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GMC Safari 1991 - 1991 ENGINE PERFORMANCE Self-Diagnostics - Gasoline
GMC Safari 1991 - INTRODUCTION
Most engine control problems result from mechanical failures, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, perform checks and inspections covered in article BASIC TESTING in this section. Failure to do so may result in lost diagnostic time.
If no faults were found while performing BASIC TESTING article 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 in this section for diagnosis by symptom (I.E. ROUGH IDLE, NO START, ETC.). If only intermittent codes are present, see INTERMITTENTS in TESTS W/O CODES article in this section.
GMC Safari 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.
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 READING TROUBLE CODES. Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.
GMC Safari 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 must be found 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 driveability can be poor.
GMC Safari 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 ECM memory. If related fault does not reoccur within 50 engine restarts, it will be erased from ECM memory. Intermittent failures may be caused by faulty sensor, connector or wiring. See INTERMITTENTS in TESTS W/O CODES article in this section.
GMC Safari 1991 - DIAGNOSTIC PROCEDURE
Diagnosis of the computerized engine control system should be performed in the following order:
1) Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See BASIC TESTING article in this section.
2) If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause SERVICE ENGINE SOON light to illuminate continuously with engine running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article in the section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes.
3) If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article in this section. From there you will be sent to the appropriate article SYSTEM/COMPONENT TESTS located in this section.
4) After repairs are made, clear trouble codes and perform FIELD SERVICE MODE check in BASIC TESTING article in this section.
GMC Safari 1991 - RETRIEVING CODES (NON-SCAN)
- Turn ignition on with engine off. 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 the article SYSTEM/COMPONENT TESTS article in this section. 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 and codes will not flash. See FIELD SERVICE MODE in BASIC TESTING article in this section. 
Fig. 1: GMC Safari 1991 - Component Locations - ALDL Diagnostic Connector Terminal ID - SERVICE ENGINE SOON light should flash codes. Each code is flashed 3 times. If codes DO NOT flash, perform DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
GMC Safari 1991 - READING TROUBLE CODES
NOTE: Trouble codes retrieved from ECM/PCM may be related to either engine or transmission. For engine related codes, use this article. For transmission related codes, see the TESTS W/CODES - TRANSMISSION article in this section Section. To identify which codes relate to transmission or engine, see TROUBLE CODE IDENTIFICATION TABLE.
The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting SERVICE ENGINE SOON light flashes or with diagnostic Scan tester connected to the ALDL connector. The tester is faster, and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN TESTER DATA TABLE and SCAN TESTER USAGE.
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, SERVICE ENGINE SOON light flashes can be read by grounding ALDL diagnostic terminal with ignition on and engine off. For example, FLASH, FLASH, pause, FLASH, longer pause, indicates 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 code. Each code is displayed 3 times and will continue as long as ALDL is grounded.
NOTE: Trouble codes will be recorded at various operating times. Some codes require sensor or switch operation for 5 seconds and others may require longer under certain conditions. Some codes may not set in a service bay operational mode.
GMC Safari 1991 TROUBLE CODE IDENTIFICATION
Code Probable Cause 12 No Engine Speed Sensor Reference Pulse To PCM/TCM 13 Open Oxygen Sensor Circuit 14 CTS Voltage Low (Sensor Or Signal Line Grounded) 15 CTS Voltage High (Sensor, Connections, Or Wires Open) 21 TPS Voltage High (Open Circuit Or Misadjusted TPS) 22 TPS Voltage Low (Circuit Grounded) 23 Intake Air Temperature Sensor Voltage High 24 Vehicle Speed Sensor Circuit Open Or Grounded 25 Intake Air Temperature Sensor Voltage Low 28 (1) Pressure Switch Manifold Range Circuit Open Or Shorted 31 Turbocharger Wastegate Overboost 32 EGR Error (Improper Vacuum Signal) 33 MAP Voltage High (Circuit Open Or Short To Voltage) 34 MAP Voltage Low (Circuit Open Or Short To Ground) 35 IAC System Fault 39 (1) TCC Stuck Off (Faulty TCC Solenoid) 42 EST Circuit Fault 43 ESC Fault 44 Lean Exhaust Indicated 45 Rich Exhaust Indicated 51 Improperly Installed/Faulty PROM/MEM-CAL 52 Fuel CAL-PAC Missing 53 System Voltage High (Charging System Problem) 54 Fuel Pump Circuit Voltage Low 55 Faulty ECM 58 (1) TTS High Temperature (Sensor Or Signal Line Grounded) 59 (1) TTS Low Temperature (Sensor, Connections, Or Wires Open) 68 (1) Overdrive Ratio Error (Engine RPM Greater Than Input Speed) 73 (1) Force Motor Commanded Amperage Differs From Return 75 (1) System Voltage Low (Charging System Problem) 81 (1) QDM Solenoid "B" Monitored Voltage Differs From Commanded 82 (1) QDM Solenoid "A" Monitored Voltage Differs From Commanded 83 (1) QDM TCC Monitored Voltage Differs From Commanded 85 (1) Undefined Gear Ratio (Input Or Output Sensor Failure) 86 (1) Shift Solenoid "B" Stuck On (Commanded Gear Not Engaged) 87 (1) Shift Solenoid "B" Stuck Off (Commanded Gear Not Engaged)
(1) Code is transmission related. For transmission code diagnosis, see TESTS W/CODES - TRANSMISSION article in this section.
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 or 55 are displayed with another code, start with 50-series code first and proceed to low profile numbered codes.
GMC Safari 1991 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
During any diagnostic procedure, determine if codes are due to hard or intermittent failure. Diagnostic charts will not usually help diagnose 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 reveals 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.
GMC Safari 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 ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.
GMC Safari 1991 - ECM LOCATION
For ECM locations, see appropriate COMPONENT LOCATIONS illustration in SYSTEM/COMPONENT TESTS article in this section.
