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Buick Riviera 1991 - G - TESTS W/CODES - 3.8L PFI 1991 ENGINE PERFORMANCE General Motors 3.8L E Body PFI - Self-Diagnostics - ECM/PCM
Buick Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 1991 - DIAGNOSTIC PROCEDURE
Diagnosis of the computerized engine control system should be performed in the following order:
- 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.
- 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.
- 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.
- After any repairs are made, clear any trouble codes and perform FIELD SERVICE MODE check in BASIC TESTING article in this section.
Buick Riviera 1991 - RETRIEVING CODES (NON-SCAN)
NOTE: For information on retrieving codes using a "Scan" tester, refer to owners manual supplied with "Scan" tester. On Reatta, Riviera, Toronado and Trofeo, codes can also be retrieved through the Electronic Climate Control Panel (ECCP). For additional information on ECCP function, see SELF-DIAGNOSTICS in the appropriate G - PCM/BCM TESTS W/ CODES article in this section.
- 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. 
Fig. 1: Buick Riviera 1991 - Component Locations - ALDL Connector Terminal Identification - 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 Riviera 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 Riviera 1991 - TROUBLE CODE DEFINITION
Buick Riviera 1991 ECM/PCM 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 24 Vehicle Speed Sensor Circuit 25 MAT Sensor Signal Voltage Low 26 Quad-Driver Error 33 MAP Sensor Signal Voltage High 34 MAF Sensor Signal Voltage Low 35 IAC Idle Speed Error 36 (2) Trans. Shift Circuit 38 Brake Switch 39 TCC 41 Cam Sensor Loss 42 EST Circuit Open Or Grounded 43 ESC Error 47 PCM-BCM Data Loss 48 Misfire Diagnosis 51 Faulty PROM, MEM-CAL or ECM/PCM 52 Faulty/Missing CALPAC or MEM-CAL 58 Anti-Theft System Fault 61 (2) Cruise Vent Solenoid 62 (2) Cruise Vacuum Solenoid 65 (2) Cruise servo position 66 Low A/C Refrigerant Charge 67 (2) Cruise Engage Switches 68 (2) Cruise System Problem
(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 Riviera 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:
- MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See CLEARING TROUBLE CODES .
- 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.
- 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.
- If SERVICE ENGINE SOON light does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.
Buick Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 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 0-255 EGR 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 (A/T); Spec. +/-50 RPM Neut. (M/T 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 Riviera 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 Riviera 1991 - DIAGNOSTIC FLOW CHARTS FOR CODES: 13 TO 68
NOTE: Schematics are supplied courtesy of General Motors Corp.
Buick Riviera 1991 - CODE 13, OPEN OXYGEN SENSOR CIRCUIT
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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.
- 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.
- This tests O2 sensor circuit wiring. Use only a high impedance (10-megohm minimum) digital voltmeter.
Buick Riviera 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 Riviera 1991 - Component Locations - Code 13, Schematic, Open Oxygen Sensor Circuit
Fig. 3: Buick Riviera 1991 - Component Locations - Code 13, Open Oxygen Sensor Circuit
Buick Riviera 1991 CODE 13 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color O2 Signal BD3 Purple O2 Ground BD2 Tan
Buick Riviera 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.
- 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.
- This tests for grounded sensor signal line between control module and coolant sensor.
Buick Riviera 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.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 5: Buick Riviera 1991 - Component Locations - Code 14, Coolant Temp Sensor Signal Voltage Low
Buick Riviera 1991 CODE 14 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal BB9 Yellow CTS Ground BA8 Black
Buick Riviera 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 Riviera 1991 - CODE 15, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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.
- 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.
- This test determines if coolant sensor ground or signal circuit is open.
Buick Riviera 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.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 7: Buick Riviera 1991 - Component Locations - Code 15, Coolant Temp Sensor Signal Voltage High
Buick Riviera 1991 CODE 15 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color CTS Signal BB9 Yellow CTS Ground BA8 Black
Buick Riviera 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 Riviera 1991 - CODE 16, SYSTEM VOLTAGE HIGH/LOW
The control module monitors battery voltage on the battery feed circuit. If the control module detects battery voltage greater than 17.3 volts or less than 9 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.
- 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 17.3 volts or less than 9 volts, service alternator.
Buick Riviera 1991 - 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: When Code 16 sets, transaxle will be forced to 3rd gear, preventing erratic shifting due to improper voltage.
