GMC Vandura G1500 1992 - 1992 ENGINE PERFORMANCE Self-Diagnostics - Diesel

GMC Vandura G1500 1992 - INTRODUCTION

The Diesel Electronic Control (DEC) system, on 6.2L with light duty emissions, electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, cold advance and glow plug operation.

Most engine control problems are NOT computer related, but result from mechanical breakdowns, poor electrical connections, or damaged vacuum hoses. Before condemning the computer system, carefully perform visual and mechanical inspections. Failure to perform these inspections can result in lost diagnostic time.

GMC Vandura G1500 1992 - SELF-DIAGNOSTIC SYSTEM

The 6.2L light duty emissions system includes a self-diagnostic system which can determine input signal circuit malfunctions. Input signal circuits determine engine function control.

GMC Vandura G1500 1992 - DIAGNOSTIC PROCEDURE Preliminary Inspection

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult-to-see vacuum hoses beneath air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chafed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed carefully and thoroughly, as it can often fix a problem without requiring further diagnosis.

GMC Vandura G1500 1992 - Diagnostic Procedure

1. Ensure all engine systems NOT related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all non-DEC system problems are repaired.
2. ALWAYS begin diagnosis with checking diagnostic circuit to determine if DEC system and ECM are working properly. See DIAGNOSTIC CIRCUIT CHECK chart in this article. If trouble codes (other than Code 12) are displayed, determine if they are hard or intermittent trouble codes.
3. A hard code is present while working on vehicle, and problem condition persists. Hard codes will cause SERVICE ENGINE SOON light to come on.
4. An intermittent code does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnostic procedures, see TESTS W/O CODES article.

GMC Vandura G1500 1992 - ENTERING OR EXITING DIAGNOSTIC MODE

1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig. 1. The Diesel Electronic Control (DEC) system will enter diagnostic mode.
2. In this mode, ECM will display Code 12 by flashing the SERVICE ENGINE SOON light once, followed by a short pause, then 2 flashes in quick succession.
3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from ALDL connector.

Fig. 1: GMC Vandura G1500 1992 - Component Locations - Identifying ALDL Connector Terminals

GMC Vandura G1500 1992 - TROUBLE CODE IDENTIFICATION

NOTE:

Trouble codes retrieved from PCM/TCM may be either engine or transmission related. Engine-related codes are covered in this article. For transmission-related codes, see G - TESTS W/ CODES - ELECTRONIC TRANSMISSION article. - TROUBLE CODE IDENTIFICATION to determine if code is engine or transmission related.

1. The DEC system codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of ECM. Code 51 indicates PROM is either improperly installed or has failed.
2. Code 52 indicates ECM has failed and must be replaced. If condition is still not corrected after replacing ECM, the following may be the cause:
   • An incorrect ECM or PROM application may cause a malfunction, which may or may not set a code.
   • The ECM connector may be the problem. Connector terminals may have to be removed from connector to be checked properly.
   • PROM failure. Although the PROM rarely fails, it could be the cause of the problem.
   • Replacement ECM may be faulty.
   • Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
   • A shorted solenoid, coil relay, or harness may be the cause of ECM failure. Use Short Circuit Tester (J-34636) to check for short circuits.

