Čeština
Dansk
Deutsch
Ελληνικά
English
English
Español
Suomi
Français
Français
עברית
Hrvatski
Magyar
Italiano
日本語
한국어
Nederlands
Polski
Português
Português
Română
Русский
Slovenčina
Slovenščina
Türkçe
中文 (中国)
GMC Safari 1990 - 1990 ENGINE PERFORMANCE Self-Diagnostics - TBI
GMC Safari 1990 - 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 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 in this article. If no fault codes or only a non-running Code 12 is present and driveability problems exist, proceed to H - TESTS W/O TESTS article 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 1990 - SELF-DIAGNOSTIC SYSTEM
The ECM is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, ECM will illuminate the SERVICE ENGINE SOON light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see RETRIEVING CODES (NON-SCAN) in this article. Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.
GMC Safari 1990 - HARD FAILURES
Hard failures cause SERVICE ENGINE SOON light to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause 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 1990 - INTERMITTENT FAILURES
Intermittent failures cause SERVICE ENGINE SOON light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, 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 1990 - 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. Appropriate diagnostic circuit check MUST be performed before using trouble code charts. See BASIC TESTING article.
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 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. 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. From there you will be sent to the appropriate area in SYSTEMS & COMPONENT TESTING article.
4) After repairs are made, clear trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.
GMC Safari 1990 - 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 SYSTEM/COMP TESTS article. Turn ignition on. Insert jumper wire from terminal "B" (diagnostic test terminal) to terminal "A" (ground) of ALDL connector. See Fig. 1.
NOTE: Inserting jumper wire into test and ground terminals of ALDL connector with engine running will cause fuel-injected vehicles to enter field service mode and codes will not flash. See FIELD SERVICE MODE in BASIC TESTING article. 
Fig. 1: GMC Safari 1990 - Component Locations - Identifying ALDL Diagnostic Connector Terminals - 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. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
GMC Safari 1990 - READING TROUBLE CODES
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, more accurate and capable of reading information which would require testing individual ECM and sensor/solenoid connector terminals with a digital voltmeter. See SCAN DATA TABLES and SCAN TESTER USAGE in this article.
NOTE: When using a "Scan" tester, there is a time delay between serial data updates. For instantaneous response, a digital voltmeter must be used.
If "Scan" tester is not available, 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 1990 ECM TROUBLE CODE DEFINITION
Code No. Circuit Affected 12 (1) No RPM reference pulse 13 Open oxygen sensor circuit open 14 Coolant Temp. Sensor (CTS) signal voltage low 15 CTS signal voltage high 21 Throttle Position Sensor (TPS) signal voltage high 22 TPS signal voltage low 23 Manifold Air Temp. (MAT) sensor signal voltage high 24 Vehicle Speed Sensor circuit 25 MAT sensor signal voltage low 32 EGR system error 33 Manifold Absolute Pressure (MAP) sensor signal voltage high 34 MAP sensor signal voltage low 35 Idle Air Control idle speed error (2.5L) 42 EST circuit open or grounded 43 ESC retard signal too low (Except 2.5L) 44 Lean exhaust indicated 45 Rich exhaust indicated 51 Faulty PROM, MEM-CAL or ECM 52 Faulty/missing CALPAC 53 System voltage high (2.5L) 54 Fuel pump voltage low 55 Faulty ECM (Except 2.5L)
(1) Display of a Code 12 is normal when no reference pulses are received by ECM (engine not running).
NOTE: Trouble code charts should only be used if SERVICE ENGINE SOON light is illuminated (indicating a current problem exists). Exceptions are Codes 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes. Anytime Codes 51, 52, 54 or 55 are displayed with another code, start with 50-series code first and proceed to low profile numbered codes.
GMC Safari 1990 - HARD OR INTERMITTENT TROUBLE CODE DETERMINATION
During any diagnostic procedure, determine if codes are hard or intermittent failure codes. 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 in this article.
GMC Safari 1990 - CLEARING TROUBLE CODES
Turn ignition switch to ON position and ground diagnostic terminal lead at ALDL connector. Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead.
GMC Safari 1990 - ECM LOCATION
The ECM is located under the dash, behind the glove box.
GMC Safari 1990 - DIAGNOSTIC MATERIALS DIAGNOSTIC AIDS
Diagnostic aids (located in many trouble code charts) are additional tips to help with diagnosis when inspected circuit is okay.