Fig. 10: Buick Riviera 1991 - Component Locations - Code 16, System Voltage High/Low
Buick Riviera 1991 - CODE 17, RPM SIGNAL PROBLEM
- Verifies that spark reference circuit is not shorted to ground or open in the ignition jumper harness.
- If a window on the harmonic balancer is lined up with the 18x Hall Effect switch, ignition module will ground spark reference signal. Starter may have to be bumped several times to obtain a voltage reading.
- Voltage reading should be lower than that obtained with engine not running, indicating a pulsed reference signal.
Buick Riviera 1991 - Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Also, check for backed out connector terminals or broken insulation spark reference circuit. If everything checks okay, try wiggling the related wiring harness and connectors while engine is idling. This may help to isolate the location of the malfunction.
Fig. 13: Buick Riviera 1991 - Component Locations - Code 17, RPM Signal Problem
Buick Riviera 1991 - CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test checks if code is the result of a hard failure or an intermittent condition.
- This test simulates conditions for a Code 22. If control module recognizes the change of state, the control module and wiring are okay.
- 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 Riviera 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.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Buick Riviera 1991 CODE 21 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal BB10 Dark Blue TPS Ground BA8 Black TPS Reference BB3 Gray
Buick Riviera 1991 - CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test checks if code is the result of a hard failure or an intermittent condition.
- This test simulates conditions for a Code 21. If control module recognizes the change of state, the control module and wiring are okay.
- 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 Riviera 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.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Buick Riviera 1991 CODE 22 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color TPS Signal BB10 Dark Blue TPS Ground BA8 Black TPS Reference BB3 Gray
Buick Riviera 1991 - CODE 23, MAT SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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).
- This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the control module and wiring are not at fault.
- 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 Riviera 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.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 19: Buick Riviera 1991 - Component Locations - Code 23, MAT Sensor Signal Voltage High
Buick Riviera 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAT Signal BB7 Tan MAT Ground BA7 Black
Buick Riviera 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 Riviera 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.
- 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.
- Before replacing control unit, check MEM-CAL for correct application.
Fig. 20: Buick Riviera 1991 - Component Locations - Code 24, Schematic, Vehicle Speed Sensor (VSS)
NOTE: Disregard Code 24 that sets while wheel are not turning.
Fig. 21: Buick Riviera 1991 - Component Locations - Code 24, Vehicle Speed Sensor (VSS)
Buick Riviera 1991 - CODE 25, MAT SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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.
- 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 Riviera 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.
NOTE: For shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 23: Buick Riviera 1991 - Component Locations - Code 25, MAT Sensor Signal Voltage Low
Buick Riviera 1991 CODE 25 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color MAT Signal BB7 Tan MAT Ground BA7 Black
Buick Riviera 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 Riviera 1991 - CODE 26, (1 OF 3) QUAD-DRIVER CIRCUIT
Each PCM Quad-Driver has a fault line which is monitored by the PCM. The PCM compares voltage values of the fault line with acceptable values in PCM memory. If the PCM 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. 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 PCM does not know which controlled circuit set the Code 26 so this chart will go through each of the circuits to determine which is at fault. This step tests SERVICE ENGINE SOON light driver and circuit.
Buick Riviera 1991 - QDM Symptoms
- 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 Riviera 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. 26: Buick Riviera 1991 - Component Locations - Code 26, Quad-Driver Circuit (1 of 3)
Buick Riviera 1991 - CODE 26, (2 OF 3) QUAD-DRIVER CIRCUIT
2) This determines which circuit is out of specification. All circuits except GD16 and GD15 should have battery voltage with ignition on, engine not running and ALDL test terminal not grounded.
Buick Riviera 1991 - Diagnostic Aids
Monitor the voltage of each terminal while moving related harness connectors, including PCM harness. If the fault is induced, the voltage will change. This may help locate intermittent problems. If code reappears with no apparent problems, replace PCM.
Fig. 27: Buick Riviera 1991 - Component Locations - Code 26, Quad-Driver Circuit (2 of 3)
Buick Riviera 1991 - CODE 26, (3 OF 3) QUAD-DRIVER CIRCUIT
3) This determines if the problem is the circuit or the component. Factory-installed PCM has an internal fuse and it's unlikely PCM needs replacing.
Fig. 28: Buick Riviera 1991 - Component Locations - Code 26, Quad-Driver Circuit (3 of 3)
Buick Riviera 1991 - CODE 31, PARK/NEUTRAL SWITCH CIRCUIT
NOTE: Complete diagnosis of Code 31 for (VIN L) E Body vehicles requires the use of the GM Tech 1 "Scan" tester which is capable of indicating status of all 4 park/neutral switch positions.
