CODES article in this section.

Buick LeSabre 1990 - 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. Appropriate DIAGNOSTIC CIRCUIT CHECK for that system MUST be performed prior to utilizing trouble code charts. See BASIC TESTING article.
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 H - TESTING W/O CODES article. 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 H - TESTING W/O CODES article. The comments there will send you to the proper system or component to check in SYSTEM/COMP TESTS article.
4. After any repairs are made, clear any trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.

Buick LeSabre 1990 - RETRIEVING CODES (NON-SCAN)

1. Turn ignition on. Do not start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness. Most ALDL connectors are located under dash on driver's side of vehicle. For exact location of ALDL, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMP TESTS article. Turn ignition on. 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. Codes will not flash if this is done. See FIELD SERVICE MODE in BASIC TESTING article.

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   Fig. 1: Buick LeSabre 1990 - 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, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

Buick LeSabre 1990 - READING TROUBLE CODES

The ECM 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 ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN DATA TABLES and SCAN TESTER USAGE 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 terminal 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.

Buick LeSabre 1990 ECM TROUBLE CODE DEFINITION

Code No.	Circuit Affected
12 (1)	No RPM reference pulse
13	Open oxygen sensor circuit
14	CTS signal voltage low
15	CTS signal voltage high
16	System voltage high
21	TPS signal voltage high
22	TPS signal voltage low
23	MAT sensor signal voltage high, M/C solenoid voltage low
24	VSS circuit
25	MAT sensor signal voltage low
26	Quad-Driver error
27, 28 & 29	Gear switch problem
31	Wastegate error (turbo), Park/Neutral switch
32	EGR system error
33	MAP sensor signal voltage high, MAF sensor signal voltage high (PFI)
34	MAP sensor signal voltage low, MAF sensor signal voltage low (PFI)
35	IAC idle speed error
38	Brake switch
39	TCC
41	C(3)I ignition - cam sensor loss, Cylinder select error (MEM-CAL)
42	EST circuit open or grounded
43	ESC retard signal too low
44	Lean exhaust indicated
45	Rich exhaust indicated
46	Anti-theft fault, Power steering pressure switch
48	Misfire diagnosis
51	Faulty PROM, MEM-CAL or ECM
52	Faulty/missing CALPAC or MEM-CAL
52	Engine oil temperature low (Corvette)
53	Faulty alternator, voltage high, EGR system malfunction, Anti-theft circuit fault
54	Fuel pump voltage low
55	Faulty ECM
61	Degraded O2 sensor
62	Gear switch error
62	Engine oil temperature high (Corvette)
63	EGR flow error
64	EGR flow error
65	EGR flow error, Fuel injector current low
66	A/C pressure sensor voltage out of specification
(1) Display of a Code 12 is normal when no reference pulses are received by ECM (engine not running).

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 Codes 51, 52, 54 or 55 are displayed with another code, start with 50-series code first, then proceed to low profile numbered codes.

Buick LeSabre 1990 - 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 each DIAGNOSTIC PROCEDURE.

Buick LeSabre 1990 - CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic terminal lead at ALDL connector. Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.

Buick LeSabre 1990 - ECM LOCATION

On most vehicles the ECM is located behind the right side of the dash, or behind the right kick panel. On the Grand Prix and Lumina, the ECM is located on the right side of the engine compartment. On Corvette, the ECM is located in the left rear corner of the engine compartment, next to the battery.

Buick LeSabre 1990 - 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 1990 - FIELD SERVICE MODE CHECK (FUEL INJECTED)

On fuel injected models, SERVICE ENGINE SOON light will indicate operational mode of engine if ALDL is grounded while engine is running. 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 (FUEL INJECTED) in BASIC TESTING article.

Buick LeSabre 1990 - 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" on EFI and terminal "M" on EFI with P-4 systems). These testers can save a great deal of time. For additional information, see SCAN TESTER USAGE and SCAN TESTER DATA tables 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, dwell meter, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (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.