GMC Vandura G1500 1992 TROUBLE CODE IDENTIFICATION

Code	Probable Cause
12 (1)	No Engine Speed Sensor Reference Pulse To PCM/TCM
14 (1)	CTS Voltage Low (Sensor Or Signal Line Grounded)
15 (1)	CTS Voltage High (Sensor, Connections, Or Wires Open)
21 (1)	TPS Voltage High (Open Circuit Or Misadjusted TPS)
22 (1)	TPS Voltage Low (Circuit Grounded)
23 (1)	TPS Not Calibrated (.25-1.3 Volts At Curb Idle)
24 (1)	Vehicle Speed Sensor Circuit Open Or Grounded
28	Pressure Switch Manifold Range Circuit Open Or Shorted
31 (1)	MAP Voltage Low (Circuit Open Or Shorted To Ground)
32 (1)	EGR Error (Improper Vacuum Signal)
33 (1)	MAP Voltage High (Circuit Open Or Shorted To Ground)
39	TCC Stuck Off (Faulty TCC Solenoid)
51 (1)	Improperly Installed/Faulty PROM
52 (1)	ECM Fault
53 (1)	Voltage Reference Overload
58	TTS High Temperature (Sensor Or Signal Line Grounded)
59	TTS Low Temperature (Sensor, Connections, Or Wires Open)
68	Overdrive Ratio Error (Engine RPM Greater Than Input Speed)
73	Force Motor Commanded Amperage Differs From Return
75	System Voltage Low (Charging System Problem)
81	QDM Solenoid "B" Monitored Voltage Differs From Commanded
82	QDM Solenoid "A" Monitored Voltage Differs From Commanded
83	QDM TCC Monitored Voltage Differs From Commanded
85	Undefined Gear Ratio (Input Or Output Sensor Failure)
86	Shift Solenoid "B" Stuck On (Commanded Gear Not Engaged)
87	Shift Solenoid "B" Stuck Off (Commanded Gear Not Engaged)
(1) Code is engine related. For information on transmission related codes, see G - TESTS W/ CODES - ELECTRONIC TRANSMISSION article.

GMC Vandura G1500 1992 - CLEARING TROUBLE CODES

CAUTION:

When battery is disconnected, vehicle computer and memory systems may lose memory data. Driveability problems may exist until computer systems have completed a relearn cycle. See COMPUTER RELEARN PROCEDURES article in the GENERAL INFORMATION Section before disconnecting battery.

NOTE:

To prevent ECM damage, ensure ignition is in OFF position when disconnecting or reconnecting power to ECM.

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require codes to be cleared before using diagnostic chart. To clear codes, remove power to ECM for 30 seconds by either disconnecting battery cable and ECM pigtail to battery, or removing ECM "B" fuse.

If no hard fault codes are present (only intermittent codes exist), proceed to TESTS W/O CODES article for intermittent diagnostic procedures.

GMC Vandura G1500 1992 - ECM LOCATION

The ECM is located under passenger's seat riser ("G" Series), or behind right side of dash (all other models).

GMC Vandura G1500 1992 - SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers, plugged into the ALDL connector, may be used to read trouble codes, and check voltages in system on serial data line. These testers can save a great deal of time. For additional information, see owner's manual included with tester.

GMC Vandura G1500 1992 - TEST EQUIPMENT

A tachometer, test light, Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, a vacuum gauge, and jumper wires are required to test and diagnose Diesel Electronic Control (DEC) system. A scan tester may also be used to access data parameters. Tester will supply a visual reading of most inputs, and some outputs, to ECM.

GMC Vandura G1500 1992 - DIAGNOSTIC CHARTS

The following diagnostic flow charts and circuit schematics are provided courtesy of General Motors.

GMC Vandura G1500 1992 - DIAGNOSTIC CIRCUIT CHECK

The ECM provides diagnostic logic to detect faults in Diesel Electronic Control (DEC) system. When ECM recognizes a fault, it turns on SERVICE ENGINE SOON light and stores codes. If condition is corrected, SERVICE ENGINE SOON light will go off. ECM recognizes errors in engine speed and EGR vacuum, as well as electrical faults involving 5-volt reference circuit.

ECM controls the following:

• Exhaust Gas Recirculation (EGR)
• Exhaust Pressure Regulation (EPR)
• Torque Converter Clutch (TCC)
• System Diagnosis
• Cold Advance & Glow Plugs

To allow proper engine control, ECM monitors the following inputs:

• Engine RPM
• Manifold Absolute Pressure (MAP)
• Throttle Position Sensor (TPS)
• Vehicle Speed Sensor (VSS)
• Coolant Temperature Sensor (CTS)

Begin all diagnosis with diagnostic circuit check. After any DEC system repair, repeat diagnostic circuit check.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests SERVICE ENGINE SOON light operation. With ignition on and engine off, light should be on.
2. Grounding ALDL diagnostic terminal "B" allows ECM to flash Code 12 and any stored codes. Light must flash on and off to be considered a proper code. If light just goes from bright to dim, it is not considered a code.

