Engine Controls - System/component Tests - 7.4l GMC Suburban I

MODEL IDENTIFICATION

Vehicle model is identified by fifth character of Vehicle Identification Number (VIN), stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION .

MODEL IDENTIFICATION

TERMINOLOGY

Due to Federal government requirements, manufacturers may use names and acronyms for systems and components different than those used in previous years. The following table will help eliminate confusion when dealing with these components and systems. Only relevant components and systems whose names have changed from current General Motors Corp. terminology have been listed.

SAE TERMINOLOGY

INTRODUCTION

Before testing separate components or systems, perform procedures in BASIC TESTING - 7.4L article. Since many computer-controlled and monitored components set a trouble code if they malfunction, also perform procedures in TESTS W/CODES - 7.4L article.

Note. Testing individual components does not isolate shorts or opens. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure. Use ohmmeter and refer to WIRING DIAGRAMS at the end of this article to isolate wiring harness shorts or opens.

Note. The following table provides the location of commonly used diagnostic information. These former "A" and "C" charts are now written in text and inserted into the appropriate location in the new Engine Performance workflow. To familiarize yourself with the Engine Performance workflow, see HOW TO USE THE ENGINE PERFORMANCE SECTION in GENERAL INFORMATION section.

GENERAL MOTORS "A" & "C" CHART REFERENCE

CONTROL UNIT

Note. To perform the following ground and power tests, use appropriate wiring diagram in WIRING DIAGRAMS section at the end of this article.

Ground Circuits

1. Using an ohmmeter, check for continuity to ground on ECM/PCM/VCM ground terminals. Resistance should be zero ohms. If not, repair open to ground.
2. Using a DVOM, touch negative lead of voltmeter to a good ground. Touch positive lead of voltmeter to each ground terminal. With vehicle running, voltmeter should indicate less than one volt. If voltmeter reading is greater than one volt, check for open, corrosion or loose connection on ground circuit.

Power Circuits

1. Using a voltmeter, check for battery voltage between ECM/PCM/VCM continuous power terminal(s) and ground. If battery voltage is not present, check for blown fuse or open fusible link. If okay, check for open in wire between ECM/PCM/VCM terminal and power source.
2. Turn ignition on. Using a voltmeter, check for battery voltage between ECM/PCM/VCM ignition power terminals and ground. If battery voltage is not present, check IGN fuse. If fuse is okay, check for an open in wire between battery and ignition switch, and between ignition switch and ECM/PCM/VCM terminal. If okay, check for a defective ignition switch.
3. Connect voltmeter between ground and ECM/PCM/VCM starter (crank) signal terminal. On vehicles with manual transmission/transaxle, depress clutch pedal. Turn ignition switch to START position. Battery voltage should be present ONLY when ignition switch is in START position.
4. If voltage is not present, check CRANK fuse or fusible link between ignition switch and ECM/PCM/VCM terminal. If fuse or fusible link is okay, check for an open in wire between ignition switch and ECM/PCM/VCM terminal, or check for a defective ignition switch.

A/C ON SWITCH/SYSTEM TEST

1. Turn ignition switch to RUN position. Move mode selector switch to OFF position. With A/C control assembly connected, measure voltage between mode selector switch Dark Green wire and ground. For wiring schematics, see mini-schematics in «A/C CLUTCH»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l__ac-clutch-relay) under MISCELLANEOUS ECM/PCM/VCM CONTROLS.
2. Battery voltage should be present. If battery voltage is present, mode selector switch is operating normally. If battery voltage is not present, check wire from selector switch to fuse for an open circuit. Also check A/C high and low pressure switches for open.
3. Check voltage between mode selector Dark Green or Dark Green/White wire and ground. Voltage should not be present. If voltage is present, replace mode selector switch.

BRAKE SWITCH

Disconnect brake switch harness connector. Using an ohmmeter, check continuity between brake switch terminals. Continuity should be present. Depress brake pedal or activate brake switch, continuity should not be present.

