Engine Controls - Tests W/codes - 7.4L: Diagnosis Chevrolet Suburban C1500

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 control module is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, control module will illuminate the SERVICE ENGINE SOON light located on instrument panel. 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 control module memory. To retrieve stored codes, see READING TROUBLE CODES or RETRIEVING CODES (NON-SCAN) . Malfunctions are recorded as HARD FAILURES or as INTERMITTENT FAILURES.

Note. Not all trouble codes will turn on the SERVICE ENGINE SOON light. All emission-related codes will turn on the SERVICE ENGINE SOON light.

DIAGNOSTIC PROCEDURE

Diagnosis of the computerized engine control system should be performed in the following order

1. Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See «BASIC TESTING - 7.4L»(ref-20095) article.
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes may cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, use DIAGNOSTIC AIDS in appropriate code chart or proceed to INTERMITTENTS in «TESTS W/O CODES - GASOLINE»(ref-20057) article.
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in «TESTS W/O CODES - GASOLINE»(ref-20057) article in the ENGINE PERFORMANCE section. From there you will be sent to «SYSTEM/COMPONENT TESTS - 7.4L»(ref-20094) article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in «BASIC TESTING - 7.4L»(ref-20095) article.

READING TROUBLE CODES

The control module 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 control module and sensor/solenoid connector terminals with a digital voltmeter. Some models REQUIRE a scan tester to pull trouble codes. See SCAN TESTER DATA 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. On models not using "P" series codes, Code 12 should always exist when DLC test terminal is grounded with key on and engine off, but it may not be indicated by all makes of scan tester.

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 posit ion 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 and is REQUIRED to clear codes on some vehicles.

Diagnostic Aids

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

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 mini-schematics are courtesy of General Motors Corp.

Verify a clean, tight connection for 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 (All Models) Open Oxygen Sensor Circuit. Scheme 291

Code 13 Flow Chart (All Models) Open Oxygen Sensor Circuit. Scheme 292

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 intake air temperature 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 (All Models) Coolant Sensor Signal Voltage Low. Scheme 293

Code 14 Flow Chart (All Models) Coolant Sensor Signal Voltage Low. Scheme 294

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 intake air temperature 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.

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

CODE 15 TERMINAL & CIRCUIT WIRING IDENTIFICATION

Code 15 Flow Chart (All Models) Coolant Sensor Signal Voltage High. Scheme 295

Check for poor connections or damaged harness.

Code 16 Schematic ("C" & "K" Series A/T) VSS Buffer Fault. Scheme 296

Code 16 Schematic ("G" Series) VSS Buffer Fault. Scheme 297

Code 16 Flow Chart (All Models) VSS Buffer Fault. Scheme 298

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 (All Models) TPS Signal Voltage High. Scheme 299

Code 21 Flow Chart (All Models) TPS Signal Voltage High. Scheme 300

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 (All Models) TPS Signal Voltage Low. Scheme 301

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 TESTS W/O CODES - GASOLINE article. 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.

Code 24 Flow Chart ("C" & "K" Series) Vehicle Speed Sensor. Scheme 302

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 TESTS W/O CODES - GASOLINE article. 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.

Code 24 Flow Chart (All Models) VSS Transmission Output Signal. Scheme 303

Code will set if control module detects one of 2 illegal combinations. Check all connections for good contact. An intermittent may be caused by a poor connection, chaffed wire insulation or a broken wire. Monitor voltage of each terminal while moving related harness connectors. If failure is induced, voltage reading will change.

VALID PSM COMBINATION (4L60-E)

VALID PSM COMBINATION (4L80-E)

Code 28 Schematic ("C", "K" & "P" Series) Transmission Range Pressure Switch. Scheme 304

Code 28 Schematic ("G" Series) Transmission Range Pressure Switch. Scheme 305

Code 28 Schematic (All Models) Transmission Range Pressure Switch. Scheme 306

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

Code 32 Schematic ("G" Series) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 307

Code 32 Flow Chart (All Models) EGR Circuit Error Backpressure EGR With Solenoid. Scheme 308

Prior to replacing control module, check resistance of all control module-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", "G", "K" & "P" Series A/T) EGR Circuit Error With EVRV Solenoid. Scheme 309

Code 32 Schematic ("P" Series M/T) EGR Circuit Error With EVRV Solenoid. Scheme 310

Code 32 Flow Chart (All Models) EGR Circuit Error With EVRV Solenoid. Scheme 311

Before replacing control module, use a DVOM and check resistance of each control module-regulated solenoid and relay.