GMC Safari 1990 - FIELD SERVICE MODE CHECK
On fuel injected models, SERVICE ENGINE SOON light will indicate operational mode of engine if ALDL is grounded while engine is running. This 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 (FUEL INJECTED) in BASIC TESTING article.
GMC Safari 1990 - 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 TESTER DATA table in this article.
The computerized engine control system is most easily diagnosed using a "Scan" tester. However, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These tools are a tachometer, dwell meter, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights 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 1990 - SCAN TESTER USAGE
NOTE: Before connecting "Scan" tester, check diagnostic system and ensure accurate information is received by "Scan" tester. Perform appropriate DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article. If vehicle does not pass diagnostic circuit check, information received by "Scan" tester may be invalid.
The "Scan" tester is a specialized tester 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 DATA table in this article.
"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" testers.
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 DATA tables in this article.
NOTE: Information obtained by "Scan" tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, 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 do 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 1990 - SCAN DATA
NOTE: Information contained in these tables 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 1/4 seconds. Not all devices and systems are used on all models. For additional information, see tester owner's manual.
GMC Safari 1990 THROTTLE BODY INJECTION
Tester Position Units Measured Nominal 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 118-138 (128 normal) Clear Flood On/Off See Tester Manual Coolant Temp. ?C 85-105? (norm. temp) 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 Counts 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 Satus 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 110 (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 1990 - DIAGNOSTIC TESTS
NOTE: The following diagnostic flow charts and mini-schematics are supplied courtesy of General Motors Corp.
GMC Safari 1990 - CHART A-5A: FUEL PUMP RELAY CIRCUIT "R & V" SERIES
GMC Safari 1990 - CODE 13: OPEN OXYGEN SENSOR CIRCUIT
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 allow system to enter closed loop operation.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Test Number 1) 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.
Test Number 2) This determines if fault is in O2 sensor, ECM or wiring.
Test Number 3) 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 1990 - 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.
Fig. 4: GMC Safari 1990 - Component Locations - Code 13: Open Oxygen Sensor Circuit
GMC Safari 1990 - 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 1.5-2.0 volts at ECM coolant sensor signal terminal.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Test Number 1) 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 6 seconds.
Test Number 2) 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 1990 - 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.
Fig. 5: GMC Safari 1990 - Component Locations - Code 14: Coolant Sensor Signal Voltage Low
GMC Safari 1990 - CODE 15: COOLANT SENSOR SIGNAL VOLTAGE HIGH
As the engine warms, the sensor resistance reduces and voltage drops. At normal engine operating temperature, voltage will be about 1.5-2.0 volts 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.
Test Number 1) 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 than 2 minutes and signal voltage indicates a coolant temperature less than -22?F (-30?C) for more than 30 seconds.
Test Number 2) 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.
Test Number 3) This determines if problem is ECM or wiring. There should be 5 volts present at sensor when measured with a DVOM.
GMC Safari 1990 - 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.
Fig. 6: GMC Safari 1990 - Component Locations - Code 15: Coolant Sensor Signal Voltage High
GMC Safari 1990 - 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.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Test Number 1) 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 3.5 volts for 8 seconds with engine running. On 2.8L, Code 21 may set if MAP sensor signal less than 2 volts.
Test Number 2) 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.
Test Number 3) This step isolates a faulty sensor, ECM or an open ground circuit.
GMC Safari 1990 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be about .43 volt (2.8L) or .45-.95 volt (on all others). 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.
Fig. 7: GMC Safari 1990 - Component Locations - Code 21: Circuit Diagram
Fig. 8: GMC Safari 1990 - Component Locations - Code 21: Flow Chart TPS Signal Voltage High
GMC Safari 1990 - 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.
Test Number 1) 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 seconds.
Test Number 2) This simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
Test Number 3) On 2.8L, check and adjust TPS. On all others, replace TPS.
Test Number 4) This simulates a high voltage signal to check for on open TPS signal circuit.
GMC Safari 1990 - Diagnostic Aids
A "Scan" tester displays throttle position in volts. Closed throttle voltage should be about .43 volt (2.8L) or .45-.95 volt (on all others). 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.
Fig. 9: GMC Safari 1990 - Component Locations - Code 22: Circuit Diagram
Fig. 10: GMC Safari 1990 - Component Locations - Code 22: Flow Chart TPS Signal Voltage Low
GMC Safari 1990 - CODE 23: MAT SIGNAL TEMP LOW (2.5L)
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.
Test Number 1) 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) and speed sensor signal is not present.
Test Number 2) This simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
Test Number 3) This checks for continuity of sensor signal and ground circuits.