ENGINE COOLANT TEMPERATURE (ECT) SENSOR

If a coolant sensor-related code is present, see appropriate TESTS W/CODES - 7.4L article. An out-of-calibration sensor may not set a trouble code. Use following procedure to test sensor calibration. Disconnect ECT sensor connector. Measure resistance between sensor terminals. Resistance should be high when engine is cold and drop as engine warms. See ECT SENSOR RESISTANCE VALUES .

ECT SENSOR RESISTANCE VALUES

Note. Intake Air Temperature (IAT) sensor is also referred to as Manifold Air Temperature (MAT) sensor.

INTAKE AIR TEMPERATURE (IAT) SENSOR

If an IAT sensor-related code is present, see the appropriate TESTS W/CODES - 7.4L article. An out-of-calibration sensor may not set a trouble code. Use following procedure to test calibration. Disconnect IAT sensor harness connector. Connect ohmmeter between sensor terminals. Sensor resistance should be as specified. See the IAT SENSOR RESISTANCE . With vehicle sitting overnight, IAT sensor and coolant sensor should have close to the same resistance reading.

IAT SENSOR RESISTANCE

KNOCK SENSOR

1. Disconnect knock sensor harness connector. Using an ohmmeter, measure knock sensor resistance between sensor terminal and engine block. Resistance should be 3300-4500 ohms. Connect voltmeter between sensor terminal and ground. Set voltmeter to 2-volt AC scale.
2. Start and idle engine. Tap on engine block near sensor. A signal should be indicated on voltmeter. If no signal is indicated, replace knock sensor. Also see «TIMING CONTROL SYSTEMS»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l) under IGNITION SYSTEM and «TESTS W/CODES - 7.4L»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l) article.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

1. MAP sensor circuit malfunction should set a related code in ECM/PCM/VCM memory. If a code is present, refer to the appropriate «TESTS W/CODES - 7.4L»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l) article. An out-of-calibration sensor may not set a trouble code. Use following procedure to test sensor calibration. If driveability problems exist, MAP sensor failure is suspected and no MAP code is present, disconnect MAP sensor connector. If driveability condition improves, check MAP vacuum hose for splits, kinks, proper routing and blockage. If no problems are found, replace MAP sensor.
2. With ignition on and engine off, check MAP sensor parameter using a scan tester connected to Data Link Connector (DLC). Voltage should be as specified in «MAP SENSOR VOLTAGE RANGE»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l) .
3. If MAP sensor voltage is as specified, go to next step. If voltage is not as specified, check for 5-volt reference supplied to sensor. Check harness integrity. If no problems are evident, replace MAP sensor.
4. Using a hand-held vacuum pump, apply 10 in. Hg to MAP sensor and note voltage change. Voltage should drop to about 1.0-2.5 volts less than specified in table. If voltage is not as specified or voltage reading does not immediately follow vacuum change, MAP sensor is faulty.

MAP SENSOR VOLTAGE RANGE

OXYGEN SENSOR (O2S)

1. Start engine and warm to operating temperature. Disconnect oxygen sensor. Connect DVOM between Purple lead of oxygen sensor and ground. Place DVOM on 2-volt scale.
2. Using another DVOM on 20-volt scale, connect DVOM in series between Purple wire from ECM/PCM/VCM and positive post of battery. This will simulate a rich condition, causing ECM/PCM/VCM to respond by leaning mixture. Reading on DVOM connected to oxygen sensor should decrease to less than .3 volt.
3. Move DVOM lead from positive battery post to negative battery post. This will simulate a lean condition, causing ECM/PCM/VCM to respond by richening mixture. Reading on DVOM connected to oxygen sensor should increase to greater than .8 volt. If reading does not change as specified, replace oxygen sensor.
4. If a second DVOM is not available, connect a jumper in Purple wire from ECM/PCM/VCM. Hold jumper in one hand and touch positive post of battery with other hand to simulate a rich condition. Touch negative post of battery to simulate a lean condition. For additional testing procedures, see «TESTS W/CODES - 7.4L»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l) article.

OXYGEN SENSOR HEATING ELEMENT

On models with oxygen sensor heating elements, disconnect 3-wire connector at oxygen sensor. Measure resistance between White wire terminals on sensor side of connector. Resistance should be 3.5-14 ohms at 68°F (20°C). If resistance is not 3.5-14 ohms, replace oxygen sensor.