Before engine reaches operating temperature, dwell should be fixed between 10-50 degrees, indicating open loop operation. With engine at operating temperature and idling, dwell meter needle should be varying between 10-50 degrees. This indicates closed loop operation. If (after reaching normal operating temperature) dwell is fixed between 10-50 degrees, less than 10 degrees or more than 50 degrees, refer to SYSTEM PERFORMANCE CHECK in BASIC TESTING article.

Buick LeSabre 1990 - 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 appropriate DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. 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 back-probe sensors and connectors. "Scan" testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN DATA tables 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 ECM. 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 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 SCAN DATA tables 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. 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 1990 - 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 & systems are used on all models.

Buick LeSabre 1990 PORT FUEL INJECTION

Tester Position	Units Measured	Nominal Value
Engine Speed	RPM	650-850
Coolant Temp.	?C	85-105? (norm. temperature)
Mani Air Temp.	?C	Varies with Air Temperature
Throttle Position	Volts	.33-.46
LV8	0	70-80
O2 Sensor	Millivolts	100 (lean) to 900 (rich)
Injector Pulse Width	Milliseconds	4.0-5.0 (Varies)
Spark Advance	Degrees	20?
Mass Air Flow	GM/SEC	4-7
Fuel Integ.	Counts	(1) 110-138
Block Learn	Counts	(1) 110-138
Knock Signal	No/Yes	No
Idle Air Control	Counts (steps)	10-30
P/N Switch	P/N/RDL	Park/Neutral
Vehicle Speed	MPH, KPH	0 (Zero)
TCC	On/Off	Off (On with command)
Battery Voltage	Volts	13.8 Volts (Varies)
Exhaust Recirc.	%	0 (Zero)
A/C Request	On/Off	OFF
Brake Switch	No/Yes	No (Yes with brake pedal depressed)
Power Steering	High Press/Normal	Normal
QDM1	High/Low	Low
QDM2	High/Low	Low (High if A/C head pressure's high)
QDM3	High/Low	Low (High w/brake pedal depressed
QDM4	High/Low	Low
P/N	Yes/No	Yes
2nd Gear	Yes/No	No
3rd Gear	Yes/No	No
4th Gear	Yes/No	No
PROM ID	Numbers	Internal ID Only
(1) A poor ECM ground at the ignition module mounting bracket to engine could cause fuel integ and block learn to read around 150; make careful physical inspection of this critical connection.

Buick LeSabre 1990 - DIAGNOSTIC CODE CHARTS: 13 - 65

NOTE:

The following diagnostic flow charts and mini-schematics are supplied courtesy of General Motors Corp.

Buick LeSabre 1990 - CODE 13: OPEN OXYGEN SENSOR CIRCUIT

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set if the following conditions occur:
   • Engine is at operating temperature.
   • No Code 21 or 22 is set.
   • At least 40 seconds engine running time after start.
   • Oxygen signal steady between .30 and .55 volt.
   • Throttle position sensor signal greater than idle.
   • All conditions must be met for 30 seconds.

   If all conditions for a Code 13 exist, system will not go into closed loop.

2. This test determines if Code 13 is caused by the sensor or wiring.
3. When performing this test, use only a high impedance (10-megohm) digital volt-ohmmeter. This tests the continuity of circuits No. 412 and 413. If circuit No. 413 is open, the ECM voltage on circuit No. 412 will be greater than .6 volt.

Buick LeSabre 1990 - Diagnostic Aids

An intermittent Code 13 may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Inspect ECM harness and connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection or damage to harness.

If connections and harness check out okay, "Scan" oxygen sensor voltage while moving related connectors and wiring harness. Engine should be warm, running at part throttle in closed loop. If the failure is induced, the oxygen sensor voltage reading will change from its normal fluctuating voltage (greater than 600 mV to less than 300 mV) to a fixed value around 450 mV. This may help to isolate the location of the malfunction.