GMC Safari 1991 - DIAGNOSTIC MATERIALS Diagnostic Aids
Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay.
GMC Safari 1991 - Field Service Mode Check
SERVICE ENGINE SOON light indicates operational mode of engine if ALDL is grounded while engine is running. Light response confirms proper fuel system operation 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.
GMC Safari 1991 - SPECIAL TOOLS (DIAGNOSTIC)
NOTE: A special Scan tester, plugged into the ALDL, can read trouble codes, check system voltages on the serial data line and save a great deal of time. For additional information, see tester owner's manual. Also, see SCAN TESTER USAGE and SCAN DATA.
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 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.
GMC Safari 1991 - SCAN TESTER USAGE
NOTE: Before connecting Scan tester, check diagnostic system and ensure accurate information is received by Scan tester. Perform DIAGNOSTIC CIRCUIT CHECK 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 can diagnose on-board computer control systems by providing almost instant access to circuit voltage information without crawling under dash or hood to back-probe sensors and connectors. Scan testers reduce diagnostic time by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER DATA TABLE.
Scan testers also furnish information on output device (solenoids and motors) status. However, status parameters are only an indication output signals have been sent to devices by the ECM. They do not indicate whether devices respond properly to that signal. This must 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 off, but it may not be indicated by all makes of Scan tester.
If trouble codes are not present, a problem may still exist. 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. Out-of-calibration sensors WILL NOT set a trouble code, but WILL cause driveability problems. 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 SCAN TESTER DATA TABLE.
NOTE: Information obtained by Scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are displayed, turn ignition off and remove tester. Turn ignition on and ground ALDL test terminal. If same codes are not flashed by SERVICE ENGINE SOON light as were indicated by Scan tester, tester cannot be used on vehicle and information obtained by it will not be guaranteed accurate.
GMC Safari 1991 - SCAN DATA
NOTE: Information contained in the following TABLE is typical of readings taken on vehicle with engine idling, upper radiator hose hot, throttle closed, transmission in Park or Neutral, closed loop status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/2 seconds. Not all devices and systems are used on all models. For additional information, see tester owner's manual.
GMC Safari 1991 SCAN TESTER DATA
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) Battery Voltage Volts 13.5-14.5 Block Learn Counts 8-138 (128 Normal) Clear Flood On/Off See Tester Manual Coolant Temp ?C 85-105? (Norm. Temperature) Crank RPM RPM 100-900 Cross Counts Counts 0-255 Desired RPM RPM ECM Desired RPM EGR Duty Cycle 0-100% 0/Closed-100/Fully Open IAC Count 0-50 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) 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 4th Gear Switch On/Off On/4th Gear
GMC Safari 1991 - TROUBLE CODE CHARTS
NOTE: The following diagnostic flow charts and mini-schematics are supplied courtesy of General Motors Corp.
GMC Safari 1991 - CODE 13, OPEN OXYGEN SENSOR CIRCUIT (EXCEPT 4.3L TURBO)
When exhaust temperature is less than 600?F (316?C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, Code 21 or 22 is not present, O2 signal voltage is steady at .35-.55 volt and throttle position sensor signal is greater than idle. All conditions must be met for at least 1 minute.
- This determines if fault is in O2 sensor, ECM or wiring.
- Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.
GMC Safari 1991 - Diagnostic Aids
Verify a clean, tight connection for ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.
GMC Safari 1991 CODE 13 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color All With 4L80-E Transmission Oxygen Sensor Signal C14 Purple Oxygen Sensor Ground C13 Tan All Others Oxygen Sensor Signal D7 Purple Oxygen Sensor Ground D6 Tan
GMC Safari 1991 - CODE 13, OPEN OXYGEN SENSOR CIRCUIT (4.3L TURBO)
When exhaust temperature is less than 600?F (316?C), the O2 sensor is open and produces no voltage. An open sensor circuit or cold sensor will not all system to entire closed loop.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 13 will set at normal operating temperature if at least 2 minutes have passed since engine start, O2 signal voltage is steady between .35-.55 volt and throttle position sensor signal is above idle. All conditions must be met for at least 1 minute.
- This checks the oxygen sensor heating element. Resistance should be 3.5-14 ohms at 662?F (350?C).
- This determines if fault is in O2 sensor, ECM or wiring.
- Use only a high-impedance Digital Volt-Ohmmeter (DVOM) while checking for continuity in signal and ground circuits. If ground circuit is open, voltage on signal circuit will be greater than .6 volt.
- If the A/C fuse is blown, check the A/C control circuit or generator for short circuits.
GMC Safari 1991 - Diagnostic Aids
Verify a clean, tight connection for ground circuit No. 413. An open circuit at sensor signal terminal or ground terminal will result in a Code 13. Both oxygen sensor and heating element must be functioning properly to enable closed loop operation.
GMC Safari 1991 - CODE 14, COOLANT SENSOR SIGNAL VOLTAGE LOW
The ECM uses coolant temperature sensor inputs in determining control of fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC). As the engine warms, the sensor resistance reduces. At normal operating temperature, voltage signal will be about .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM coolant sensor signal terminal.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This tests if code was set because of a hard failure or intermittent condition. Code 14 sets if signal voltage indicates a coolant temperature greater than 275?F (135?C) for more than 3 seconds on 4.3L Turbo or 6 seconds on all other models.
- This simulates conditions for a Code 15. If the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.
GMC Safari 1991 - Diagnostic Aids
After the engine is started, the temperature should rise steadily to about 194?F (90?C), then stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor (if equipped) should read close to each other, when measured with a Scan tester.