THROTTLE POSITION (TP) SENSOR

1. Install jumper wires to enable connection of a DVOM in parallel between TP sensor harness connectors. Connect DVOM positive lead to Dark Blue wire terminal. Connect negative lead to Black wire terminal. (Scheme 241)
2. Turn ignition on, engine off. Slowly depress accelerator pedal. Signal voltage should gradually change from less than one volt at closed throttle to about 5.0 volts at wide open throttle position. If reading is not as specified, replace TP sensor.
3. TP sensor circuit malfunction should set a related trouble code. For further information, see «TESTS W/CODES - 7.4L»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wcodes-74l) article. Also see TP Sensor ADJUSTMENT in «ADJUSTMENTS - 6.5L DIESEL»(/gmc/suburban/i-1992-1999/remont/adjustments/#engine-controls-adjustments-65l-diesel) article.

Scheme 241

TRANSMISSION RANGE SWITCH

A problem in transmission range switch circuit will set related diagnostic trouble code. See TESTS W/CODES - 7.4L article.

VEHICLE SPEED SENSOR (PM GENERATOR)

Disconnect vehicle speed sensor harness connector (located in transmission/transaxle). Place gear selector in Neutral. Raise vehicle drive wheels off the ground. Turn drive wheels by hand (greater than 3 MPH). Measure AC signal voltage between sensor terminals. Voltage reading should vary from 0.1 to 0.5 volt AC as wheel is turned. If reading is not as specified, replace vehicle speed sensor. If a code is set, refer to TESTS W/CODES - 7.4L article.

A/C Relays

See MISCELLANEOUS ECM/PCM/VCM CONTROLS .

Fuel Pump Relay

1. If a prolonged crank is required to start vehicle, fuel pump relay may be faulty. To verify, start engine. With engine running, disconnect oil pressure switch (fuel pump back-up circuit). If engine stalls, fuel pump relay is faulty. If vehicle continues to run, relay is okay. Check for other causes of prolonged crank.
2. To test fuel pump relay, disconnect fuel pump relay. Refer to «COMPONENT LOCATIONS»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l). Apply battery voltage and ground to fuel pump relay winding terminals (control and ground). (Scheme 242)and (Scheme 243).
3. Using an ohmmeter, check continuity between fuel pump relay control and ground terminals. Continuity should exist. If continuity does not exist, fuel pump relay is defective.
4. To by-pass fuel pump relay on vehicle (fuel pump not operating), turn ignition off. Disconnect fuel pump relay connector. Using a fused jumper wire, connect fuel pump test connector to positive side of battery. Fuel pump should run.
5. If fuel pump runs, check for faulty connections to relay or replace defective relay. To locate fuel pump test connector, refer to «COMPONENT LOCATIONS»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l).

Scheme 242

Scheme 243

SOLENOIDS

Note. All ECM/PCM/VCM-controlled solenoids should have at least 20 ohms of resistance (except fuel injectors).

Electronic Air Control (EAC) Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS .

EGR Control Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS .

Idle Air Control (IAC) Valve

See IDLE CONTROL SYSTEM under FUEL SYSTEM.

Torque Converter Clutch (TCC) Solenoid

See MISCELLANEOUS ECM/PCM/VCM CONTROLS .

FUEL DELIVERY

Note. For fuel system pressure testing, see the BASIC TESTING - 7.4L article.

Fuel Pressure Regulator (Port Fuel Injection)

Fuel pressure regulator is a vacuum-controlled diaphragm type, which uses manifold vacuum to modify fuel pressure to compensate for engine load fuel requirements. Connect fuel pressure gauge to fuel pressure service port. Start engine and note fuel pressure. Disconnect vacuum hose from fuel pressure regulator. Fuel pressure should increase 4-10 psi (.28-.70 kg/cm 2 ). If pressure does not increase 4-10 psi (.28-.70 kg/cm 2 ), check for presence of manifold vacuum at signal line. If vacuum is not present, check for kinked, cut or split vacuum hose or plugged throttle body vacuum port. If vacuum is present and no pressure change occurs, replace fuel pressure regulator.