Fig. 2: Buick LeSabre 1990 - Component Locations - Code 13: Open O2 Sensor Ckt Diag

Fig. 3: Buick LeSabre 1990 - Component Locations - Code 13: Open O2 Sensor Flow Chart

Buick LeSabre 1990 - CODE 14: COOLANT TEMP SENSOR SIGNAL VOLT LOW

As the engine warms, sensor resistance becomes less and monitored voltage drops. At normal engine operating temperature, voltage will measure about 1.5-2.0 volts.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 14 will set if monitored voltage indicates a coolant temperature greater than 284?F (140?C) for .4 second or more.
2. This test determines if circuit No. 410 is shorted to ground, which will cause the conditions for Code 14. If Code 14 is set, the ECM uses a default coolant value of about 120?F (49?C). However, the "Scan" tester will read the actual temperature indicated by the sensor.

Buick LeSabre 1990 - Diagnostic Aids

The "Scan" tester displays engine temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90?C, then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check the following conditions:

• Poor Connection Inspect ECM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness.
• Intermittent If connections and harness check out okay, use "Scan" tester to check the coolant temperature reading while moving related connectors and wiring harness. If the failure is induced, coolant temperature display will change. This may help to isolate the location of the malfunction.
• Shifted Sensor - The TEMPERATURE VS. RESISTANCE VALUES table may be used to test the coolant sensor at various temperature levels to evaluate the possibility of a shifted (out-of-calibration) sensor which may result in driveability problems.

Fig. 4: Buick LeSabre 1990 - Component Locations - Code 14: Coolant Temp Sensor Volt Lo Ckt Diag

Fig. 5: Buick LeSabre 1990 - Component Locations - Code 14: Coolant Temp Sensor Volt Lo Flow Chart

Buick LeSabre 1990 - CODE 15: COOLANT TEMP SENSOR SIGNAL VOLT HIGH

At operating temperature, voltage will measure about 1-2 volts at ECM terminal.

NOTE:

Test numbers refer to test numbers on diagnostic chart. The "Scan" tester reads engine temperature in degrees centigrade.

1. Code 15 will set if the monitored voltage indicates a coolant temperature less than -36?F (-38?C) for at least 4 seconds with engine running.
2. This test simulates a Code 14. If the ECM recognizes the low signal voltage (high temperature) and "Scan" tester reads 140?C, the ECM and wiring are okay.
3. This test determines if circuit No. 410 is open. Using a digital volt-ohmmeter, there should be 5 volts present at sensor connector. If Code 15 is set, the ECM uses a default coolant value of about 120?F (49?C). However, the "Scan" tester will read the actual temperature indicated by the sensor.

Buick LeSabre 1990 - Diagnostic Aids

After engine is started the temperature should rise steadily to about 90?C, then stabilize when thermostat opens. An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation. Check the following conditions:

• Poor Connection Inspect ECM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness.
• Intermittent If connections and harness check out okay, use "Scan" tester to check coolant temperature while moving related connectors and wiring harness. If the failure is induced, the coolant temperature display will change. This may help to isolate the location of the malfunction.
• Shifted Sensor The TEMPERATURE TO RESISTANCE VALUES table may be used to test the coolant sensor at various temperature levels to evaluate the possibility of a shifted (out-of-calibration) sensor which may result in driveability complaints.

A faulty connection, or an open circuit No. 410 or 452 will result in a Code 15. If Code 23 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor for good contact.

Fig. 6: Buick LeSabre 1990 - Component Locations - Code 15: Coolant Temp Sensor Volt Hi Ckt Diag

Fig. 7: Buick LeSabre 1990 - Component Locations - Code 15: Coolant Temp Sensor Volt Hi Flow Chart

Buick LeSabre 1990 - CODE 16: SYSTEM VOLTAGE HIGH

The ECM monitors battery voltage on circuit No. 440. If the ECM 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. Starting engine with battery charger connected may set Code 16.

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 1990 - Diagnostic Aids

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.

NOTE:

Starting engine with battery charger connected may set Code 16.

