Engine Controls - Tests W/codes - 7.4L: Diagnosis GMC Cab & Chassis C2500

SELF-DIAGNOSTIC SYSTEM

All vehicle are equipped with either an Electronic Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM). Unless specifically stated, references to ECM also apply to PCM or VCM 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. This light is also referred to as the Malfunction Indicator Light (MIL). When malfunction is detected and MIL is turned on, a corresponding trouble code will be stored in ECM memory. To retrieve stored codes, see READING TROUBLE CODES or RETRIEVING CODES (NON-SCAN). Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

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 appropriate BASIC TESTING article in the ENGINE PERFORMANCE section below. «BASIC TESTING - GASOLINE»(ref-19753) (for "C" & "K" Series) «BASIC TESTING - GASOLINE (For "G" & "P" Series)»(ref-19757)
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause Malfunction Indicator Light (MIL) 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 appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE section below. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes. «TESTS W/O CODES - GASOLINE (for "C" & "K" Series)»(ref-19726) «TESTS W/O CODES - GASOLINE (For "G" & "P" Series)»(ref-19728)
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE section above. From there you will be sent to the appropriate area in appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section below. «SYSTEM/COMPONENT TESTS - GASOLINE (for "C" & "K" Series)»(ref-19749) «SYSTEM/COMPONENT TESTS - GASOLINE (For "G" & "P" Series)»(ref-19756)
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in appropriate BASIC TESTING article in the ENGINE PERFORMANCE section below. «BASIC TESTING - GASOLINE (for "C" & "K" Series)»(ref-19753) «BASIC TESTING - GASOLINE (For "G" & "P" Series)»(ref-19757)

READING TROUBLE CODES

Note. Trouble codes retrieved from ECM/PCM/VCM may be related to either engine or transmission. For engine-related codes, use this article. For transmission-related codes, see appropriate G - ELECTRONIC TRANSMISSION TESTS W/ CODES article in the ENGINE PERFORMANCE section below. To identify whether codes relate to transmission or engine, see TROUBLE CODE IDENTIFICATION table.

1. «TESTS W/CODES - ELECTRONIC TRANSMISSION (for "C" & "K" Series)»(ref-19766)
2. AUTO TRANS DIAGNOSIS - 4L60-E & 4L80-E (For "G" & "P" Series)

The ECM stores component failure information under a related trouble code which can be recalled for diagnosis and repair. Read trouble codes by counting Malfunction Indicator Light (MIL) flashes or with diagnostic scan tester connected to the Data Link Connector (DLC). 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, MIL flashes can be read by grounding DLC 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 DLC 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.

TROUBLE CODE IDENTIFICATION

Note. Trouble code charts should only be used if Malfunction Indicator Light (MIL) 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 Code 51, 52 or 55 is displayed with another code, start with 50-series code first and proceed to lower numbered codes.

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

1. MANUALLY enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes. See CLEARING TROUBLE CODES.
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. MIL should go out. Run warm engine at specified curb idle for 2 minutes and note MIL.
3. If MIL 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.
4. If MIL does not come on, all stored trouble codes were intermittent failures. Exceptions are noted under DIAGNOSTIC PROCEDURE.

CLEARING TROUBLE CODES

Turn ignition switch to ON position and ground diagnostic test terminal "B" at DLC. see scheme 1 Turn ignition switch to OFF position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove diagnostic terminal ground lead. Codes may also be cleared using the General Motors Tech 1 scan tester.

Diagnostic Aids

Diagnostic aids (located in many trouble code charts) are provided as additional tips to help with diagnosis when inspected circuit is okay.

SPECIAL TOOLS (DIAGNOSTIC)

Note. A special scan tester, plugged into the DLC, 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.

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.

TROUBLE CODE CHARTS

Note. The following diagnostic mini-schematics are supplied courtesy of General Motors Corp.

Verify a clean, tight connection for the sensor ground. An open circuit at sensor signal terminal or ground terminal will result in a Code 13.

CODE 13 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 13 Schematic (7.4L) Open Oxygen Sensor Circuit. Scheme 91

Code 13 Flow Chart (7.4L) Open Oxygen Sensor Circuit. Scheme 92

After engine is started, temperature should rise steadily to about 194°F (90°C), then stabilize when thermostat opens. If engine is allowed to cool overnight, coolant temperature sensor and MAT sensor (if equipped) should read close to each other, when measured with a scan tester.

CODE 14 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 14 Schematic (7.4L) Coolant Sensor Signal Voltage Low. Scheme 93

Code 14 Flow Chart (7.4L) Coolant Sensor Signal Voltage Low. Scheme 94

After engine starts, temperature should rise steadily to about 194°F (90°C) and stabilize when thermostat opens. If engine is allowed to cool overnight, 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.

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

TEMPERATURE-TO-RESISTANCE VALUES (1) (2)

Code 15 Flow Chart (7.4L) Coolant Sensor Signal Voltage High. Scheme 95

Check for poor connections or damaged harness.

Code 16 Schematic ("G" Series) VSS Buffer Fault. Scheme 96

Code 16 Flow Chart ("G" Series) VSS Buffer Fault. Scheme 97

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate to about 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.

CODE 21 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 21 Schematic (7.4L) TPC Signal Voltage High. Scheme 98

Code 21 Flow Chart (7.4L) TPC Signal Voltage High. Scheme 99

A scan tester displays throttle position in volts. Closed throttle voltage should be less than 1.0 volt. TPS voltage should increase at a steady rate to about 4.5 volts as throttle angle increases. Code 22 will also set if TPS signal or ground circuits are open or grounded.