GMC Safari 1991 CODE 14 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 4.3L Turbo CTS Signal GE16 Yellow CTS Ground BB6 Black All W/4L80-E Transmission CTS Signal D16 Yellow CTS Ground D3 Black 2.8L, 3.1L, 4.3L Except Turbo, 5.0L & 5.7L C & K Series CTS Signal C10 Yellow CTS Ground D2 Black All Others CTS Signal C10 Yellow CTS Ground A11 Black
GMC Safari 1991 TEMPERATURE-TO-RESISTANCE VALUES (1) (2)
?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 40 (4) 7500 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
(2) Temperatures are approximates.
Fig. 6: GMC Safari 1991 - Component Locations - Code 14 Schematic Coolant Sensor Signal Voltage Low
Fig. 7: GMC Safari 1991 - Component Locations - Code 14 Flow Chart Coolant Sensor Signal Voltage Low
GMC Safari 1991 - CODE 15, COOLANT SENSOR SIGNAL VOLTAGE HIGH
As the engine warms, the sensor resistance reduces and voltage drops. At normal operating temperature, voltage signal will be about .5-1.1 volts (4.3L Turbo) or 1.5-2.0 volts (all other models) at ECM coolant sensor signal terminal. If sensor signal circuit opens, ECM will see -40?F (-40?C) and deliver fuel for this temperature.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if code was set as a result of a hard failure or intermittent condition. Code 15 will set if engine is running for more 50 seconds and signal voltage indicates a coolant temperature less than -22?F (-30?C) for more than 30 seconds.
- This simulates conditions for a Code 14. If the ECM recognizes the grounded circuit and displays a high temperature, the ECM and wiring are okay.
- This determines if problem is ECM or wiring. There should be 5 volts present at sensor when measured with a DVOM.
GMC Safari 1991 - Diagnostic Aids
After engine starts, temperature should rise steadily to about 194?F (90?C) and stabilize when thermostat opens. If the engine is allowed to cool overnight, the coolant temperature sensor and MAT sensor (if equipped) should read close to each other when measured with a Scan tester. Code 15 will also set if sensor signal or ground circuit is open.
GMC Safari 1991 CODE 15 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 4.3L Turbo CTS Signal GE16 Yellow CTS Ground BB6 Black All W/4L80-E Transmission CTS Signal D16 Yellow CTS Ground D3 Black 2.8L, 3.1L, 4.3L Except Turbo, 5.0L & 5.7L C & K Series CTS Signal C10 Yellow CTS Ground D2 Black All Others CTS Signal C10 Yellow CTS Ground A11 Black
GMC Safari 1991 TEMPERATURE-TO-RESISTANCE VALUES (1) (2)
?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 40 (4) 7500 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
(2) Temperatures are approximates.
Fig. 8: GMC Safari 1991 - Component Locations - Code 15 Schematic Coolant Sensor Signal Voltage High
GMC Safari 1991 - CODE 21, TPS SIGNAL VOLTAGE HIGH
The Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from about .50 volt at idle to 4.5 volts at wide open throttle. On models with non-adjustable TPS, each time TPS voltage drops to less than 1.25 volts and stops, ECM assumes this is zero degrees throttle angle and measures throttle percentage angle from this point.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 21 and checks if fault is a hard failure or an intermittent condition. Code 21 will set if TPS voltage is greater than 2.5 volts 2-10 seconds with engine running. On 2.8L, Code 21 may set if MAP sensor signal less than 2 volts.
- This test simulates conditions for Code 22. If the ECM recognizes the low voltage signal and sets Code 22, the ECM and power and signal circuits are not at fault.
- This step isolates a faulty sensor, ECM or an open ground circuit.
GMC Safari 1991 - Diagnostic Aids
A Scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt (4.3L Turbo) or less than 1.25 volts (all other models). TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 21 will also result if ground circuit is open or TPS signal circuit is shorted to voltage.
GMC Safari 1991 CODE 21 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 4.3L Turbo TPS Signal GF13 Dark Blue TPS Ground BB5 Black TPS Reference BA5 Gray All W/4L80-E Transmission TPS Signal C5 Dark Blue TPS Ground D3 Black TPS Reference C4 Gray 2.8L, 3.1L, 4.3L Except Turbo, 5.0L & 5.7L C & K Series TPS Signal C13 Dark Blue TPS Ground D2 Black TPS Reference C14 Gray All Others TPS Signal C13 Dark Blue TPS Ground A11 Black TPS Reference C14 Gray
Fig. 10: GMC Safari 1991 - Component Locations - Code 21 Schematic TPS Signal Voltage High
Fig. 11: GMC Safari 1991 - Component Locations - Code 21 Schematic TPS Signal Voltage High
GMC Safari 1991 - CODE 22, TPS SIGNAL VOLTAGE LOW
The Throttle Position Sensor (TPS) provides a varying voltage signal depending on throttle valve angle. Signal voltage varies from less than about .50 volt at idle to 4.5 volts at wide open throttle.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 22 and tests if the fault is a hard failure or an intermittent condition. Code 22 will set if engine is running, TPS voltage is less than .2 volt for 2-4 seconds.
- This simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
- On 2.8L, check and adjust TPS. On all others, replace TPS.
- This simulates a high voltage signal to check for on open TPS signal circuit.
GMC Safari 1991 - Diagnostic Aids
A Scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt (4.3L Turbo) or less than 1.25 volts (all other models). TPS voltage should increase at a steady rate to 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.