Fuel Pressure Regulator (Central & Throttle Body Injection)

Fuel pressure regulator is mechanically controlled by internal spring pressure. Regulator is adjusted at factory and is not serviceable. If fuel pressure is too low, check for restricted delivery line. Also, check fuel pump pressure and volume. If fuel pressure is too high, check for restricted fuel tank return line or fuel filter. If no faults are found and pressure is too high or too low, replace fuel pressure regulator.

Fuel Pump Oil Pressure Switch (Back-Up Circuit)

To test fuel pump oil pressure switch back-up circuit, start engine. With engine running, disconnect fuel pump relay. If engine stalls, fuel pump oil pressure switch is faulty. If vehicle continues to run, switch is okay.

See RELAYS, SOLENOIDS, MOTORS & MODULES .

Fuel Pump Relay By-Pass Procedure

If fuel pump will not energize, relay may be by-passed to test fuel pump. Turn ignition off. Using a fused jumper wire, apply battery voltage to fuel pump test connector. Fuel pump should turn on. For fuel pump test connector location, refer to COMPONENT LOCATIONS .

Fuel Injector(s)

Disconnect fuel injector harness connector. Measure resistance across injector terminals. Resistance should be as specified. See FUEL INJECTOR RESISTANCE .

FUEL INJECTOR RESISTANCE

See ENGINE SENSORS & SWITCHES .

Note. If injectors are dirty, they should be cleaned using approved injector cleaning procedure before performing port fuel INJECTOR BALANCE TEST.

C-2, Port Fuel Injector Balance Test

The injector balance test is used to pulse the injector for a precise amount of time, spraying a measured amount of fuel in the intake manifold. As each injector is pulsed, a drop in fuel rail pressure occurs. This pressure drop can be recorded and compared to other injectors. An injector with a pressure drop of 1.5 psi (.11 kg/cm 2 ) or more, greater than or less than other injectors, should be considered faulty.

Note. Allow engine to cool to avoid irregular readings due to "hot soak" fuel boiling. To prevent flooding, INJECTOR BALANCE TEST should not be repeated more than once without starting and running engine.

1. With ignition off, connect Fuel Pressure Gauge (J-34730-1) to pressure tap. Unplug harness connector at all injectors. Connect Injector Tester (J-34730-3) to one of the injectors.
2. Follow manufacturer's instructions when installing adapter harness. Ignition should be turned off at least 10 seconds to complete ECM/PCM/VCM shutdown cycle.
3. Turn ignition on. Fuel pump should run at least 2 seconds after ignition is turned on. Bleed air from gauge and hose to ensure accurate gauge reading. Repeat this procedure until all air is bled from system. Turn ignition off for at least 10 seconds.
4. Turn ignition on again to bring fuel pressure to maximum. Record initial pressure reading. Energize tester one time and note pressure drop at lowest point.
5. Disregard any slight pressure drop after low point is reached. Subtracting second pressure reading from initial reading indicates amount of injector pressure drop.
6. Repeat step 4) on each injector and compare pressure drop. Recheck injectors not within pressure drop range. Replace injector(s) failing second check. If injectors are all okay, plug in harness connectors and review SYMPTOMS in «TESTS W/O CODES - GASOLINE»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-tests-wo-codes-gasoline) article.

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" to "B" and "C" to "D". (Scheme 244) Resistance should be 40-80 ohms. If okay, go to next step. If resistance is not as specified, replace IAC valve.
2. Check resistance between IAC terminals "B" to "C" and "A" to "D". Resistance should be infinite. If resistance is not as specified, replace IAC valve.

Note. Functional testing of Idle Air Control (IAC) valve requires a scan tester capable of cycling ECM/PCM/VCM output devices (bidirectional) or a special IAC Driver and Noid Light Set (222L or J-37027). Flow charts in TESTS W/CODES - 7.4L article may refer to Tech 1 tester, General Motors' bidirectional tester.

Scheme 244

IGNITION SYSTEM

Note. For basic ignition system checks, see the BASIC TESTING - 7.4L article.