Buick Riviera 1991 - CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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.
- 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 Riviera 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.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 30: Buick Riviera 1991 - Component Locations - Code 33, MAP Sensor Signal Voltage High
Buick Riviera 1991 - CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW
NOTE: Test numbers refer to test numbers on diagnostic chart.
- 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.
- 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.
- "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 Riviera 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.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 32: Buick Riviera 1991 - Component Locations - Code 34, MAP Sensor Signal Voltage Low
Buick Riviera 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.
- This test determines if code is the result of an intermittent or hard failure.
- 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.
- Verifies both ignition voltage and a good ground are available.
Buick Riviera 1991 - Diagnostic Aids
The MAF sensor produces a frequency signal, which cannot be easily measured. Check for the following:
Buick Riviera 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 Riviera 1991 - Harness
Inspect MAF sensor harness to ensure it is not too close to high voltage wires, such as spark plug wires.
Buick Riviera 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.
NOTE: For shared sensor reference and shared sensor ground tie-offs, see appropriate diagram in WIRING DIAGRAMS.
Fig. 33: Buick Riviera 1991 - Component Locations - Code 34, Schematic, MAF Sens Signal Vltge Low
Fig. 34: Buick Riviera 1991 - Component Locations - Code 34, MAF Sens Signal Vltge Low
Buick Riviera 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.
- 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.
- 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.
- 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 Riviera 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 Riviera 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 Riviera 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 Riviera 1991 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
Buick Riviera 1991 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Buick Riviera 1991 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Fig. 35: Buick Riviera 1991 - Component Locations - Code 35, Schematic, IAC Idle Speed Error
Fig. 36: Buick Riviera 1991 - Component Locations - Code 35, IAC Idle Speed Error
Buick Riviera 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.
- 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.
- 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 Riviera 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 Riviera 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 Riviera 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 Riviera 1991 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
Buick Riviera 1991 - IAC Valve Connections
Inspect carefully for loose or corroded connections.
Buick Riviera 1991 - PCV Valve
An incorrect PCV valve may cause incorrect idle speed.
Fig. 38: Buick Riviera 1991 - Component Locations - Code 35, IAC Idle Speed Error Using Tech I
Buick Riviera 1991 - CODE 36, WITH 4T60E TRANS SHIFT CONTROL PROBLEM
The 4T60E transaxle is electronically shifted. Within transaxle are 4 solenoids. Solenoid A is used for 1st and 4th gear operation only. Solenoid b is used for 1st and 2nd gear operation. The remaining 2 solenoids are for TCC operation only. All PRNDL indications are ignored as far as transaxle shifting is concerned except manual low. Code 36 will set if solenoid "B" failed in the off position, which will cause the transaxle to be in 3rd gear and desired gear is 1st, TPS is greater than 5 percent, VSS is greater than 5 MPH and no code 21, 22 or 24 is present. Code will also set if solenoid "B" failed in the on position, which will cause transaxle to be in 1st gear and desired gear is 4th, PRNDL is in 3rd or 4th, TPS is greater than 10 percent and no Code 31, 21 or 22 is present.
Buick Riviera 1991 - Diagnostic Aids
When Code 36 is set, transaxle will be forced into 3rd gear. If code sets due to a grounded circuit No. 1223, 1st and 2nd gear operation will be available only. If circuit No. 1223 is open, 3rd and 4th gear operation will be available only. If fault goes away, normal operation will be resumed for the duration of the key cycle.
Fig. 39: Buick Riviera 1991 - Component Locations - Code 36, Schematic, W/4T60E, Shift Ctrl Problem
Fig. 40: Buick Riviera 1991 - Component Locations - Code 36, W/4T60E, Shift Ctrl Problem
Buick Riviera 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.
- Jumpering the brake switch determines if the ECM and wiring for the brake switch are okay.
- Determines if brake switch is out of adjustment or is faulty.
- Verifies the voltage to the brake switch.
Buick Riviera 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. 41: Buick Riviera 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.
NOTE: If Code 26 is set with Code 38, see Code 26 chart.
Fig. 42: Buick Riviera 1991 - Component Locations - Code 38, Brake Switch Circuit
Buick Riviera 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.
- Tests fuse, brake switch and battery power circuit to the TCC solenoid.
- Tests for ECM driver operation at TCC harness connector.
- Tests for ECM driver operation at ECM terminal.
Buick Riviera 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. 44: Buick Riviera 1991 - Component Locations - Code 39, W/4T60 (440-T4) Trans. TCC Ckt
Buick Riviera 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.