GMC Safari 1990 - 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. When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if signal and ground circuits become open.
Fig. 11: GMC Safari 1990 - Component Locations - Code 23: Mat Sensor Temp Low (2.5L)
GMC Safari 1990 - CODE 24: VEHICLE SPEED SENSOR (EXCEPT 3.1L)
ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS 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.
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, circuit No. 437 voltage is constant and all of these conditions are met for 10 seconds. These conditions are met during a road load deceleration. The 2.8L may set this code while accelerating or at highway speeds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Test Number 1) Monitors ECM voltage on VSS circuit. With wheels turning, pulsing action will result in a varying voltage. Variation will be greater at low speeds to an average of 4-6 volts are about 20 MPH.
Test Number 2) A voltage of less than one volt at ECM 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 is faulty. If voltage remains less than 10 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.
Test Number 3) A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty.
Test Number 4) This is normal voltage which indicates a possible intermittent condition.
GMC Safari 1990 - 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.
Fig. 12: GMC Safari 1990 - Component Locations - Code 24: Vehicle Speed Sensor (Except 3.1L)
GMC Safari 1990 - CODE 24: VEHICLE SPEED SENSOR (3.1L)
ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS 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. A Code 24 will set when MPH reads less than 2 MPH and transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, circuit No. 437 voltage is constant and all of these conditions have been met for 30 seconds.
NOTE: Test numbers refer to test numbers on diagnostic chart.
Test Number 1) Disregard Code 24 that sets with drive wheels not turning. If speedometer is not functioning properly, service speedometer.
Test Number 2) 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 ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect wire at vehicle speed sensor. 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 1990 - 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 dash while vehicle is in motion.
GMC Safari 1990 - CODE 25: MAT SENSOR TEMP. HIGH (2.5L)
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.
Test Number 1) This checks if the code is a hard failure or an intermittent condition. Code 25 will set if a VSS signal is present and MAT sensor temperature is greater than 302?F (150?C) is sensed for more than 2 seconds.
GMC Safari 1990 - 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.
Fig. 14: GMC Safari 1990 - Component Locations - Code 25: MAT Sensor Temp High (2.5L)
GMC Safari 1990 - CODE 32: EGR SYSTEM ERROR 2.5L/5.0L/4.3L EXCEPT S & T) & 5.7L (LESS THAN 8500 GVWR)
ECM controls a solenoid regulating 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.
Test Number 1) When test terminal "B" of the ALDL connector is grounded, EGR solenoid should be energized, allowing vacuum to the EGR valve. Vacuum should hold.
Test Number 2) SERVICE ENGINE SOON light should also flash while diagnostic terminal is grounded. If SERVICE ENGINE SOON does not operate it may indicate the Quad-Driver damage by low TCC circuit resistance.
Test Number 3) 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.
Test Number 4) Determines if fault lies in electrical control part of the system, connector or solenoid.
Test Number 5) This system uses a negative backpressure EGR valve. Valve should hold vacuum with engine off.
Test Number 6) When engine is started, backpressure should cause vacuum to bleed off and valve should fully close.
GMC Safari 1990 - Diagnostic Aids
The Code 32 chart is an EGR system functional check. If the EGR system works properly and a Code 32 has set, check for other causes for an increased open throttle block learn value.
- Check for a blocked/restricted intake and EGR passages.
- Check vacuum lines.
- Check MAP sensor function. A MAP sensor may shift enough in calibration to change air/fuel mixture without setting a MAP related code.
Fig. 15: GMC Safari 1990 - Component Locations - Code 32: EGR System Error Circuit Diagram
GMC Safari 1990 - CODE 32: EGR SYSTEM ERROR 2.8L, 3.1L, 4.3L (S & T), 5.7L (GREATER THAN 8500 GVWR) & 7.4L
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.
Test Number 1) 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.
Test Number 2) Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
Test Number 3) 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.
Test Number 4) 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 1990 - Diagnostic Aids
The Code 32 chart is a functional check of the EGR system. If the EGR system works properly and a Code 32 has set, check for other causes for an increased open throttle block learn value.
- Check for a blocked/restricted intake and EGR passages.
- Check vacuum lines.
- Check MAP sensor function. A MAP sensor may shift enough in calibration to change air/fuel mixture without setting a MAP related code.
GMC Safari 1990 - 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.
Test Number 1) 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 for a time greater than 6 seconds and if the TPS voltage indicates throttle is closed.
Test Number 2) This step simulates conditions for a Code 34. If the ECM recognizes and sets Code 34 (low MAP signal), the ECM and 5-volt reference and MAP signal circuits are not at fault.