CODE 22 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 22 Flow Chart (7.4L) TPS Signal Voltage Low. Scheme 100

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE section below. 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.

1. «TESTS W/O CODES - GASOLINE (for "C" & "K" Series)»(ref-19726)
2. «TESTS W/O CODES - GASOLINE (For "G" & "P" Series)»(ref-19728)

Code 24, Schematic ("C" & "K" Series Exc. M/T) Vehicle Speed Sensor. Scheme 101

Code 24, Schematic ("P" Series M/T) Vehicle Speed Sensor. Scheme 102

Code 24, Flow Chart Vehicle Speed Sensor. Scheme 103

Code 24 will set when no vehicle speed is detected at vehicle start off. Code 72 will set when VSS signal is present and is lost. Check all connections, especially those at transmission pass-through connector. If code is intermittent, see INTERMITTENTS in appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE 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.

1. «TESTS W/O CODES - GASOLINE (for "C" & "K" Series)»(ref-19726)
2. «TESTS W/O CODES - GASOLINE (For "G" & "P" Series)»(ref-19728)

Code 24 Schematic ("G" Series A/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 104

Code 24 Schematic ("P" Series M/T) Vehicle Speed Sensor Transmission Output Signal. Scheme 105

Code 24 Flow Chart Vehicle Speed Sensor Transmission Output Signal. Scheme 106

Before replacing ECM, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 9.5-10.5 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "B" and "C" should start at 700 ohms and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series A/T) EGR Circuit Error With Linear EGR. Scheme 107

Code 32 Schematic ("G" Series) EGR Circuit Error With Linear EGR. Scheme 108

Code 32 Schematic ("P" Series 7.4L) EGR Circuit Error With Linear EGR. Scheme 109

Code 32 Flow Chart EGR Circuit Error With Linear EGR. Scheme 110

Prior to replacing ECM, check resistance of all ECM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("C" & "K" Series M/T) EGR Circuit Error. Scheme 111

Code 32 Flow Chart ("C" & "K" Series M/T) EGR Circuit Error. Scheme 112

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms.

Code 32 Schematic ("G" Series A/T) EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 113

Code 32 Flow Chart EGR Ckt. Error Using Manifold Vac., 2-Terminal Sol. & Back Press. EGR. Scheme 114

Prior to replacing PCM, check resistance of all PCM-controlled solenoids and relays. Replace any with a resistance value less than 20 ohms. EVRV solenoid cannot be checked with a DVOM due to solid state circuitry.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 115

Code 32 Schematic ("G" Series) EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 116

Code 32 Flow Chart EGR Circuit Error Models Using Ported Vacuum & EVR Solenoid. Scheme 117

With ignition switch in 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 accuracy of suspected sensor. Readings should be within .4 volt of each other. Code 33 will also result if ground circuit is open or MAP signal circuit is shorted to voltage or to 5-volt reference circuit.

CODE 33 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 33 (7.4L) MAP Sensor Signal Voltage High. Scheme 118

Code 33 Flow Chart (7.4L) MAP Sensor Signal Voltage High. Scheme 119

With ignition switch in ON position and engine off, manifold pressure is equal to atmospheric pressure and signal voltage will be high. Comparing BARO readings with a known good vehicle using the same sensor is a good way to check accuracy of suspected sensor. Readings should be within .4 volt of each other. 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 Code 22 is stored, check MAP signal circuit for short to ground.

CODE 34 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 34 Flow Chart (7.4L) MAP Sensor Signal Voltage Low. Scheme 120

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.

If BARO is low, a 96 second reset will result. If Code 36 is set, Code 35 will usually also be set. Repair actuator circuit first. If IAC counts are zero, idle speed control actuator will disengage and raise idle speed.

Code 36 Schematic ("C" & "K" Series) Idle Speed Actuator Fault. Scheme 121

Code 36 Schematic ("P" Series) Idle Speed Actuator Fault. Scheme 122

Code 36 Flow Chart Idle Speed Actuator Fault. Scheme 123

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in appropriate TESTS W/O CODES article in the ENGINE PERFORMANCE section.

1. «TESTS W/O CODES - GASOLINE (for "C" & "K" Series)»(ref-19726)
2. «TESTS W/O CODES - GASOLINE (For "G" & "P" Series)»(ref-19728)

Code 42 Schematic ("P" Series A/T) Ignition Control. Scheme 124

Code 42 Schematic ("G" Series) Ignition Control. Scheme 125

Code 42 Schematic ("P" Series M/T) Ignition Control. Scheme 126

Code 42 Flow Chart Ignition Control. Scheme 127

ECM/PCM applies and monitors a 5-volt DC signal to 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 DC signal to ECM/PCM. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 128

Code 43 Flow Chart ("C", "G", "K" & "P" Series A/T) Knock Sensor W/O Spark Module (Single Sensor). Scheme 129

Code 43 can be caused by a faulty knock sensor connection at ESC module or ECM. Also, check controller-to-ECM signal line for an open or short to ground.

Code 43 Schematic ("C" & "K" Series) Knock Sensor W/ Spark Control Module. Scheme 130

Code 43 Schematic ("P" Series) Knock Sensor W/ Spark Control Module. Scheme 131

Code 43 Flow Chart (7.4L) Knock Sensor W/ Spark Control Module. Scheme 132

Using scan tester, observe Block Learn Memory (BLM) value at different RPMs. If Code 44 conditions exist, block learn value will be around 150-172.

Code 45, rich exhaust, is most likely caused by one of the following