Fig. 8: Buick LeSabre 1990 - Component Locations - Code 16: System Volt Hi Ckt Diag

Fig. 9: Buick LeSabre 1990 - Component Locations - Code 16: System Volt Hi Flow Chart

Buick LeSabre 1990 - CODE 21: TPS SIGNAL VOLT HIGH

1. A Code 21 will set if engine is running and the following conditions occur:
   • TPS signal voltage is greater than 4.9 volts at any time.

   or if:

   • Mass Airflow Sensor (MAF) signal is low.
   • Throttle Position Sensor (TPS) signal voltage is greater than 1.5-2.5 volts.
   • Code 34 is not present.
   • Both conditions must be met for 5 seconds.

   With closed throttle, ignition on or engine at idle, voltage should be less than .33-.46 volt. If voltage is not okay, check TPS adjustment. See ADJUSTMENTS article in this section.

2. With the TPS sensor disconnected, Code 22 should set and TPS voltage should go low if the ECM and wiring are okay.
3. Probing circuit No. 452 with a test light checks the sensor ground circuit. A faulty sensor ground circuit will cause a Code 21.

Buick LeSabre 1990 - Diagnostic Aids

An open in circuit No. 452 will result in a Code 21. Check the following conditions:

• Poor Connection Inspect ECM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness.
• Intermittents If connections and harness check out okay, monitor TPS voltage on "Scan" tester while moving related connectors and wiring harness. If failure is induced, coolant temperature display will change. This may help isolate malfunction.
• TPS Scaling Observe TPS voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TPS voltage when throttle was closed, to greater than 4.5 volts when throttle is held at wide open throttle position.

Fig. 10: Buick LeSabre 1990 - Component Locations - Code 21: TPS Signal Volt Hi Ckt Diag

Fig. 11: Buick LeSabre 1990 - Component Locations - Code 21: TPS Signal Volt Hi Flow Chart

Buick LeSabre 1990 - CODE 22: TPS SIGNAL VOLT LOW

1. Code 22 will set if engine is running and TPS signal voltage is less than .1 volt for 4 seconds.
2. Simulates Code 21. If ECM recognizes the high signal voltage, the ECM and wiring are okay.
3. With closed throttle, ignition on or engine at idle, voltage should be .33-.46 volt. If voltage is not okay, check TPS adjustment.
4. Simulates a high signal voltage. Checks circuit No. 417 for open.

Buick LeSabre 1990 - Diagnostic Aids

The "Scan" tester reads throttle position in volts. Voltage should increase at a steady rate as throttle is moved from idle to wide open throttle. Some testers may also read throttle angle in percent (zero at closed throttle, 100 at wide open throttle). An open or short to ground in circuit No. 416 or No. 417 will result in a Code 22. Check for the following conditions:

• Poor Connections Inspect ECM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness.
• Intermittents If connections and harness check out okay, monitor TPS voltage using "Scan" tester, while moving related connectors and wiring harness. If the failure is induced, the coolant temperature display will change. This may help to isolate the location of the malfunction.
• TPS Scaling Observe TPS voltage display while depressing accelerator pedal with engine stopped and ignition on. Display should vary from closed throttle TPS voltage (or zero percent) when throttle was closed, to greater than 4.5 volts (or 100 percent) when throttle is held wide open.

Fig. 12: Buick LeSabre 1990 - Component Locations - Code 22: TPS Signal Volt Lo Ckt Diag

Fig. 13: Buick LeSabre 1990 - Component Locations - Code 22: TPS Signal Volt Lo Flow Chart

Buick LeSabre 1990 - CODE 23: MAT SENSOR SIGNAL VOLT HIGH

The MAT sensor is used in conjunction with the MAF sensor so the ECM can accurately compensate airflow readings based on temperature. Code 23 will set if a signal voltage indicates a manifold air temperature less than -40?F (-40?C) for 4 seconds. Due to conditions necessary to set a Code 23, SERVICE ENGINE SOON light will stay on only while the fault is present.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. The "Scan" tester may not be used to diagnose this fault, due to the ECM transmitting default values. A Code 23 will set, due to an open sensor, wire or connection. This test determines if the wiring and ECM are okay.
2. If the resistance is greater than 25,000 ohms, inspect air cleaner for presence of ice. If okay, replace the sensor. INTERMITTENTS is located in H - TESTING W/O CODES article.