Fig. 2: GMC Vandura G1500 1992 - Component Locations - Diagnostic Circuit Check, Schematic, "C" & "K" Series

Fig. 3: GMC Vandura G1500 1992 - Component Locations - Diagnostic Circuit Check, Schematic, "G" Series

Fig. 4: GMC Vandura G1500 1992 - Component Locations - Diagnostic Circuit Check, Flow Chart

GMC Vandura G1500 1992 - SERVICE ENGINE SOON LIGHT INOPERATIVE

When engine is started, ECM grounds terminal A10 to turn off SERVICE ENGINE SOON light. When ALDL terminal "B" is grounded, ECM alternately grounds and opens terminal A10 to flash a code.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests for an open ECM fuse or an open in SERVICE ENGINE SOON light circuit, including instrument panel connector and printed circuit board. Light on is normal response.
2. This tests for a shorted ECM. A grounded ECM terminal A10 will cause SERVICE ENGINE SOON light to go off. If light comes on after disconnecting ECM, ECM is shorted. Light on is normal response.
3. This tests for a grounded circuit between terminal "C" of remote lamp driver and ECM terminal A10. It also checks for an open circuit No. 439 to terminal "B" of remote lamp driver module, a bad ground, or a faulty remote lamp driver. Because of voltage drop through remote lamp driver upper resistor, a normal voltage reading should be 6-11 volts. If voltage is greater than 11 volts, there is no voltage drop in remote lamp driver module due to bad ground or faulty module.
4. This tests for an open wire to remote lamp driver terminal "B". Normal reading should be close to battery voltage.
5. This tests for a grounded circuit between terminal "C" of remote lamp driver and ECM terminal A10. Light on is normal response.

Fig. 5: GMC Vandura G1500 1992 - Component Locations - SVC Soon Light Inop, Schematic, "C" & "K" Series

Fig. 6: GMC Vandura G1500 1992 - Component Locations - SVC Soon Light Inop, Schematic, "G" Series

Fig. 7: GMC Vandura G1500 1992 - Component Locations - SVC Soon Light Inop, Flow Chart

GMC Vandura G1500 1992 - ECM CHECK, SERVICE ENGINE SOON LIGHT ON AT ALL TIMES OR WON'T FLASH CODE 12

The ECM check determines why SERVICE ENGINE SOON light remains on, or does not flash Code 12. Normally, ECM will not recognize a fault for at least 10 seconds after engine start-up. If SERVICE ENGINE SOON light remains on, ECM has lost battery power, ground, or the signal that turns SERVICE ENGINE SOON light off. When engine is started, ECM grounds terminal A10 to turn off SERVICE ENGINE SOON light. ECM alternately grounds and opens terminal A10 to flash trouble code.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests for a short to battery voltage in wire to remote lamp driver terminal "C", or a faulty remote lamp driver. Normal reading is 9-11 volts.
2. This tests if problem is related to ECM, or remote lamp driver. Normally, grounding remote lamp driver terminal "C" should turn light off. If light goes out when terminal "C" is grounded, problem is related to ECM and its wiring. If light does not go out, problem is related to remote lamp driver and its wiring.
3. Normally, grounding ECM terminal A10 should turn light off. If light stays on, there is an open in circuit between ECM harness terminal A10 and remote lamp driver terminal "C".
4. This tests for open ECM circuit No. 451 to diagnostic terminal "B" in ALDL connector. The light should flash Code 12 when ECM terminal A6 is grounded.
5. This tests for proper voltage supply to ECM. Both readings should be more than 9 volts. Voltage to ECM terminal C14 comes from ignition switch. Terminal C16 has constant battery voltage for long term memory.
6. This tests for a bad ground to ECM. Terminal C2 is connected in the ECM.
7. This test distinguishes between a faulty ECM and PROM. Normal response is for Code 51 to flash even though PROM is not installed in the ECM. If Code 51 is not flashed, ECM is faulty.