GMC Safari 1991 CODE 22 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 4.3L Turbo TPS Signal GF13 Dark Blue TPS Ground BB5 Black TPS Reference BA5 Gray All W/4L80-E Transmission TPS Signal C5 Dark Blue TPS Ground D3 Black TPS Reference C4 Gray 2.8L, 3.1L, 4.3L Except Turbo, 5.0L & 5.7L C & K Series TPS Signal C13 Dark Blue TPS Ground D2 Black TPS Reference C14 Gray All Others TPS Signal C13 Dark Blue TPS Ground A11 Black TPS Reference C14 Gray
Fig. 12: GMC Safari 1991 - Component Locations - Code 22 Schematic TPS Signal Voltage Low
Fig. 13: GMC Safari 1991 - Component Locations - Code 22 Flow Chart TPS Signal Voltage Low
GMC Safari 1991 - CODE 23, MAT SENSOR TEMP. LOW (2.5L & 4.3L TURBO)
The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor. When temperatures are low, sensor resistance is high and the ECM will see a high-monitored voltage signal. As temperature increases, sensor resistance decreases and voltage sensed by the ECM drops.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This checks if Code 23 is a hard failure or an intermittent condition. Code 23 will set if engine is running for one minute, MAT sensor temperature is less than -22?F (-30?C) for 12 seconds and speed sensor signal is not present.
- This simulates conditions for a Code 25. If the Scan tester displays a high temperature, the ECM and wiring are not at fault.
- This checks for continuity of sensor signal and ground circuits.
GMC Safari 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 also result if signal and ground circuits become open.
GMC Safari 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 2.5L Signal C12 Tan MAT Ground D2 Black/Red 4.3L Turbo Signal GF16 Tan MAT Ground BB5 Black
GMC Safari 1991 TEMPERATURE-TO-RESISTANCE VALUES (1) (2)
?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 40 (4) 7500 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
(2) Temperatures are approximates.
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (EXCEPT 3.1L, C & K SERIES & MODELS W/4L80-E TRANSMISSION)
The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 5 seconds. These conditions are met during a road load deceleration.
- This determines if DRAC is receiving the AC signal from the VSS.
- This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an aver age of 4-6 volts at about 20 MPH.
GMC Safari 1991 - Diagnostic Aids
Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment. If no problem is found while using flow chart, see INTERMITTENTS in TESTS W/O CODES article in this section.
Fig. 16: GMC Safari 1991 - Component Locations - Code 24 Schematic (4.3L Turbo) Vehicle Speed Sensor
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (C & K SERIES EXCEPT W/4L80-E TRANSMISSION)
The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the Digital Ratio Adapter Controller (DRAC) which alternately grounds this circuit when it is receiving voltage pulses from the Vehicle Speed Sensor (VSS). Scan tester reading should closely match speedometer reading.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 24 will set if voltage on circuit No. 437 is constant, engine speed is greater than 1200 RPM, vehicle speed is less than 2 MPH vehicle is not in Park or Neutral and all conditions have been met for at least 10 seconds. These conditions are met during a road load deceleration.
- This test monitors DRAC voltage on circuit No. 437. With wheels turning, pulsing voltage should be present. Voltage variation will be greater at a low speed to an aver age of 4-6 volts at about 20 MPH.
GMC Safari 1991 - Diagnostic Aids
Scan tester reading should closely match speedometer reading with drive wheels turning. If vehicle is equipped with an automatic transmission, check park/neutral switch adjustment. If no problem is found while using flow chart, see INTERMITTENTS in TESTS W/O CODES article in this section.
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (C & K SERIES 2WD W/4L80-E TRANSMISSION)
VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth cut in the outside diameter of the rear internal gear induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, input speed is at least 3000 RPM and output speed is less than 200 RPM for at least one second. If input speed sensor is not operational at start-up, this can cause VSS to read zero.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Test verifies VSS voltage at PCM.
- Test verifies operation of VSS.
- Test checks circuits No. 821 and 822 up to the instrument cluster.
GMC Safari 1991 - Diagnostic Aids
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section. While Code 24 is set, Scan tester will display an RPM derived from input speed.
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (2WD W/4L80-E TRANS. EXCEPT C & K SERIES)
VSS output sensor circuit consists of a magnetic induction type speed sensor, Digital Ratio Adapter Controller (DRAC), and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Signal is transmitted to the instrument cluster DRAC where it is converted to a square wave signal which is passed on to the PCM, cruise control, speedometer, and anti-lock brake system. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 200 RPM for at least 1.5 seconds. If input speed sensor is not operational at start-up, this can cause VSS to read zero.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Test verifies VSS voltage at PCM.
- Test verifies VSS signal to DRAC.
- Test verifies VSS signal at sensor.
GMC Safari 1991 - Diagnostic Aids
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section. While Code 24 is set, Scan tester will display an RPM derived from input speed.
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (4WD WITH 4L80-E TRANSMISSION)
VSS output sensor is a magnetic induction type. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor when drive wheels are turning. Since vehicle speed is taken from the transfer case on 4WD vehicles, output speed sensor signal on these units goes directly to PCM. Code 24 will set if gear selector is not in Park or Neutral, engine speed is at least 3000 RPM and output speed is less than 200 RPM for at least 1.5 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Test verifies VSS voltage at PCM.
- Test verifies VSS signal at sensor.
GMC Safari 1991 - Diagnostic Aids
Check all connections, especially those at the transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in the TESTS W/O CODES article in this section. While Code 24 is set, Scan tester will display an RPM derived from input speed. If input speed sensor is not operational at start-up, this can cause VSS to read zero.
GMC Safari 1991 - CODE 24, VEHICLE SPEED SENSOR (3.1L ENGINE)
ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS is connected to speed sensor buffer which alternately grounds and opens circuit No. 437 when wheels are turning. This pulsing action takes place about 2000 times per mile. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH. The ECM uses VSS information in calculations to determine vehicle adjustments. Scan tester reading should closely match speedometer reading when wheels are turning.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- A Code 24 sets when MPH is less than 2 MPH, transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, TPS is greater than 5 percent, circuit No. 437 voltage is constant and all of these conditions are met for 30 seconds. These conditions are met during a road load deceleration.
- A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty. A voltage of less than one volt at IP connector indicates circuit No. 437 wire is shorted to ground. Disconnect vehicle speed sensor connector. If voltage is now greater than 10 volts, vehicle speed sensor buffer is faulty. If voltage remains less than 8 volts, circuit is grounded. If circuit is not grounded, check for faulty ECM connector or ECM. Before replacing the ECM, PROM should be checked for correct application.