C-4, Ignition Control Circuit

An open or short to ground in Ignition Control (IC) or by-pass circuit will cause ECM/PCM/VCM to turn on Malfunction Indicator Light (MIL) and confirm fault by setting a related trouble code. Refer to TESTS W/CODES - 7.4L article.

C-5, Knock Sensor Circuit (Models Using External Spark Controller Module)

1. An open or short circuit on ESC wire to ECM/PCM/VCM will cause a loss of 12-volt ESC controller signal. This will cause ECM/PCM/VCM to fully retard ignition timing.
2. If a scan tester is available, connect tester to Data Link Connector (DLC). Using a metal object, tap on engine next to knock sensor and note knock parameter. Knock should be indicated on scan tester.
3. If a scan tester is not available, backprobe ECM/PCM/VCM knock sensor signal terminal with a DVOM. With engine idling, 8-12 volts should be present at this terminal. Using a metal object, tap on engine close to knock sensor. Voltage signal at ECM/PCM/VCM terminal should drop to zero volts, and return when knock signal ceases.
4. If signal does not respond as described, check knock sensor-to-controller-signal. On vehicles equipped with automatic transmission, it may be necessary to place transmission in Drive for timing change to occur. See «KNOCK SENSOR»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l__knock-sensor) under ENGINE SENSORS & SWITCHES.

C-5, Knock Sensor Circuit (Models Using Knock Sensor With Internal Spark Controller Module)

1. An open or short circuit on knock sensor wire to ECM/PCM/VCM will set a related trouble code. A false detonation signal will not cause ECM/PCM/VCM to set a code.
2. If a scan tester is available, connect it to Data Link Connector (DLC). Tap on engine next to knock sensor and note "knock" parameter. Knock should be indicated on scan tester.
3. If a scan tester is not available, connect tachometer to engine. Start engine and hold RPM above idle. Using a metal object, tap on engine close to knock sensor. A noticeable decrease in engine RPM should occur. If no RPM decrease occurred, check knock sensor to ECM/PCM/VCM circuit.
4. On vehicles equipped with automatic transmission, it may be necessary to place transmission in Drive for timing change to occur. See «KNOCK SENSOR»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l__knock-sensor) under ENGINE SENSORS & SWITCHES.

System Test (Vacuum-Controlled Valve)

Start and run engine to normal operating temperature. With engine at idle, RPM should drop as EGR valve is opened by pushing up on underside of EGR diaphragm.

1. Disconnect EGR solenoid electrical harness connector and vacuum hoses. Connect a hand-held vacuum pump to solenoid vacuum source port. Connect vacuum gauge to solenoid EGR port. Pump up vacuum pump. Vacuum should not be present at port to EGR valve.
2. Activate EGR solenoid with a remote ground and 12-volt power supply. Vacuum should now be present at vacuum gauge. Solenoid should have at least 20 ohms of resistance.

Negative Backpressure EGR Valve

With engine off, disconnect vacuum hose to EGR valve. Connect vacuum pump to EGR and apply 10 in. Hg. If EGR diaphragm does not move up and stay up for 20 seconds, replace EGR valve.

Linear EGR Valve (Digital Valve)

1. Install scan tester. Ensure P/N switch is operating properly. See «ENGINE SENSORS & SWITCHES»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l) . With engine at normal operating temperature, command EGR pintle position to zero percent. Increase engine speed to 2000 RPM. If scan tester reads actual EGR pintle position at greater than 3 percent, EGR valve is stuck open. Replace EGR valve.
2. If scan tester reads actual EGR pintle position at 3 percent or less, command a 25 percent position step increase (i.e. 0-25 percent, 25-50 percent, 50-75 percent, etc.). Observe MAP reading and actual EGR pintle position for 3 seconds. EGR should increase by about 25 percent position and MAP reading should also increase.
3. If actual EGR pintle position is stable and within 10 percent of desired EGR pintle position command after 2 seconds, go to next step. If actual EGR pintle position is not as specified, go to step 5).
4. MAP reading should have increased when EGR pintle responded. If MAP did not respond, check EGR passages and EGR valve for blockage. If MAP responded, set desired EGR pintle position to 100 percent. If EGR pintle position sets to 100%, EGR is okay. If not, replace EGR valve.
5. Turn engine off. Check EGR electrical circuit and connecting components. Turn ignition on, check for 5-volt reference voltage on harness connector terminal "D" (Gray wire). If 5-volt reference voltage is not present, check ECM/PCM/VCM. See «CONTROL UNIT»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l__control-unit) under COMPUTERIZED ENGINE CONTROLS. If circuits are okay, replace EGR valve.