NOTE:

A "SCAN" tool may not be used to diagnosis this fault, Due to the ECM transmitting "DEFAULT" (Substitute) values when the fault is present.

Fig. 14: Buick LeSabre 1990 - Component Locations - Code 23: MAT Sensor Volt Hi Ckt Diag

Fig. 15: Buick LeSabre 1990 - Component Locations - Code 23: MAT Sensor Volt Hi Flow Chart

Buick LeSabre 1990 - CODE 24: VEHICLE SPEED SENSOR (VSS)

The Vehicle Speed Sensor (VSS) is a PM generator type located in the transmission or transaxle. As the vehicle moves, VSS produces an AC voltage signal which is routed to the buffer inside the ECM. In the buffer, the AC signal is changed from a sine wave to a square wave and amplified. The square wave is an on/off pulsed signal. The length of time between pulses determines vehicle speed.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Code 24 will set if vehicle speed equals less than 3 MPH when the following conditions occur:
   • Engine is running greater than 1000 RPM.
   • Vehicle is in high gear.
   • No Code 28, 29 or 31 is present.
   • All conditions met for 40 seconds.
2. Before replacing ECM, check MEM-CAL for proper application.

Buick LeSabre 1990 - Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect ECM harness connectors for improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection and damaged harness. If connections and harness check out okay, raise drive wheels (support drive axles at lower control arms to prevent damage to CV joints). Block other wheels and run engine at greater than 3 MPH in low gear. Use "Scan" tester to check vehicle speed, while moving related connectors and wiring harness. If failure is induced, vehicle speed display will change. This may help isolate the malfunction.

Fig. 16: Buick LeSabre 1990 - Component Locations - Code 24: VSS Ckt Diag

Fig. 17: Buick LeSabre 1990 - Component Locations - Code 24: VSS Flow Chart

Buick LeSabre 1990 - CODE 25: MAT SENSOR SIGNAL VOLT HIGH

Code 25 will set if the following conditions occur:

• Signal voltage indicates manifold air temperature is greater than 275?F (135?C).
• Vehicle speed is greater than 35 MPH.
• Both of the above requirements are met for at least 16 seconds.

Due to conditions necessary to set a Code 23, SERVICE ENGINE SOON light will stay on only while the fault is present.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. The "Scan" tester may NOT be used to diagnose this fault, due to the ECM transmitting default values. ECM and wiring are good if voltage is greater than 4 volts.
2. If resistance is less than 185 ohms, replace sensor. See MAT SENSOR TEMPERATURE TO RESISTANCE VALUES table. INTERMITTENTS is located in H - TESTING W/O CODES article.

Fig. 18: Buick LeSabre 1990 - Component Locations - Code 25: MAT Sensor Volt Hi Ckt Diag

Fig. 19: Buick LeSabre 1990 - Component Locations - Code 25: MAT Sensor Volt Hi Flow Chart

Buick LeSabre 1990 - CODE 26: 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. This tests SERVICE ENGINE SOON light driver and 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 1990 - Hot Light Diagnosis

Hot light is powered by the 10-amp GAUGE fuse. Light will turn on when ECM provides a ground for the circuit. If circuit ground 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 does not illuminate, check ignition switch ground. Turn ignition off.
2. Disconnect ECM connectors. Jumper terminal YC9 to ground. Turn ignition on. If hot light does not illuminate, check the following:
   • 10-amp GAUGE fuse.
   • Open circuit between fuse and hot light.
   • Open circuit between hot light and ECM terminal.
3. This determines which circuit is out of specification. All circuits except YC11 and YC9 should have battery voltage with engine running and ALDL test terminal not grounded.
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.