GMC Safari 1991 - Diagnostic Aids
A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use Scan tester and check for proper signal in Drive, while wiggling shifter. Scan tester MPH reading should closely match speedometer when vehicle is moving. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
Fig. 27: GMC Safari 1991 - Component Locations - Code 24 Schematic (3.1L) Vehicle Speed Sensor
Fig. 28: GMC Safari 1991 - Component Locations - Code 24 Flow Chart (3.1L) Vehicle Speed Sensor
GMC Safari 1991 - CODE 25, MAT SENSOR TEMP. HIGH (2.5L & 4.3L TURBO)
The ECM applies and monitors a voltage signal (4-6 volts) to MAT sensor. When manifold air is cold, the sensor resistance is high and ECM sees a high signal voltage. As air warms, resistance decreases and voltage sensed by the ECM drops. Sensor resistance can be measured at sensor terminals with harness disconnected.
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 VSS signal is present (2.5L) and monitored MAT sensor temperature is greater than 302?F (150?C) for 2.5L or 275?F (135?C) for 4.3L Turbo.
GMC Safari 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 also result if sensor signal circuit is shorted to ground.
GMC Safari 1991 CODE 23 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 2.5L Signal C12 Tan MAT Ground D2 Black/Red 4.3L Turbo Signal GF16 Tan MAT Ground BB5 Black
GMC Safari 1991 TEMPERATURE-TO-RESISTANCE VALUES (1) (2)
?F (?C) Ohms 210 (100) 185 160 (70) 450 100 (38) 1800 70 (20) 3400 40 (4) 7500 20 (-7) 13,500 0 (-18) 25,000 -40 (-40) 100,700
(1) Measure resistance across sensor terminals.
(2) Temperatures are approximates.
GMC Safari 1991 - CODE 31, TURBO OVERBOOST (4.3L TURBO)
On turbocharged engines, exhaust gases pass from exhaust manifold through turbocharger, turning turbine blades. Compressor side of turbocharger also turns, pulling air through air filter and pushing it into intake manifold under pressure.
Wastegate is normally closed, but will open to by-pass exhaust gases when an overboost condition exists. This is determined through monitored MAP sensor signals. Under normal driving conditions, ECM energizes the wastegate control (solenoid) valve. This blocks off intake manifold pressure from wastegate actuator diaphragm. When overboost condition exists, ECM will de-energize the wastegate control (solenoid) valve, allowing intake manifold pressure to act upon actuator diaphragm, opening wastegate.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 31 will set when Code 33 is not set and excessive manifold boost pressure is sensed for 2 seconds. This step tests the ability of the actuator to open wastegate. Code 31 may be caused by circuit No. 471 shorted to ground, a sticking wastegate or actuator, control (solenoid) valve stuck in the closed position, a cut or pinched hose, or a faulty ECM.
- After pressure is applied to valve and then removed, actuator should slowly close wastegate. If not, vent in control (solenoid) valve could be plugged.
- With ignition on and ALDL test terminal "B" grounded, control (solenoid) valve should be energized. This closes off the manifold to wastegate actuator.
- This checks the electrical control portion of the system. With ignition on and engine not running, solenoid should not be energized.
Fig. 31: GMC Safari 1991 - Component Locations - Code 31 Schematic (4.3L Turbo) Turbo Overboost
Fig. 32: GMC Safari 1991 - Component Locations - Code 31 Flow Chart (4.3L Turbo) Turbo Overboost
GMC Safari 1991 - CODE 32, EGR SYSTEM ERROR (4.3L C, G, K, & P SERIES W/4L80-E TRANSMISSION)
PCM controls a solenoid regulating vacuum to the EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by PCM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
- When jumper wire is removed from terminal "B", vacuum to the EGR valve should bleed through a vent in the solenoid and EGR valve should close. Vacuum gauge may or may not bleed off vacuum, however, this does not indicate a problem.
- Determines if fault lies in electrical control part of the system, connector or solenoid.
- This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
- When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.
GMC Safari 1991 - Diagnostic Aids
Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.
GMC Safari 1991 - CODE 32, EGR SYSTEM ERROR (ALL W/4L80-E TRANS. EXCEPT 4.3L)
PCM controls a solenoid that regulates vacuum to the EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by PCM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.
ECM checks EGR operation when engine speed is greater than 1600 RPM, MAP sensor signal indicate cruise condition and throttle position is constant.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- With ignition on and engine off, solenoid should not be energized or allow vacuum to pass to EGR valve. When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
- Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
- Vehicle must be driven during this test to produce sufficient load to operate EGR. Lightly accelerating (approximately 1/4 throttle) will produce a large and stable enough reading to determine if ECM is commanding system on.
GMC Safari 1991 - Diagnostic Aids
Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.
GMC Safari 1991 - CODE 32, EGR SYSTEM ERROR (ALL W/O 4L80-E TRANS EXCEPT 3.1L, & 4.3L TURBO)
ECM controls a solenoid that regulates vacuum to the EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by ECM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.
ECM checks EGR operation when vehicle speed is greater than 50 MPH, engine vacuum indicates cruise and throttle is steady.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- With ignition on and engine off, solenoid should not be energized or allow vacuum to pass to EGR valve. When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
- Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
- SERVICE ENGINE SOON light should also flash while diagnostic terminal is grounded. If it does not this may indicate the Quad-Driver has been damaged by low TCC circuit resistance.
- Vehicle must be driven during this test to produce sufficient load to operate EGR. Lightly accelerating (approximately 1/4 throttle) will produce a large and stable enough reading to determine if ECM is commanding system on.
GMC Safari 1991 - Diagnostic Aids
Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.