Fig. 8: GMC Vandura G1500 1992 - Component Locations - ECM SVC Soon Light On All The Time, Schematic, "C" & "K"

Fig. 9: GMC Vandura G1500 1992 - Component Locations - ECM SVC Soon Light On All The Time, Schematic, "G"

Fig. 10: GMC Vandura G1500 1992 - Component Locations - ECM SVC Soon Light On All Time, Flow Chart

GMC Vandura G1500 1992 - EPR SOLENOID ELECTRICAL CHECK

The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to EPR valve. When energized, EPR solenoid allows vacuum to close EPR valve, increasing exhaust backpressure for proper EGR operation. The solenoid is supplied with 12 volts by ignition switch. ECM completes ground circuit to energize solenoid, and turn EPR on when needed (EGR operation command).

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests for short to ground, or a faulty ECM signal to EPR solenoid. Test light should normally be off.
2. This tests for signal to energize EPR solenoid with engine at idle. If test light is on, electrical circuits to solenoid are okay.
3. This tests for voltage, or open circuit from terminal "B" of EPR solenoid to ECM terminal C12.

Fig. 11: GMC Vandura G1500 1992 - Component Locations - EPR Sol Elec Check, Schematic

Fig. 12: GMC Vandura G1500 1992 - Component Locations - EPR Sol Elec Check, Flow Chart

GMC Vandura G1500 1992 - EPR VACUUM CIRCUIT CHECK

The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to EPR valve. When energized, EPR solenoid allows vacuum to close EPR valve, increasing exhaust backpressure for proper EGR operation.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests for normal EPR vacuum at idle. Since electrical circuit was already proven okay in EPR SOLENOID ELECTRICAL CHECK chart, absence of vacuum is due to no vacuum source (vacuum pump), or a restriction or leak in vacuum hose to valve, including a leak in solenoid.
2. EPR solenoid is de-energized, no vacuum should be present.
3. This tests for normal operation of EPR valve. When vacuum is applied to vacuum valve, valve actuator should move and hold.

Fig. 13: GMC Vandura G1500 1992 - Component Locations - EPR VAC Check, Schematic

Fig. 14: GMC Vandura G1500 1992 - Component Locations - EPR Vac Check, Flow Chart

GMC Vandura G1500 1992 - CODE 12, NO REFERENCE PULSE

Code 12 indicates ECM is on, and sees no reference pulse from engine speed sensor. Code display is normal with ignition on and engine not running. Code 12 is not stored and will only flash when fault is present. Code 12 with engine running could indicate an open or ground in engine speed sensor reference circuit.

The engine speed sensor is a camshaft-driven pick-up, mounted at center rear of engine. Sensor is supplied with a 5-volt reference voltage by ECM, and allows ECM to measure engine RPM by the number of times voltage is pulsed. The engine speed sensor pulses reference voltage 4 times per revolution.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This tests for a good 5-volt reference. Normally, ECM should be at about 5 volts for fully charged batteries.
2. This tests for proper ECM voltage to the engine speed sensor. If circuit to ECM is complete, normal voltage will be about 5 volts with harness disconnected from sensor.
3. This tests for good sensor ground circuit No. 452 from sensor to ECM. Since result of test 2) indicates an open exists, this test will determine if open is in wire or at ECM.

Fig. 15: GMC Vandura G1500 1992 - Component Locations - CODE 12, Schematic, No Reference Pulse "C" & "K"

Fig. 16: GMC Vandura G1500 1992 - Component Locations - CODE 12, Schematic, No Reference Pulse "G"

Fig. 17: GMC Vandura G1500 1992 - Component Locations - CODE 12, Flow Chart, No Reference Pulse

GMC Vandura G1500 1992 - CODE 14, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE LOW

The coolant temperature sensor uses a thermistor to control signal voltage to ECM. The ECM applies and monitors voltage over circuit No. 410 to sensor. When engine is cold, sensor resistance is high. The ECM will sense a high signal voltage. As engine warms, sensor resistance becomes less and voltage drops. At normal engine operating temperature, voltage will be 1.5-2.0 volts.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 14 will set if signal voltage indicates a coolant temperature greater than 275?F (135?C) for 3 minutes.
2. This test determines if circuit No. 410 is shorted to ground, causing conditions for Code 14.