- Tests fuse, brake switch and battery power circuit to the TCC solenoid.
- Tests for ECM driver operation at TCC harness connector.
- Tests for ECM driver operation at ECM terminal.
Buick Riviera 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. 45: Buick Riviera 1991 - Component Locations - Code 39, Schematic, W/4T60, TCC Ckt Using Tech 1
Fig. 46: Buick Riviera 1991 - Component Locations - Code 39, W/4T60, TCC Ckt Using Tech 1
Buick Riviera 1991 - CODE 41, CAM SENSOR CIRCUIT All Updated Information
The Code 41 flow chart has been updated as per the General Motors Technical Service Bulletin (TSB) listed below. All outdated information has been discarded.
- Buick 91-6E-13
- Oldsmobile 91-T-45
Code 41 will set if the engine is running and a cam sensor signal is not received by the ECM for the last 2-5 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Verifies proper operation of circuits No. 633, 644 and 645.
- Tests circuit No. 630 from the C(3)I ignition module to the ECM.
- If harmonic balancer windows are interfacing with the cam sensor, the voltage reading may be zero. Bumping the engine with the starter will cause the condition to go away.
- If voltage reading is varying around a midpoint of 10.5 volts and connections are good, the ECM is faulty.
Buick Riviera 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:
Buick Riviera 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 Riviera 1991 - Intermittents
If connections and harness are okay, connect a digital volt-ohmmeter (10-megohm) between ground and circuit No. 630 terminal at ECM. monitor DVOM while moving related connectors and wiring harness. Voltage reading will change if failure is induced. This may help isolate malfunction.
Fig. 49: Buick Riviera 1991 - Component Locations - Code 41, Flow Chart, All With Type 1 Ignition
Fig. 50: Buick Riviera 1991 - Component Locations - Code 41, Flow Chart, All With Type 2 Ignition
Buick Riviera 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.
- 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.
- 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.
- 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.
- 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.
- 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 Riviera 1991 - Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.
Buick Riviera 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 Riviera 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. 53: Buick Riviera 1991 - Component Locations - Code 42, EST Ckt Open/Grounded
Buick Riviera 1991 - CODE 43, ESC ERROR WITHOUT ESC MODULE
- If tapping on engine does not produce a knock signal on "Scan" tester, try tapping the engine closer to the sensor before proceeding.
- 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.
- This test determines if the knock sensor is faulty or if the ESC portion of MEM-CAL is faulty.
Buick Riviera 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. 54: Buick Riviera 1991 - Component Locations - Code 43, Schematic, ESC Error Without ESC Module
Fig. 55: Buick Riviera 1991 - Component Locations - Code 43, ESC Error Without ESC Module
Buick Riviera 1991 - CODE 47, PCM-BCM DATA LOSS
Check for momentary loss of BCM's 7 volts on circuit No. 807, 750 or 555. Also, ensure that MEM-CAL is properly installed.
Buick Riviera 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 Riviera 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 Riviera 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 Riviera 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 Riviera 1991 - Component Locations - Code 48, Flow Chart Misfire Diagnosis
Buick Riviera 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 Riviera 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 Riviera 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 Riviera 1991 - CODE 58, PASS-KEY FUEL ENABLE CIRCUIT
Personal Automotive Security System (PASS-KEY) is designed to disable vehicle operation if an incorrect ignition key or starting procedure is used. If correct key and starting technique is used, PASS-KEY decoder module sends a fuel enable signal to the PCM and also energizes starter enable relay. If the proper signal does not reach the PCM on circuit No. 229, the PCM will not pulse the injectors and the vehicle will not start and Code 58 will set.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- If vehicle will not crank with Code 58 stored, problem affects entire PASS-KEY system and is not isolated to fuel enable circuit.
- The PCM applies and monitors a 5-volt signal on circuit No. 229. Decoder module will pulse this signal to ground when proper key and starting technique is used. This test ensures that PCM is supplying the 5-volt signal and that circuit is not open or shorted to ground.
- Checks PWM signal from PASS-KEY module. Since the 5 volts supplied by the PCM is being pulsed to ground, voltage on circuit No. 229 should measure about 2.5 volts.
- Checks for a faulty PCM or intermittent condition by clearing code. Since the PCM ignores the absence of a fuel enable signal only when Code 58 is stored, vehicle should not start if the problem is present and Code 58 is not.