GMC Safari 1991 - CODE 32, EGR SYSTEM ERROR (3.1L ENGINE)
ECM operates a Electronic Vacuum Regulator Valve (EVRV) to control vacuum to the EGR valve. Regulator normally allows only a small amount of vacuum to pass to EGR valve. When ECM grounds EVRV drive circuit, EVRV opens, allowing enough vacuum to pass through to open EGR valve. ECM monitors EGR effectiveness by periodically checking fuel integrator counts. This is done when vehicle speed is greater than 50 MPH, MAP sensor signal indicates cruise condition, throttle position is constant and all conditions have been met twice for 60 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- With ignition on and engine off, EVRV should not be energized or allow enough vacuum through to open EGR valve. When test terminal "B" of the ALDL connector is grounded, EVRV will energize, allowing vacuum to the EGR valve.
- Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
Fig. 40: GMC Safari 1991 - Component Locations - Code 32 Schematic (3.1L) EGR System Error
Fig. 41: GMC Safari 1991 - Component Locations - Code 32 Flow Chart (3.1L) EGR System Error
GMC Safari 1991 - CODE 32, EGR SYSTEM ERROR (4.3L TURBO)
ECM controls a solenoid that regulates vacuum to the EGR valve. The normally closed solenoid prevents vacuum from passing until it is energized by ECM. A properly operating EGR will directly affect fuel integrator counts. With EGR valve open, integrator counts will be less than without EGR operation. If monitored integrator counts do not change with EGR commanded, Code 32 will set.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Diagnostic chart covers checks for the entire EGR system. If no trouble is found while performing chart routine, fault is an intermittent electrical problem or a sticky EGR valve.
GMC Safari 1991 - Diagnostic Aids
Vacuum switch in the EGR solenoid is not used. If EGR valve sticks open, a rough idle will result. If EGR solenoid vent filter becomes plugged, EGR valve will remain open or close slowly. An inoperative check valve in the ported vacuum supply line will result in faulty EGR system operation.
Fig. 42: GMC Safari 1991 - Component Locations - Code 32 Schematic (4.3L) EGR System Error
Fig. 43: GMC Safari 1991 - Component Locations - Code 32 Flow Chart (4.3L) EGR System Error
GMC Safari 1991 - CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If MAP sensor fails, ECM will substitute a fixed MAP value and use the TPS input to control fuel delivery.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 33 and determines if it is a hard failure or an intermittent condition. Code 33 will set when voltage signal reading is too high and TPS voltage indicates throttle is closed.
- This step simulates conditions for a Code 34. If the ECM recognizes and indicates low MAP signal, the ECM and 5-volt reference and MAP signal circuits are not at fault.
GMC Safari 1991 - Diagnostic Aids
With the ignition switch in the "ON" position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage is high. Comparing 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 also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.
GMC Safari 1991 CODE 33 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color Astro & Safari 4.3L & 2.5L S Series MAP Signal C12 (1) Lt. Green MAP Ground D2 Black/Red MAP Reference C14 Gray All W/4L80-E Transmission MAP Signal C11 Lt. Green MAP Ground D2 Purple MAP Reference D4 Green 2.8L, 3.1L & V8 C & K Series MAP Signal C11 (1) Lt. Green MAP Ground A11 Purple MAP Reference C14 Gray 4.3L Turbo MAP Signal GF15 Lt. Green MAP Ground BB6 Black MAP Reference BA4 Gray 4.3L Non-Turbo & V8 Except C & K Series MAP Signal C11 (1) Lt. Green MAP Ground D2 Purple MAP Reference C14 Gray
(1) May have a Black trace.
Fig. 44: GMC Safari 1991 - Component Locations - Code 33 Schematic Map Sensor Signal Voltage High
Fig. 45: GMC Safari 1991 - Component Locations - Code 33 Flow Chart Map Sensor Signal Voltage High
GMC Safari 1991 - CODE 34, MAP SENSOR SIGNAL VOLTAGE LOW
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the ECM will substitute a fixed MAP value and use TPS input to control fuel delivery.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This confirms Code 34 and determines if code was a hard failure or an intermittent condition. 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 harness terminals "B" to "C" will determine if problem is sensor, ECM or wiring. If ECM recognizes and indicates high MAP signal, ECM and wiring are okay.
- Scan tester may not display 12 volts. The important thing is the ECM recognizes the voltage as greater than 4 volts (high MAP voltage signal), indicating the ECM and MAP signal circuit are not at fault.
GMC Safari 1991 - Diagnostic Aids
With the ignition switch in the ON position and engine off, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparing 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. If 5-volt reference circuit is not shorted to ground and a stored Code 22 is stored, check MAP signal circuit for short to ground.
GMC Safari 1991 CODE 34 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color Astro & Safari 4.3L & 2.5L S Series MAP Signal C12 (1) Lt. Green MAP Ground D2 Black/Red MAP Reference C14 Gray All W/4L80-E Transmission MAP Signal C11 Lt. Green MAP Ground D2 Purple MAP Reference D4 Green 2.8L, 3.1L & V8 C & K Series MAP Signal C11 (1) Lt. Green MAP Ground A11 Purple MAP Reference C14 Gray 4.3L Turbo MAP Signal GF15 Lt. Green MAP Ground BB6 Black MAP Reference BA4 Gray 4.3L Non-Turbo & V8 Except C & K Series MAP Signal C11 (1) Lt. Green MAP Ground D2 Purple MAP Reference C14 Gray
(1) May have a Black trace.
Fig. 46: GMC Safari 1991 - Component Locations - Code 34 Schematic Map Sensor Signal Voltage Low
Fig. 47: GMC Safari 1991 - Component Locations - Code 34 Flow Chart Map Sensor Signal Voltage Low
GMC Safari 1991 - CODE 35, IDLE SPEED ERROR 2.5L & 4.3L TURBO
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 changing engine speed. If no engine speed change 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 tester light flash while extending or retracting. If IAC valve is retracted beyond control range (about 1500 RPM), it may take many flashes in the extend position before engine speed reduces. This is normal on some engines. Fully extending the IAC may cause engine to stall. This may be normal.