GMC Vandura G1500 1992 - Diagnostic Aids

Check circuit No. 410 routing for a potential short to circuit No. 452, or ground. Scan tester displays engine temperature in degrees Celsius. After starting engine, temperature should rise to about 90?C and stabilize when thermostat opens. Test coolant sensor at various temperature levels. See COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart. An out-of-calibration sensor can cause poor driveability.

Fig. 18: GMC Vandura G1500 1992 - Component Locations - CODE 14, Schematic, CTS Signal Voltage Low

Fig. 19: GMC Vandura G1500 1992 - Component Locations - CODE 14, Flow Chart, CTS Signal Voltage Low

GMC Vandura G1500 1992 - CODE 15, COOLANT TEMPERATURE SENSOR SIGNAL VOLTAGE HIGH

The coolant temperature sensor uses a thermistor which controls signal voltage to the ECM. The ECM applies and monitors voltage over circuit No. 410 to sensor. When engine is cold, sensor resistance is high. The ECM will sense a high signal voltage. As engine warms, sensor resistance reduces and voltage drops. At normal engine operating temperature, voltage should be 1.5-2.0 volts.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. Code 15 will set if engine has run for more than 5 minutes and coolant temperature is less than -22?F (-30?C).
2. This test simulates Code 14. If ECM recognizes low signal voltage (high temperature), and scan tester reads 130?C or greater, ECM and wiring are okay.
3. This test determines if circuit No. 410 is open. Voltage at sensor connector should be 5 volts when measured with a DVOM.

GMC Vandura G1500 1992 - Diagnostic Aids

Scan tester displays engine temperature in degrees Celsius. After starting engine, temperature should rise to about 90?C and stabilize when thermostat opens. If Code 12 or 21 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor connector. Test coolant sensor at various temperature levels. See COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart. An out-of-calibration sensor can cause poor driveability.

Fig. 20: GMC Vandura G1500 1992 - Component Locations - CODE 15, Schematic, CTS Signal Voltage High

Fig. 21: GMC Vandura G1500 1992 - Component Locations - CODE 15, Flow Chart, CTS Signal Voltage High

GMC Vandura G1500 1992 - CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH

The Throttle Position Sensor (TPS), a variable mechanical resistor, informs ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest, and output to ECM will be close to 5 volts. When Code 21 is set, EPR is turned off. Code 21 indicates ECM has sensed high voltage at terminal A2 for at least 2 minutes, with engine speed less than 1120 RPM.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test confirms Code 21 fault is present.
2. This tests for 5-volt reference signal at TPS harness connector, and separates an electrical circuit problem from a faulty TPS. If circuit is okay, normal voltage reading will be 5 volts.
3. This checks if low reference voltage is due to an open wire, or ECM.

Fig. 22: GMC Vandura G1500 1992 - Component Locations - CODE 21, Schematic, TPS Signal Voltage High

Fig. 23: GMC Vandura G1500 1992 - Component Locations - CODE 21, Flow Chart, TPS Signal Voltage High

GMC Vandura G1500 1992 - CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW

The Throttle Position Sensor (TPS), a variable mechanical resistor, informs ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest, and output to ECM will be close to 5 volts. When Code 21 is set, EPR is turned off. Code 22 indicates ECM has sensed low voltage at terminal A2 for at least 2 minutes, with engine speed more than 1250 RPM.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test confirms Code 22 fault is present.
2. This test simulates Code 21 (high voltage). If ECM recognizes high signal voltage, ECM and wiring are okay. If signal voltage is still low, Code 23 will set because test was performed at less than 1250 RPM.