Code 32 Schematic (7.4L "P" Series) EGR Circuit Error. Scheme 312

Code 32 Flow Chart - 1 Of 2 (All Models) EGR Circuit Error. Scheme 313

Code 32 Flow Chart - 1 Of 2 (All Models) EGR Circuit Error. Scheme 314

Before replacing control module, use an ohmmeter and check EGR valve resistances. Resistance between terminals "A" and "E" should be 7.8-8.6 ohms. Resistance between terminals "B" and "D" should be greater than 3000 ohms. Resistance between terminals "D" and "C" should start at .7 ohm and increase to about 4000 ohms as pintle is slowly moved inward.

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error. Scheme 315

Code 32 Schematic ("C" & "K" Series) EGR Circuit Error. Scheme 316

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 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 Schematic (All Models) MAP Sensor Signal Voltage High. Scheme 317

Code 33 Flow Chart (All Models) MAP Sensor Signal Voltage High. Scheme 318

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 (All Models) MAP Sensor Signal Voltage Low. Scheme 319

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", "G", "K" & "P" Series A/T) Idle Speed Actuator Fault. Scheme 320

Code 36 Flow Chart ("C", "G", "K" & "P" Series A/T) Idle Speed Actuator Fault. Scheme 321

If code is intermittent, check customer driving habits (driving with foot resting on brake pedal) or for unusual traffic conditions (stop and go expressway traffic).

Codes 37/38 Schematic (All Models) TCC Brake Switch On/Off. Scheme 322

Codes 37/38 Flow Chart (All Models) TCC Brake Switch On/Off. Scheme 323

If TCC is mechanically stuck on, vehicle speed is zero MPH, brakes are applied and D2 is selected, TCC fluid will mechanically apply TCC causing an engine stall.

Code 39 Schematic (With 4L80-E Transmission) TCC Stuck Off. Scheme 324

Code 39 Flow Chart (With 4L80-E Transmission) TCC Stuck Off. Scheme 325

The scan tester cannot help diagnose a Code 42 problem. See INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 42 Schematic ("C", "K" & "P" Series) Ignition Control. Scheme 326

Code 42 Schematic ("G" Series) Ignition Control. Scheme 327

Code 42 Flow Chart (All Models) Ignition Control. Scheme 328

Control module applies and monitors a 5-volt DC signal to knock sensors. 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 control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (All Models) Knock Sensor Without Spark Module (Dual Sensors). Scheme 329

Code 43 Schematic (All Models) Knock Sensor Without Spark Module (Dual Sensors). Scheme 330

Control module 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 control module. Knock signal intensity is dependent upon knock signal level.

Code 43 Schematic (All Models) Knock Sensor Without Spark Module (Single Sensor). Scheme 331

Code 43 Flow Chart (All Models) Knock Sensor Without Spark Module (Single Sensor). Scheme 332

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

Code 43 Schematic ("C" & "K" Series) Knock Sensor With Spark Control Module. Scheme 333

Code 43 Schematic ("P" Series) Knock Sensor With Spark Control Module. Scheme 334

Code 43 Flow Chart (All Models) Knock Sensor With Spark Control Module. Scheme 335

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

Jump-starting engine or charging battery with a battery charger may set code. If code is set when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link.

Code 52/53 Schematic ("C","K" & "P" Series) System Voltage High. Scheme 336

Code 52/53 Schematic ("G" Series) System Voltage High. Scheme 337

Code 52/53 Flow Chart (All Models) System Voltage High. Scheme 338

Check harness routing for a potential short to ground in TFT signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

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

Code 58 Schematic ("C", "K" & "P" Series) Transmission Fluid Temperature High. Scheme 339

Code 58 Schematic ("G" Series) Transmission Fluid Temperature High. Scheme 340

Code 58 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 341

Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, displayed temperature should rise steadily to about 100°C then stabilize. A faulty connection or an open in ground circuit or signal circuit will result in a Code 59. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in firm shifts or TCC enabled complaint. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

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

Code 59 Flow Chart (All Models) Transmission Fluid Temperature Low. Scheme 342

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal. The 3-2 control solenoid feedback normally oscillates on/off when duty cycle is applied.