Vapor Canister Purge Valve ("P" Series)

1. Install a short length of hose to lower port of purge valve. Blow into hose. Little or no air should pass into canister (a small amount of air will pass if the canister has a constant purge hole).
2. Using a hand-held vacuum pump, apply 15 in. Hg to vacuum control (upper) port. If vacuum does not hold for 20 seconds, replace canister.
3. With vacuum still applied, again try to blow through hose connected to lower port. An increased flow of air should be observed. If airflow does not increase, replace canister.

Fuel Tank Pressure Control Valve (Series - High-Altitude)

1. Connect a hand-held vacuum pump to "control vac" port of tank pressure control valve. Apply approximately 15 in. Hg. to "control vac" port.
2. Attach a short piece of hose to fuel tank side of valve. Blow through hose. Air must pass through fuel tank pressure control valve. If air does not pass through fuel tank pressure control valve, replace defective valve.

Thermostatic Vacuum Switch

1. With engine coolant temperature less than 100°F (38°C), apply vacuum to manifold side of thermostatic vacuum switch. Switch should hold vacuum.
2. With engine coolant temperature greater than 122°F (50°C), vacuum should drop off. If thermostatic vacuum switch fails either test, replace switch.

Required Service

The PCV system may require service for obstructions if any of the following conditions exist

1. Rough Idle
2. Stalling or Slow Idle Speed
3. Oil Leaks
4. Oil in Air Cleaner
5. Sludge in Engine

A leaking PCV valve or hose could cause

1. Rough Idle
2. Stalling
3. High Idle Speed

If engine idles roughly, check for clogged PCV valve and for plugged or broken PCV hoses BEFORE adjusting idle. Check correct PCV valve application to ensure the correct valve is fitted. Replace PCV valve if required.

Checking PCV Valve Function

1. Remove PCV valve from rocker cover. Run engine at idle. Place thumb over open end of valve to check for vacuum. If there is no vacuum at valve, check for obstruction in manifold port, hoses or PCV valve. Repair or replace as necessary.
2. Turn engine off. Remove PCV valve. Shake valve and listen for rattle of check valve inside PCV valve. If a clear rattle is not heard, replace PCV valve.
3. Visually inspect valve for varnish or deposits that may make PCV valve sticky, restricted or incompletely seated. Replace if necessary.
4. Engine must be sealed for PCV system to function as designed. If leakage, sludging or dilution of oil is noted and PCV system is functioning properly, check engine for cause and repair as required to ensure PCV system will continue to function properly.
5. Since an engine operating without any crankcase ventilation can be damaged, it is important to replace PCV valve and air cleaner breather at regular intervals (at least every 30,000 miles). Check all hoses and clamps for failure or deterioration.

Damper Door - Wax Pellet Check

1. Remove air cleaner assembly from vehicle and allow to cool to less than 40°F (4°C). Damper door should be closed to outside (cold) air.
2. Reinstall air cleaner assembly. Start engine and observe damper door. As air cleaner assembly warms up, wax pellet should expand, closing off hot air delivery and opening cold air delivery.
3. If door does not respond as indicated, ensure door is not binding and calibrated damper spring is installed properly.

MISCELLANEOUS ECM/PCM/VCM CONTROLS

Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.

Disconnect harness connector to TCC solenoid. Measure resistance between TCC solenoid terminals. (Scheme 245) Solenoid resistance should be greater than 20 ohms.

Scheme 245

Note. Some solenoids have an internal pressure switch in series with solenoid winding and will not show continuity until transmission hydraulic pressure is applied.