Fig. 24: GMC Vandura G1500 1992 - Component Locations - CODE 22, Schematic, TPS Signal Voltage Low

Fig. 25: GMC Vandura G1500 1992 - Component Locations - CODE 22, Flow Chart, TPS Signal Voltage Low

GMC Vandura G1500 1992 - CODE 23, THROTTLE POSITION SENSOR MISADJUSTED

The Throttle Position Sensor (TPS), a variable mechanical resistor, informs ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest, and output to ECM will be close to 5 volts. Code 23 indicates ECM has noted that voltage, at terminal A2, is not .25-1.35 volts for at least 30 seconds, with engine speed at 550-650 RPM.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This confirms Code 23 fault is present.
2. This test determines if sensor signal line is shorted to ground.
3. See THROTTLE POSITION SENSOR in N - REMOVE/INSTALL/OHAUL article.

GMC Vandura G1500 1992 - Diagnostic Aids

Disregard Code 23 if SERVICE ENGINE SOON light goes out when throttle is returned to idle.

Fig. 26: GMC Vandura G1500 1992 - Component Locations - CODE 23, Schematic, TPS Misadjusted

Fig. 27: GMC Vandura G1500 1992 - Component Locations - CODE 23, Flow Chart, TPS Misadjusted

GMC Vandura G1500 1992 - CODE 24, VEHICLE SPEED SENSOR CIRCUIT

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 connects to Digital Ratio Adapter Controller (DRAC), which alternately grounds and opens circuit No. 437 when drive wheels are turning. The circuit is opened and closed approximately 2000 times per mile when drive wheels are turning. ECM calculates vehicle speed based on the time between pulses. Scan tester reading should closely match speedometer reading with drive wheels turning.

Code 24 will set after the following conditions are met for at least 10 seconds: circuit No. 437 is constant, engine speed is more than 2000 RPM, vehicle speed signal at ECM terminal A9 is less than 5 MPH.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This test monitors ECM voltage on circuit No. 437. With wheels turning, the pulsating action causes varying voltage. Voltage variation will be greater at low wheel speeds, and an average of 4-6 volts at about 20 MPH.
2. Less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at DRAC. If voltage now reads more than 10 volts, the DRAC is faulty. If voltage remains less than 10 volts, then circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts reading at the ECM connector indicates that circuit No. 437 is open or DRAC is faulty.
4. Normal voltage indicates a possible intermittent condition.

GMC Vandura G1500 1992 - Diagnostic Aids

With drive wheels turning, scan tester and speedometer reading should closely match.

Fig. 28: GMC Vandura G1500 1992 - Component Locations - CODE 24, Schematic, Vehicle Speed Sensor Circuit

Fig. 29: GMC Vandura G1500 1992 - Component Locations - CODE 24, Schematic, Vehicle Speed Sensor Circuit

Fig. 30: GMC Vandura G1500 1992 - Component Locations - CODE 24, Flow Chart, Vehicle Speed Sensor Circuit

GMC Vandura G1500 1992 - CODE 31, MAP SENSOR SIGNAL VOLTAGE LOW

The Manifold Absolute Pressure (MAP) sensor monitors vacuum in EGR circuit. It senses actual vacuum in EGR vacuum line and sends a signal back to ECM. The signal is compared to EGR duty cycle calculated by ECM. If there is a difference in ECM command and what vacuum is at EGR valve as sensed by MAP sensor, ECM makes minor adjustments. When a major difference is sensed, ECM recognizes a fault and sends a full EGR signal.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This confirms Code 31 fault is present.
2. If ECM recognizes and sets Code 33 (high MAP signal), the ECM, MAP sensor and wiring are okay.
3. If ECM recognizes and sets Code 33 (high MAP signal), the ECM and wiring are okay.
4. This tests for 5-volt reference signal to MAP sensor. Normally, about 5 volts should be present at MAP sensor terminal "C" with ignition on.
5. This tests for an open in EGR solenoid circuit.