- Steps 1) and 2) verified proper IAC valve operation. This step checks IAC circuits. Each light on the node light should flash Red and Green, while the IAC valve is cycled. While the color sequence is not important, if either light is off or does not flash Red and Green, check circuits beginning with poor terminal contacts.
GMC Safari 1991 - IAC Valve Reset Procedure
Turn ignition off for 10 seconds. Start and run engine for 5 seconds. Turn ignition off another 10 seconds.
GMC Safari 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. With ignition on, ground ALDL test terminal "B". Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, inspect vehicle for vacuum leaks.
GMC Safari 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 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.
GMC Safari 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 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 leaking/sticky injectors. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore. A silicone-contaminated oxygen sensor will produce lean air/fuel mixture. Oxygen sensor output would be fixed greater than 800 mv (.8 volt). This may also set Code 45.
GMC Safari 1991 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
GMC Safari 1991 - IAC Valve Connections
Carefully inspect connections for looseness or corrosion.
GMC Safari 1991 - PCV Valve
The wrong PCV valve may cause incorrect idle speed.
GMC Safari 1991 CODE 35 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color 2.5L A Coil High C5 Lt. Blue/Wht A Coil Low C6 Lt. Blue/Blk B Coil High C4 Lt. Grn./Wht B Coil Low C3 Lt. Grn./Blk 4.3L Turbo A Coil High GE6 Lt. Blue/Wht A Coil Low GE5 Blue/Blk B Coil High GE4 Lt. Grn./Blk B Coil Low GE3 Lt. Grn
GMC Safari 1991 - CODE 42, ELECTRONIC SPARK TIMING (EST)
Code 42 indicates ECM has seen an open or short to ground in HEI EST or by-pass circuits.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
- This tests for a normal EST ground path through the ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
- As test light voltage touches circuit No. 424, the module should switch. This causes ohmmeter to "over-range" with meter in 100-200 ohm range. A higher ohm range will indicate over 5000 ohms. This test assures the module switched.
- If module did not switch, this step tests for a short in circuit No. 423, an open in circuit No. 424 and a faulty ignition module connection or module.
- This step confirms Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.
GMC Safari 1991 - Diagnostic Aids
The Scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES article in this section.
Fig. 50: GMC Safari 1991 - Component Locations - Code 42 Schematic (2.5L) Electronic Spark Timing
Fig. 52: GMC Safari 1991 - Component Locations - Code 42 Schematic (3.1L) Electronic Spark Timing
Fig. 55: GMC Safari 1991 - Component Locations - Code 42 Flow Chart Electronic Spark Timing
GMC Safari 1991 - CODE 43, ESC (ALL EXCEPT 4.3L TURBO & MODELS W/4L80-E TRANS)
NOTE: Test numbers refer to test numbers on diagnostic chart.
- If conditions for a Code 43 exist, Scan tester will display YES. A knock signal should not exist at idle unless an internal or system problem exists.
- Determines if system is functioning. Usually, a knock signal can be made by tapping on the exhaust manifold. If knock signal is not made, try tapping on engine block near sensor. On models with automatic transmission, it may be necessary to place gear selector lever in Drive.
- Because Code 43 sets when the signal voltage on the spark retard line remains low, this test should cause the signal on that line to go high. The 12-volt signal should be seen by the ECM as a no knock signal if the ECM and wiring are okay.
- This test determines if the knock signal is detected on the sensor-to-controller line or if the ESC module is at fault.
- If sensor line is routed too close to secondary ignition wires, the ESC module may see the interference as a knock signal.
- This checks ground circuit to module. An open ground will cause the voltage on the monitored line to remain constant at about 12 volts. This would cause the Code 43 functional test to fail.
- This should generate a knock signal to the controller. This determines if the ESC controller is operating correctly.
GMC Safari 1991 - Diagnostic Aids
Code 43 can be caused by a faulty knock sensor connection at the ESC module or ECM. Also, check the controller-to-ECM signal line for an open or short to ground.
Fig. 56: GMC Safari 1991 - Component Locations - Code 43 Schematic (3.1L) Electronic Spark Control
GMC Safari 1991 - CODE 43, ESC (4.3L TURBO & MODELS W/ 4L80-E TRANS)
ECM/PCM applies and monitors a 5-volt DC signal to the knock sensor. Internal knock sensor circuitry pulls this DC signal down to about 2.5 volts. When knock sensor detects detonation, it generates an AC signal which rides back on the DC signal to the ECM/PCM. Knock signal intensity is dependent upon knock signal level.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 43 will set when vehicle reaches normal operating temperature (but not overheating), high engine load is indicated by MAP sensor and voltage on circuit No. 496 is greater than 3.5 volts DC or less than 1.5 volts DC. This step determines if system is functioning properly at the current time.
- This step determines the state of the 5-volt reference signal applied to the sensor.
- Checks knock sensor internal resistance.
GMC Safari 1991 - CODE 44, LEAN EXHAUST INDICATION (EXCEPT 4.3L TURBO)
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 44 sets when O2 sensor signal at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".
GMC Safari 1991 - Diagnostic Aids
Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, the block learn value will be around 150.
GMC Safari 1991 - O2 Sensor Wire
O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.
GMC Safari 1991 - Fuel Contamination
Water, even small amounts, near the in-tank fuel pump inlet can reach fuel injector, causing a lean exhaust and setting Code 44.
GMC Safari 1991 - Fuel Pressure
System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING article in this section.
GMC Safari 1991 - Exhaust Leaks
If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.
If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES in this section.