Code 66 Schematic (All Models) 3-2 Control Solenoid Circuit Fault 4l60-E Transmission. Scheme 343

Code 66 Flow Chart (All Models) 3-2 Control Solenoid Circuit Fault 4l60-E Transmission. Scheme 344

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Some slight TCC slippage is normal.

Code 67 Schematic (All Models) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 345

Code 67 Flow Chart (All Models) TCC Solenoid Circuit Check - 4L60-E Transmission. Scheme 346

Check for spread connectors at pass-thru connector. Code 68 will set when going to default (2nd gear). If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 68 Flow Chart (All Models) Overdrive Ratio Error - 4L80-E Transmission. Scheme 347

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Schematic (All Models) Torque Converter Clutch Stuck On 4L60-E Transmission. Scheme 348

Code 69 Flow Chart (All Models) Torque Converter Clutch Stuck On 4L60-E Transmission. Scheme 349

If TCC is mechanically stuck on, TCC fluid will apply TCC, causing an engine stall.

Code 69 Flow Chart (All Models) Torque Converter Clutch Stuck On 4L80-E Transmission. Scheme 350

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

Code 72 Schematic ("C", "G", "K" & "P" Series) VSS Circuit Loss Transmission Output Signal. Scheme 351

Code 72 Flow Chart (All Models) VSS Circuit Loss Transmission Output Signal. Scheme 352

Check for poor connections at control module and transmission pass-thru connector.

Code 73 Schematic (All Models) Pressure Control Solenoid (Current Error). Scheme 353

Code 73 Flow Chart (All Models) Pressure Control Solenoid (Current Error). Scheme 354

Check for poor connections in related circuits.

Code 74 Schematic (All Models) Transmission Input Speed Error 4L80-E Transmission. Scheme 355

Code 74 Flow Chart (All Models) Transmission Input Speed Error 4L80-E Transmission. Scheme 356

Charging battery with a charger and jump starting engine may set this code. If code sets when an accessory is operated, check for poor connections or excessive current draw. Also, check for poor connections at starter solenoid or fusible link. Minimum voltage allowed for Code 75 to set is on a graduated scale and changes with temperature from a low of 7.3 volts at -40°F (-40°C) to a voltage of 11.7 volts at 304°F (150°C).

Code 75 Schematic (All Models) System Voltage Low. Scheme 357

Code 75 Flow Chart (All Models) System Voltage Low. Scheme 358

Check harness routing for a potential short to ground in signal circuit. Scan tester displays transmission fluid temperature in degrees Centigrade. After transmission is running, temperature display should rise steadily to about 100°C then stabilize. Test transmission sensor at various temperature levels to determine if sensor is out of calibration. See TRANSMISSION SENSOR - TEMP TO RESISTANCE chart. An out-of-calibration sensor could result in delayed shifts or TCC enabled complaint.

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

Code 79 Flow Chart (All Models) Transmission Fluid Temperature High. Scheme 359

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 81 Schematic (All Models) Transmission 2-3 Error. Scheme 360

Code 81 Flow Chart (All Models) Transmission 2-3 Error. Scheme 361

Check all connections, especially at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 82 Schematic (All Models) Transmission 1-2 Error. Scheme 362

Code 82 Flow Chart (All Models) Transmission 1-2 Error. Scheme 363

Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 83 Schematic (All Models) TCC Solenoid Circuit Fault. Scheme 364

Code will set when an unknown gear ratio is detected for any gear but 4th. Check all connections, especially those at transmission pass-thru connector. If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article. Compare scan tester gear ratio reading to specifications in GEAR RATIO table.

GEAR RATIO

Code 85 Schematic (All Models) Undefined Gear Ratio. Scheme 365

Code 85 Flow Chart (All Models) Undefined Gear Ratio. Scheme 366

If code is intermittent, see INTERMITTENTS in TESTS W/O CODES - GASOLINE article.

Code 86 Flow Chart (All Models) Low Gear Ratio Error. Scheme 367

Check all connections, especially those at transmission pass-thru connector. Fault may be an internal transmission problem.

Code 87 Flow Chart (All Models) High Gear Ratio Error. Scheme 368