Fig. 31: GMC Vandura G1500 1992 - Component Locations - CODE 31, Schematic, MAP Sensor Signal Voltage Low

Fig. 32: GMC Vandura G1500 1992 - Component Locations - CODE 31, Schematic, MAP Sensor Signal Voltage Low

Fig. 33: GMC Vandura G1500 1992 - Component Locations - CODE 31, Flow Chart, MAP Sensor Signal Voltage Low

GMC Vandura G1500 1992 - CODE 32, EGR CIRCUIT LOOP ERROR

During normal operation, the ECM compares its EGR duty cycle signal with Manifold Absolute Pressure (MAP) signal and makes adjustments in duty cycle. If actual EGR control pressure (line vacuum) varies from what ECM has previously determined pressure should be, and this variance continues for 10 seconds or more, a Code 32 will be set and ECM will shut down EGR.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This determines if Code 32 can reset.
2. This tests EGR solenoid electrical control circuit. Test light should flicker faintly if ECM harness and connections are okay. A faintly flickering light is a slightly pulsating glow as opposed to a bright steady glow from a continuous ground path.

GMC Vandura G1500 1992 - Diagnostic Aids

A vacuum leak may cause a Code 32. Check all vacuum hoses and components connected to vacuum hoses for leaks. Also check cruise control and A/C systems vacuum hoses and components.

Fig. 34: GMC Vandura G1500 1992 - Component Locations - CODE 32, Schematic, EGR Circuit Loop Error

Fig. 35: GMC Vandura G1500 1992 - Component Locations - CODE 32, Schematic, EGR Circuit Loop Error

Fig. 36: GMC Vandura G1500 1992 - Component Locations - CODE 32, Flow Chart, EGR Circuit Loop Error

GMC Vandura G1500 1992 - CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH

The Manifold Absolute Pressure (MAP) sensor monitors vacuum in EGR circuit. It senses actual vacuum in EGR vacuum line and sends a signal back to ECM. The signal is compared to EGR duty cycle calculated by ECM. If there is a difference in ECM command and what vacuum is at EGR valve as sensed by MAP sensor, the ECM makes minor adjustments. When a major difference is sensed, the ECM recognizes a fault and sends a full EGR signal.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This step confirms Code 33 fault is present.
2. If ECM recognizes and sets Code 33 (low MAP signal), ECM and wiring are okay.
3. This determines if solenoids are stuck closed.
4. This determines whether a short circuit to ground exists in solenoid circuit, or if ECM is faulty.

Fig. 37: GMC Vandura G1500 1992 - Component Locations - CODE 33, Schematic, MAP Sensor Signal Voltage High

Fig. 38: GMC Vandura G1500 1992 - Component Locations - CODE 33, Schematic, MAP Sensor Signal Voltage High

Fig. 39: GMC Vandura G1500 1992 - Component Locations - CODE 33, Flow Chart, MAP Sensor Signal Voltage High

GMC Vandura G1500 1992 - CODE 51, PROM PROBLEM

Ensure all pins are fully inserted in socket. If pins are fully inserted, replace PROM and recheck. If problem is not corrected, replace ECM.

GMC Vandura G1500 1992 - CODE 52, ECM FAULT

Ensure ECM connectors are fully inserted. Clear ECM memory. Start engine and check for SERVICE ENGINE SOON light. If light and Code 52 reappear, replace ECM. After repairs, clear ECM memory to confirm no SERVICE ENGINE SOON light.

GMC Vandura G1500 1992 - CODE 53, VOLTAGE REFERENCE OVERLOAD

The 5-volt reference circuit is overloaded (grounded circuit). It takes 10 seconds before Code 53 will set.

NOTE:

Test numbers refer to numbers on diagnostic chart.

1. This confirms Code 53 is still present.
2. This confirms 5-volt reference signal from ECM.
3. This tests if a short circuit to ground exists in circuit No. 416 or ECM.

Fig. 40: GMC Vandura G1500 1992 - Component Locations - CODE 53, Schematic, Voltage Reference Overload

Fig. 41: GMC Vandura G1500 1992 - Component Locations - CODE 53, Flow Chart, Voltage Reference Overload