Buick LeSabre 1990 - Diagnostic Aids

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

Fig. 20: Buick LeSabre 1990 - Component Locations - Code 26: Quad-Driver Ckt Diag

Fig. 21: Buick LeSabre 1990 - Component Locations - Code 26: Quad-Driver Steps 1 & 2 Flow Chart (2 of 3)

Fig. 22: Buick LeSabre 1990 - Component Locations - Code 26: Quad-Driver Step 3 Flow Chart (2 of 3)

Fig. 23: Buick LeSabre 1990 - Component Locations - Code 26: Quad-Driver Step 4 Flow Chart (3 of 3)

Buick LeSabre 1990 - 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 1990 - 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 1990 - Component Locations - Code 27/28 or 29: Gear Switch Ckt Diag

Fig. 25: Buick LeSabre 1990 - Component Locations - Code 27/28 or 29: Gear Switch Circuit Flow Chart

Buick LeSabre 1990 - 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 circuit No. 434 indicates an open for 4 consecutive starts or if conditions occur as follows:

• No Code 38 exists.
• Circuit No. 434 indicates ground.
• Transmission is in high gear.
• TCC is locked on 4-speed only.
• Vehicle speed is greater than 45 MPH and TPS is less than 15 percent (.94 volt) on 3-speed only.
• All above conditions have been met for 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 1990 - Component Locations - Code 31: Park/Neutral Switch Ckt Diag

Fig. 27: Buick LeSabre 1990 - Component Locations - Code 31: Park/Neutral Switch Flow Chart

Buick LeSabre 1990 - CODE 34: MASS AIRFLOW SENSOR

Code 34 is set when engine is running with no MAF sensor signal for greater than 4 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This tests if ECM recognizes a problem.
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 1990 - Diagnostic Aids

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

• Poor Connections Inspect ECM pin YD10 and harness connectors for backed out terminals, improper connector mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connection.
• Harness Inspect MAF sensor harness to ensure it is not too close to high voltage wires, such as spark plug wires.
• 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.

Fig. 28: Buick LeSabre 1990 - Component Locations - Code 34: Mass Airflow Sensor Ckt Diag

Fig. 29: Buick LeSabre 1990 - Component Locations - Code 34: Mass Airflow Sensor Flow Chart

Buick LeSabre 1990 - CODE 38: BRAKE SWITCH CIRCUIT

Code 38 will set if no Code 24 is present, status at circuit No. 420 has not changed from high to low and vehicle speed has been greater than 35 MPH and back to zero MPH, 22 or more 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. Verifies the voltage to the brake switch.
3. Tests the brake switch for proper operation or misadjustment.

Buick LeSabre 1990 - Diagnostic Aids

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

• Fuse, circuit No. 639, brake switch or wire before the splice.
• A single Code 38 is the result of a wire or circuit No. 420 problem between the splice and the ECM, poor connection to the ECM, or possibly the ECM, itself.

Fig. 30: Buick LeSabre 1990 - Component Locations - Code 38: Brake Switch Ckt Diag

Fig. 31: Buick LeSabre 1990 - Component Locations - Code 38: Brake Switch Flow Chart

Buick LeSabre 1990 - CODE 39: TCC CIRCUIT (W/440-T4 TRANS)

Code 39 will set when no Code 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 availability of battery voltage at circuit No. 420.
3. Electrical circuits have checked out okay. If there is a TCC engagement problem still, problem is possibly caused by an internal transmission hydraulic failure.

Buick LeSabre 1990 - Diagnostic Aids

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

• Fuse, circuit No. 639, brake switch or wire before the splice.

Buick LeSabre 1990 - A single Code 39 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. 32: Buick LeSabre 1990 - Component Locations - Code 39: TCC W/440-T4 Trans Ckt Diag

Fig. 33: Buick LeSabre 1990 - Component Locations - Code 39: TCC W/440-T4 Trans Flow Chart

Buick LeSabre 1990 - CODE 41: CAM SENSOR CIRCUIT

Quick reference:

• See Fig. 35 for Code 41 flow chart.
• See Fig. 34 for Code 41 supplemental wiring.