Converter Lock-Up Signal At Transmission

1. Warm engine to operating temperature. Raise vehicle and support drive wheels. Support suspension where necessary to prevent damage to drive axles.
2. Disconnect converter clutch connector at transmission. Connect a test light across converter clutch harness terminals. (Scheme 245) Start engine and place transmission in Drive. Accelerate vehicle to 45 MPH and note test light.
3. If test light is not on, check solenoid power supply wire of harness for open or short to ground. Check ground circuit for open between harness connector and ECM/PCM/VCM. If harness is okay, see CONVERTER LOCK-UP SIGNAL FROM ECM/PCM/VCM below.

Converter Lock-Up Signal From ECM/PCM/VCM

1. Warm engine to operating temperature. Raise vehicle and support drive wheels. Support suspension where necessary to prevent damage to drive axles.
2. Connect a test light to battery voltage. Touch TCC control driver terminal with test light. (Scheme 245) Accelerate vehicle to 45 MPH and note test light. If test light does not illuminate, problem is a faulty ECM/PCM/VCM connector or ECM/PCM/VCM. On some models, lock-up signal may be checked at Data Link Connector (DLC) terminal "F" instead of at ECM/PCM/VCM terminal.

Shift Light (Manual Transmission)

1. These tests assume a shift light problem exists. Use this procedure only if the light will not illuminate, or illuminates all the time.
2. Turn ignition on, with engine off. Note shift light. Shift light should not be on. If shift light is on, check for a short to ground between bulb and ECM/PCM/VCM or for bad ECM/PCM/VCM.
3. With ignition on and engine off, ground test terminal "B" of Data Link Connector (DLC). Malfunction Indicator Light (MIL) should start to flash and shift light should come on. If MIL comes on, go to next step. If MIL does not flash, perform DIAGNOSTIC CIRCUIT CHECK as described in «BASIC TESTING - 7.4L»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-basic-testing-74l) article.
4. If shift light does not come on, ground Tan/Black light driver wire at ECM/PCM/VCM terminal using a jumper wire. For circuit identification, see «SHIFT LIGHT CIRCUIT IDENTIFICATION»(/gmc/suburban/i-1992-1999/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-74l) below, and (Scheme 246).
5. If shift light still does not come on, check for blown GAGES fuse, blown bulb or open circuit between fuse and ECM/PCM/VCM. If shift light comes on when grounding ECM/PCM/VCM terminal with a jumper wire, problem is a bad ECM/PCM/VCM connection or bad ECM/PCM/VCM.

SHIFT LIGHT CIRCUIT IDENTIFICATION

Scheme 246

A/C CLUTCH (C-10) & ELECTRIC COOLING FAN (C-12)

Note. For additional information on electric cooling fans, see appropriate ENGINE COOLING FAN article in ENGINE COOLING section below.

1. «ENGINE COOLING FAN»(ref-20107) (for "C" & "K" Series Pickup, Commercial Van, Sierra Pickup & Van)
2. «ENGINE COOLING FAN»(/gmc/suburban/i-1992-1999/remont/cooling-fan/#engine-cooling-fan) (for Suburban, Tahoe & Yukon)

A/C Clutch Relay

1. Disconnect A/C clutch relay harness connector. Using proper mini-schematic and an ohmmeter, check continuity between A/C clutch relay winding terminals. (Scheme 247) Continuity should exist. Check continuity between clutch drive circuit terminals of relay. Continuity should not exist.
2. Using jumper wires, apply ground and battery voltage to relay winding. Continuity should now exist between clutch drive circuit terminals of relay. Replace A/C clutch relay if continuity does not exist.

COOLING FAN (C-12)

Note. For additional information on electric cooling fans, see appropriate ELECTRIC COOLING FANS article in ENGINE COOLING below.