GMC Safari 1991 CODE 44 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color All W/4L80-E Transmission O2 Sensor Signal C14 Purple O2 Sensor Ground C13 Tan All Others O2 Sensor Signal D7 Purple O2 Sensor Ground D6 Tan
GMC Safari 1991 - CODE 44, LEAN EXHAUST INDICATION (4.3L TURBO)
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Code 44 sets when O2 sensor signal at ECM is less than .2 volt for 50 seconds or more and system is operating in "closed loop".
GMC Safari 1991 - Diagnostic Aids
Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, the block learn value will be around 150.
GMC Safari 1991 - O2 Sensor Wire
O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.
GMC Safari 1991 - Fuel Contamination
Water, even small amounts, near the in-tank fuel pump inlet can reach fuel injector, causing a lean exhaust and setting Code 44.
GMC Safari 1991 - Fuel Pressure
System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. For fuel pressure checking procedure, see BASIC TESTING article in this section.
GMC Safari 1991 - Exhaust Leaks
If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.
If Code 44 is intermittent, see INTERMITTENTS in the article TESTS W/O CODES in this section.
GMC Safari 1991 - CODE 45, RICH EXHAUST INDICATION (EXCEPT 4.3L TURBO)
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation. Code 45 indicates a rich exhaust and diagnosis should begin with: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Tests if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than 5 percent, O2 sensor signal at ECM is greater than .7 volt for 60 seconds or more.
GMC Safari 1991 - Diagnostic Aids
Code 45, rich exhaust, is most likely caused by one of the following:
GMC Safari 1991 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING article in this section. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.
GMC Safari 1991 - Ignition Ground
If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by ECM on EST harness reference line. Additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help diagnose problem.
GMC Safari 1991 - Fuel Canister
Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.
GMC Safari 1991 - MAP Sensor
If ECM senses higher than normal manifold pressure (low vacuum) the system can go rich. Disconnecting the MAP sensor allows ECM to substitute a fixed value for the MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.
GMC Safari 1991 - TPS
An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.
GMC Safari 1991 - O2 Sensor Contamination
O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White-powdery coating to cover O2 sensor. The false high signal voltage produced (or low oxygen content sensed) is interpreted by the ECM as a rich mixture, causing the ECM to set Code 45.
GMC Safari 1991 - EGR Problem
EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
GMC Safari 1991 CODE 45 ECM TERMINAL & CIRCUIT WIRING IDENTIFICATION
Application ECM Terminal Wire Color All W/4L80-E Transmission O2 Sensor Signal C14 Purple O2 Sensor Ground C13 Tan All Others O2 Sensor Signal D7 Purple O2 Sensor Ground D6 Tan
GMC Safari 1991 - CODE 45, RICH EXHAUST INDICATION (4.3L TURBO)
Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is less than 600?F (316?C). An open sensor circuit or cold sensor causes "open loop" operation. Code 45 indicates a rich exhaust and diagnosis should begin with: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.
NOTE: Test numbers refer to test numbers on diagnostic chart.
- Tests if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is greater than 5 percent, O2 sensor signal at ECM is greater than .7 volt for 60 seconds or more.
GMC Safari 1991 - Diagnostic Aids
Code 45, rich exhaust, is most likely caused by one of the following:
GMC Safari 1991 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING article in this section. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.
GMC Safari 1991 - Ignition Ground
If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses picked up by ECM on EST harness reference line. Additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help diagnose problem.
GMC Safari 1991 - Fuel Canister
Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.
GMC Safari 1991 - MAP Sensor
If ECM senses higher than normal manifold pressure (low vacuum) the system can go rich. Disconnecting the MAP sensor allows ECM to substitute a fixed value for the MAP sensor. If rich condition disappears, replace MAP sensor and continue testing.
GMC Safari 1991 - TPS
An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.
GMC Safari 1991 - O2 Sensor Contamination
O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White-powdery coating to cover O2 sensor. The false high signal voltage produced (or low oxygen content sensed) is interpreted by the ECM as a rich mixture, causing the ECM to set Code 45.
GMC Safari 1991 - EGR Problem
EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in TESTS W/O CODES article in this section.
GMC Safari 1991 - CODE 51, FAULTY PROM/MEM-CAL
Ensure all pins are fully inserted in socket. If okay, replace PROM/MEM-CAL, clear memory and recheck. If Code 51 reappears, replace ECM.
GMC Safari 1991 - CODE 52, FAULTY CALPAK (EXCEPT 2.5L)
Ensure all pins are fully inserted in socket. If okay, replace CALPAK, clear memory and recheck. If Code 51 reappears, replace ECM.
GMC Safari 1991 - CODE 53, SYSTEM OVERVOLTAGE (2.5L)
This code indicates a basic charging system problem. Code 53 will set when voltage at ECM terminal is greater than 17.1 volts for 2 seconds. Check and repair charging system.
GMC Safari 1991 - CODE 54, FUEL PUMP CIRCUIT
Code 54 will set if the ECM does not see 12 volts on the fuel pump signal voltage monitor during the first 2 seconds after ignition is turned on.
Fig. 70: GMC Safari 1991 - Component Locations - Code 54 Diagnostic Flow Chart (S & T Series Only)
Fig. 71: GMC Safari 1991 - Component Locations - Code 54 Schematic (Astro, Safari) Fuel Pump Circuit
Fig. 75: GMC Safari 1991 - Component Locations - Code 54 Schematic (3.1L) Fuel Pump Circuit
GMC Safari 1991 - CODE 55, ECM ERROR (EXCEPT 2.5L)
Ensure ECM grounds are good and MEM-CAL is properly latched. If okay, replace ECM. Clear codes and confirm closed loop operation. Check operation of "SERVICE ENGINE SOON" light.
GMC Safari 1991 - SUMMARY
If hard fault codes are not present and driveability symptoms OR INTERMITTENT CODES EXIST, PROCEED TO TESTS W/O CODES article in this section for diagnosis by symptom (i.e. ROUGH IDLE, NO START, etc.), or intermittent diagnostic procedures.