Buick LeSabre 1990 - 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 1990 - 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 1990 - 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. 34 for circuit identification.
2. Step validates the integrity of Circuit 630 from the ignition module to the ECM. See Fig. 35 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 1990 - 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 1990 - Schematic & Flow Chart

Fig. 34: Buick LeSabre 1990 - Component Locations - Code 41 Supplemental Wiring Schematic, Cam Sensor Signal

Fig. 35: Buick LeSabre 1990 - Component Locations - Code 41 Flow Chart, Cam Sensor Signal

Buick LeSabre 1990 - CODE 42: EST IGNITION CIRCUIT

To set a Code 42 one of the following conditions must occur:

• Engine speed greater than 600 RPM with no EST pulse for 100-200 milliseconds (open or grounded circuit No. 423).
• ECM commanding by-pass mode (open or grounded circuit No. 424).

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 terminal BC7, 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 shift 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 1990 - Diagnostic Aids

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

• 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.
• 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. 36: Buick LeSabre 1990 - Component Locations - Code 42: EST Ignition Ckt Diag

Fig. 37: Buick LeSabre 1990 - Component Locations - Code 42: EST Ignition Circuit Flow Chart

Buick LeSabre 1990 - CODE 43: ELECTRONIC SPARK CONTROL

The ECM supples a 5-volt reference on knock sensor signal line. Internal sensor circuitry will cause this voltage to pull down to about 2.5 volts DC. The knock sensor produces an AC voltage signal when knock occurs. This AC voltage signal will ride on the DC signal back to the ECM. Code 43 will set if voltage on circuit No. 496 is greater than 3.5 volts or less than 1.5 volts and this condition is met for 20 seconds.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. If a Code 43 is detected, the ECM will retard spark timing by 10 degrees. If an audible knock is heard from the engine, repair the internal engine problem. Normally, no knock should be heard at idle.
2. The ECM has a 5-volt pull-up resistor which places a 5-volt reference on the knock sensor signal line.
3. This tests if the knock sensor is faulty or if the ESC portion of the MEM-CAL is faulty.

Buick LeSabre 1990 - Diagnostic Aids

Check circuit No. 496 for a potential open or short to ground. Also, check for proper installation of the MEM-CAL.

Fig. 38: Buick LeSabre 1990 - Component Locations - Code 43: Elect Spark Control Ckt Diag

Fig. 39: Buick LeSabre 1990 - Component Locations - Code 43: Elect Spark Control Flow Chart

Buick LeSabre 1990 - CODE 44: LEAN EXHAUST INDICATION

Code 44 is set when the O2 sensor signal voltage on circuit No. 412 remains less than .25 volt for up to 4 1/2 minutes and the system is operating in closed loop.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. Running the engine at 1000 RPM will keep the O2 sensor hot so an accurate voltage display is maintained. Opening the sensor wire should result in a voltage display of .35-.55 volt. If the display is still fixed at less than .35 volt, the fault is a short to ground in circuit No. 412 or a faulty ECM.

Buick LeSabre 1990 - Diagnostic Aids

Using the "Scan" tester, observe block learn values at different RPM and airflow conditions. "Scan" tester also displays block cells, so the block learn values can be checked in each cell to determine when Code 44 may have been set. If the conditions for Code 44 exist, the block learn values will be around 150. Check the following:

• Oxygen sensor pigtail may be mispositioned and contacting the exhaust manifold.
• Check for intermittent ground in wire between connector and sensor.
• Check for a false lean condition caused by faulty Mass Airflow (MAF) sensor. Disconnect the MAF sensor and if the lean condition stops, replace the MAF sensor.
• Perform injector balance test, see SYSTEM/COMP TESTS article.
• Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set Code 44.
• System will be lean if pressure is too low. Monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm problem (if necessary).
• An exhaust leak can cause outside air to be pulled into the exhaust across the oxygen sensor. Vacuum or crankcase leaks can also cause a lean condition.
• If all of the above check okay, replace oxygen sensor.