1. «ENGINE COOLING FAN»(ref-20107) (for "C" & "K" Series Pickup, Commercial Van, Sierra Pickup & Van)
2. «ENGINE COOLING FAN»(/gmc/suburban/i-1992-1999/remont/cooling-fan/#engine-cooling-fan) (for Suburban, Tahoe & Yukon)

Cooling Fan Relay

1. Disconnect cooling fan relay harness connector. Using an ohmmeter, check continuity of relay winding. (Scheme 247)- (Scheme 250). Continuity should exist. Check continuity across power delivery terminals of relay. With relay not energized, continuity should not exist.
2. With ohmmeter still attached to power delivery terminals of relay, apply battery voltage and ground to energize relay winding. Continuity should now exist between cooling fan relay power delivery terminals. Replace cooling fan relay if continuity does not exist.

Cooling Fan Motor

Disconnect cooling fan motor harness connector. Apply battery voltage to one of the fan motor terminals and jumper the other terminal to ground. Fan motor should activate. If fan motor does not activate, replace faulty fan motor.

Note. For a more specific system testing, refer to the following C-10 or C-12 diagnostic charts. If any chart other than a C-10 or C-12 chart is referenced, see appropriate TESTS W/CODES - 7.4L article.

DIAGNOSTIC CHARTS

Note. In the following diagnostic tests, illustrations, schematics and flow charts are courtesy of General Motors Corp.

A/C REQUEST SIGNAL CIRCUIT DIAGNOSIS

Turning on A/C system supplies battery voltage to A/C compressor clutch and Powertrain Control Module (PCM) terminal E12 through circuit No. 59, increasing idle speed. PCM does not control A/C compressor clutch. If A/C compressor clutch is malfunctioning, repair basic A/C system problem. If A/C system is operating properly and idle speed drops too low when A/C compressor turns on or flares too high when A/C compressor turns off, check for an open circuit No. 59 to PCM. If circuit is okay, repair faulty PCM connection (terminal E12) or replace faulty PCM.

A/C Clutch Circuit Schematic - 7.4L VIN H ("C" & "K" Series) A/C Request Signal Circuit Diagnosis. Scheme 247

A/C Clutch Circuit Schematic - 7.4L VIN H ("G" Series) A/C Request Signal Circuit Diagnosis. Scheme 248

A/C Clutch Circuit Schematic - 7.4L VIN H ("L" & "M" Series) A/C Request Signal Circuit Diagnosis. Scheme 249

A/C Clutch Circuit Schematic - 7.4L VIN H ("P" Series) A/C Request Signal Circuit Diagnosis. Scheme 250

Auxiliary Cooling Fan Schematic (Models With A/C) Auxiliary Cooling Fan Circuit Diagnosis. Scheme 251

Auxiliary Cooling Fan Diagnostic Flow Chart (Models With A/C) Auxiliary Cooling Fan Circuit Diagnosis. Scheme 252

Scheme 253

Scheme 254

Scheme 255

Scheme 256

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 1 Of 3). Scheme 257

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 2 Of 3). Scheme 258

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 3 Of 3). Scheme 259

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 1 Of 3). Scheme 260

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 2 Of 3). Scheme 261

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 3 Of 3). Scheme 262

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 1 Of 3). Scheme 263

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 2 Of 3). Scheme 264

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 3 Of 3). Scheme 265

Wiring Diagram ("P" Series - Cutaway - 7.4L - 1 Of 3). Scheme 266

Wiring Diagram ("P" Series - Cutaway - 7.4L - 2 Of 3). Scheme 267

Wiring Diagram ("P" Series - Cutaway - 7.4L - 3 Of 3). Scheme 268

Wiring Diagram (Van - 7.4L With 4L80-E - 1 Of 3). Scheme 269

Wiring Diagram (Van - 7.4L With 4L80-E - 2 Of 3). Scheme 270

Wiring Diagram (Van - 7.4L With 4L80-E - 3 Of 3). Scheme 271

See also:
• BASIC TESTING - 7.4L
• TESTS W/CODES - 7.4L
• ADJUSTMENTS - 6.5L DIESEL
• TESTS W/O CODES - GASOLINE
• ENGINE COOLING FAN
• MODEL IDENTIFICATION
• A/C CLUTCH
• MISCELLANEOUS ECM/PCM/VCM CONTROLS
• KNOCK SENSOR
• CONTROL UNIT