Buick Riviera 1991 - PASS-KEY Diagnosis
PASS-KEY is a sophisticated system which interfaces the PASS-KEY decoder module, PCM and starter with a power source, starter enable relay, ignition switch, instrument cluster and Remote Accessory Control (RAC) module.
NOTE: Testing and servicing PASS-KEY decoder module requires special test equipment and documentation.
Before replacing the decoder module, check the system for the following common problems.
- Check ignition key pellet sensing contacts in the ignition lock cylinder. Look into cylinder lock. If contacts are damaged, replace cylinder lock.
- Check fuse No. 11 in instrument panel fuse block.
- Check fuse No. 4 in right underhood fuse block.
- Check fuse No. 8 in relay center.
- Check security indicator bulb in the instrument panel.
- A defective resistor pellet within the ignition key or incorrect resistance value of key (15 different assigned values) will cause vehicle not to start. Key must be correct electrically and mechanically. If incorrect key is used to try to start vehicle, decoder will not allow vehicle to start for 2-4 minutes, even if correct key is inserted.
Fig. 57: Buick Riviera 1991 - Component Locations - Code 58, Schematic, Pass-Key Fuel Enable Ckt
Fig. 58: Buick Riviera 1991 - Component Locations - Code 58, Pass-Key Fuel Enable Ckt
Buick Riviera 1991 - CODE 61, CRUISE VENT SOLENOID
Cruise switch within turn signal lever receives ignition voltage from a 15-amp fuse on circuit No. 639. Cruise switches are inputs to the cruise control portion of the PCM. Cruise servo, vent and vac solenoids are output lines and are controlled by high side drivers in the PCM. With ignition on, PCM looks at these output lines and will set a code depending upon actual status of these lines compared with commanded status. In this manner PCM can detect opens or shorts to ground or voltage.
Fig. 59: Buick Riviera 1991 - Component Locations - Code 61, Schematic, Cruise Vent Solenoid
Fig. 60: Buick Riviera 1991 - Component Locations - Code 61, Cruise Vent Solenoid
Buick Riviera 1991 - CODE 62, CRUISE VAC SOLENOID
Cruise switch within turn signal lever receives ignition voltage from a 15-amp fuse on circuit No. 639. Cruise switches are inputs to the cruise control portion of the PCM. Cruise servo, vent and vac solenoids are output lines and are controlled by high side drivers in the PCM. With ignition on, PCM looks at these output lines and will set a code depending upon actual status of these lines compared with commanded status. In this manner PCM can detect opens or shorts to ground or voltage.
Fig. 61: Buick Riviera 1991 - Component Locations - Code 62, Schematic, Cruise VAC Solenoid
Fig. 62: Buick Riviera 1991 - Component Locations - Code 62, Cruise VAC Solenoid
Buick Riviera 1991 - CODE 65, CRUISE SERVO POSITION
The PCM supplies 5 volts to the cruise control servo position sensor. Depending upon actual servo position, the voltage on the servo position sensor circuit will indicate to PCM the position of the servo. Code 65 will set if circuit No. 399 is open or if circuit No. 398 is open, or shorted to ground or voltage.
Fig. 63: Buick Riviera 1991 - Component Locations - Code 65, Schematic, Cruise Servo Position
Fig. 64: Buick Riviera 1991 - Component Locations - Code 65, Cruise Servo Position
Buick Riviera 1991 - CODE 67, CRUISE ENGAGE SWITCHES
Code 67 will set if circuit No. 397 or 86 is open, SET/COAST or RESUME/ACCEL switch is closed for an extended amount of time, or any of the 3 cruise engage switch circuits are shorted to voltage.
Fig. 67: Buick Riviera 1991 - Component Locations - Code 67, Cruise Engage Switches
Buick Riviera 1991 - CODE 68, SYSTEM PROBLEM USING TECH 1
The PCM-integrated cruise control system is designed to monitor itself to ensure that the desired cruise position and actual cruise position are equal to each other. Code 68 will set when actual servo position sensor signal is 12 percent greater than desired servo position sensor signal for 1.5 seconds.
Fig. 68: Buick Riviera 1991 - Component Locations - Code 68, Schematic, System Problem Using Tech 1
NOTE: Diagnosing Code 68 using this chart requires the use of the GM Tech 1 Bi-Directional Scan Tool.
Fig. 69: Buick Riviera 1991 - Component Locations - Code 68, System Problem (Using Tech 1)
Buick Riviera 1991 - Diagnostic Aids
Outside interference such as CB antenna lead near PCM wiring harness may cause a false servo position sensor signal, setting Code 68.