Fig. 40: Buick LeSabre 1990 - Component Locations - Code 44: Lean Exhaust Ckt Diag

Fig. 41: Buick LeSabre 1990 - Component Locations - Code 44: Lean Exhaust Flow Chart

Buick LeSabre 1990 - CODE 45: RICH EXHAUST INDICATION

Code 45 is set if no Code 21 or 22 is set and the oxygen sensor signal voltage or circuit No. 412 remains greater than .75 volt for 2 minutes in closed loop, engine time after start is one minute or more and TPS voltage is .7-1.4 volts.

Buick LeSabre 1990 - Diagnostic Aids

Using the "Scan" tester, observe block learn values at different RPM and airflow conditions. Some "Scan" testers also display block cells, so block learn values can be checked in each cell to determine when Code 45 may have been set. If the conditions for Code 45 exists, block learn values will be around 115.

System will go rich if pressure is too high. The ECM can compensate for some increase, but if pressure is too high, a Code 45 may set. The following conditions could create a rich exhaust indication:

• Check for a leaking or sticking injector.
• Check charcoal canister. It may be full of raw fuel.
• Check Mass Airflow (MAF) sensor. Disconnecting the MAF sensor allows the ECM to set a fixed value for the sensor. Substitute another MAF sensor if the rich condition is gone with the sensor disconnected.
• Check fuel pressure regulator diaphragm for leaks by checking vacuum line fuel.
• Check for intermittent TPS output. It can give a false indication of engine acceleration.
• Check for EGR staying open, especially at idle. It will cause the oxygen sensor to indicate a rich exhaust.

Fig. 42: Buick LeSabre 1990 - Component Locations - Code 45: Rich Exhaust Ckt Diag

Fig. 43: Buick LeSabre 1990 - Component Locations - Code 45: Rich Exhaust Flow Chart

Buick LeSabre 1990 - 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 circuit No. 495 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 1990 - 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. 44: Buick LeSabre 1990 - Component Locations - Code 46: P/S Pressure Switch Ckt Diag & Flow Chart

Buick LeSabre 1990 - 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 1990 - 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.
• 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.
• 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.

Buick LeSabre 1990 - CODE 51, MEM-CAL ERROR

Ensure all pins are fully inserted in ECM socket. If okay, replace MEM-CAL. Clear memory and recheck for Code 51. If code reappears, replace ECM.

CAUTION:

To prevent possible electrostatic discharge (static electricity) damage to ECM or MEM-CAL, DO NOT touch component leads or remove integrated circuit from carrier. See N - REMOVE/INSTALL/OHAUL article.

Buick LeSabre 1990 - CODE 63, 64 OR 65, EGR FLOW CHECK 3.8L

On closed throttle coast down, the ECM (on closed throttle coast down) will cycle the integral EGR solenoids on and off individually and look for a resulting change in engine RPM and O2 sensor activity.

NOTE:

Test numbers refer to test numbers on diagnostic charts.

1. A change should be noticed in engine RPM as each solenoid is cycled on. A small change should occur when solenoid No. 1 is energized. Largest change should be noticed when solenoid No. 2 is energized. Engine may even stall.
2. Check for an open in circuits No. 697, No. 698 and No. 699 or a faulty ECM connection or ECM.

NOTE:

If EGR valve shows signs of excessive heat (melting), check for exhaust system blockage. See EXHAUST SYSTEM BACKPRESSURE in BASIC TESTING article Repair as necessary. If exhaust system has been damaged, check for a fuel injector constantly injecting fuel. Repair as necessary and check oil for fuel contamination.

Buick LeSabre 1990 - Diagnostic Aids

An intermittent code may be caused by poor connections at ECM or EGR valve or harness damage. Also, to help locate intermittents in the harness, connect a digital volt-ohmmeter to the suspected circuit. Wiggle the harness and related wiring while monitoring voltage. If the reading changes abruptly, the intermittent has been induced.

Fig. 45: Buick LeSabre 1990 - Component Locations - Code 63/64 or 65: EGR Flow Check Ckt Diag

Fig. 46: Buick LeSabre 1990 - Component Locations - Code 63/64 or 65: EGR Flow Check Flow Chart