GMC Safari 1990 - 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.
Fig. 19: GMC Safari 1990 - Component Locations - Code 33: MAP Sensor Signal Voltage High Flow Chart
GMC Safari 1990 - 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.
Test Number 1) 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.
Test Number 2) Jumpering harness terminals "B" to "C" will determine if problem is sensor, ECM or wiring.
Test Number 3) "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 1990 - 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.
Fig. 21: GMC Safari 1990 - Component Locations - Code 34: MAP Sensor Signal Voltage Low Flow Chart
GMC Safari 1990 - CODE 35: IDLE SPEED ERROR (2.5L)
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.
Test Number 1) IAC tester 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.
Test Number 2) 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.
Test Number 3) 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 1990 - 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 1990 - 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. 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 1990 - 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 1990 - 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 1990 - Throttle Body
Remove IAC and inspect bore for evidence of IAC valve dragging.
GMC Safari 1990 - IAC Valve Connections
Carefully inspect connections for looseness or corrosion.
GMC Safari 1990 - PCV Valve
The wrong PCV valve may cause incorrect idle speed.
Fig. 22: GMC Safari 1990 - Component Locations - Code 35: Idle Speed Error (2.5L) Circuit Diagram
Fig. 23: GMC Safari 1990 - Component Locations - Code 35: Idle Speed Error (2.5L) Flow Chart
GMC Safari 1990 - CODE 42: ELECTRONIC SPARK TIMING (EST) WITH HEI IGNITION
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.
Test Number 1) This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
Test Number 2) 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.
Test Number 3) 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.
Test Number 4) 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.
Test Number 5) This step confirms Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.
GMC Safari 1990 - Diagnostic Aids
The "Scan" tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in H - TESTING W/O CODES article.
Fig. 24: GMC Safari 1990 - Component Locations - Code 42: EST With HEI Ignition Circuit Diagram
GMC Safari 1990 - CODE 43: ELEC SPARK CONTROL SIGNAL VOLTAGE LOW (EXCEPT 2.5L)
- If conditions for a Code 43 exist, Scan tester will display YES. A knock signal should 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.
- 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 be about 12 volts. This would cause the Code 43 functional test to fail.
- Touch sensor-to-controller wire with a test light connected to 12 volts. This should generate a knock signal to the controller. This determines if the ESC controller is operating correctly.
GMC Safari 1990 - 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.
GMC Safari 1990 - CODE 44: LEAN EXHAUST INDICATION
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.
Test Number 1) 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 1990 - 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 1990 - O2 Sensor Wire
O2 sensor wire may be mispositioned and touching exhaust manifold. Check for ground between sensor and wire connector.
GMC Safari 1990 - 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 1990 - 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.
GMC Safari 1990 - 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 H - TESTING W/O CODES article.
Fig. 28: GMC Safari 1990 - Component Locations - Code 44: Flow Chart Lean Exhaust Indication
GMC Safari 1990 - CODE 45: RICH EXHAUST INDICATION
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.
Test Number 1) 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 1990 - Diagnostic Aids
Code 45, rich exhaust, is most likely caused by one of the following:
GMC Safari 1990 - Fuel Pressure High
If fuel pressure is too high, air/fuel ratio will be rich. For fuel pressure checking procedure, see BASIC TESTING article. The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.
GMC Safari 1990 - 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 1990 - Fuel Canister
Charcoal canister fuel saturation will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.
GMC Safari 1990 - 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 1990 - TPS
An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.
GMC Safari 1990 - 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 1990 - 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 H - TESTING W/O CODES article.
Fig. 29: GMC Safari 1990 - Component Locations - Code 45: Flow Chart Rich Exhaust Indication
GMC Safari 1990 - 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 1990 - 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 1990 - 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 1990 - CODE 54: FUEL PUMP CIRCUIT (EXCEPT 3.1L)
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. 32: GMC Safari 1990 - Component Locations - Code 54: Fuel Pump Circuit Flow Chart
GMC Safari 1990 - CODE 54: FUEL PUMP CIRCUIT (3.1L)
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. 33: GMC Safari 1990 - Component Locations - Code 54: Fuel Pump Circuit Circuit Diagram (3.1L)
Fig. 34: GMC Safari 1990 - Component Locations - Code 54: Fuel Pump Circuit Flow Chart (3.1L)
GMC Safari 1990 - 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 1990 - SUMMARY
If hard fault codes are not present and driveability symptoms 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.