Engine Control System & Component Tests - 5.7L: Other Chevrolet Tahoe I

CONTROL UNIT

Note. To perform the following ground and power tests, use appropriate wiring diagram in the WIRING DIAGRAMS article.

Ground Circuits

1. Using an ohmmeter, check for continuity to ground on PCM/VCM ground terminals. Resistance should be zero ohms. If not, repair open to ground.
2. Using a voltmeter, 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 more than one volt, check for open, corrosion or loose connection on ground circuit.

Power Circuits

1. Using a voltmeter, check for battery voltage between 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 PCM/VCM terminal and power source.
2. Turn ignition on. Using a voltmeter, check for battery voltage between 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 PCM/VCM terminal. If okay, check for a defective ignition switch.
3. Connect voltmeter between ground and 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 PCM/VCM terminal. If fuse or fusible link is okay, check for an open in wire between ignition switch and PCM/VCM terminal, or check for a defective ignition 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 the TESTS W/CODES - 5.7L 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 table.

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 TESTS W/CODES - 5.7L 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 IAT SENSOR RESISTANCE table. 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 100 k/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»(ref-20719-S00998409482001010300000) under IGNITION SYSTEM and the «TESTS W/CODES - 5.7L»(ref-20681) article.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

1. MAP sensor circuit malfunction should set a related code in PCM/VCM memory. If a code is present, see the «TESTS W/CODES - 5.7L»(ref-20681) 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 tool connected to Data Link Connector (DLC). Voltage should be as specified in «MAP SENSOR VOLTAGE RANGE»(ref-20719-S36122882662001010300000) table.
3. If MAP sensor voltage is as specified, go to next step. If voltage is not as specified, check 5-volt reference supply 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 wire 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 PCM/VCM and positive post of battery. This will simulate a rich condition, causing 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 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 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 the «TESTS W/CODES - 5.7L»(ref-20681) 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 25)
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 the «TESTS W/CODES - 5.7L»(ref-20681) article. Also see TP SENSOR ADJUSTMENT in the «ADJUSTMENTS»(ref-20562) article.

Scheme 25

TRANSMISSION RANGE SWITCH

A problem in transmission range switch circuit will set related diagnostic trouble code. See the TESTS W/CODES - 5.7L 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 (more 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 the TESTS W/CODES - 5.7L article.

RELAYS

Note. To perform the following tests, use the WIRING DIAGRAMS article.

A/C Clutch Relay

See MISCELLANEOUS 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»(ref-20719-S22239130952001010300000) . Apply battery voltage and ground to fuel pump relay winding terminals (control and ground).
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»(ref-20719-S22239130952001010300000) .

SOLENOIDS

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

Canister Purge Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS .

Idle Air Control (IAC) Valve

See IDLE CONTROL SYSTEM under FUEL SYSTEM.

FUEL DELIVERY

Note. For fuel system pressure testing, see the BASIC TESTING - 5.7L article.

Fuel Pressure Regulator (CSI)

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 (fuel pump 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 FUEL PUMP RELAY under MODULES, MOTORS, RELAYS & SOLENOIDS .

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 CONTROL

Note. To check Central Sequential Port Injectors (CSI), see Central Sequential Port Injection Balance Test (CSI) and Injector Coil Test (CSI).

See ENGINE SENSORS & SWITCHES .

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

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" to "B" and "C" to "D". (Scheme 26) 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 bidirectional scan tool capable of cycling PCM/VCM output devices or a special IAC Driver and Noid Light Set (222L or J-37027). Text in TESTS W/CODES - 5.7L article may refer to Tech 1 tester, General Motor's bidirectional scan tool.

Scheme 26

ENHANCED IGNITION SYSTEM CHECK

Note. Before clearing DTCs, use scan tool Capture Info to save Freeze Frame and Failure Records for reference. Control module's data is deleted once Clear Info function is used.

1. Perform Powertrain On-Board Diagnostic (OBD) System Check.
2. Check spark plug wires for open circuits, cracks or improper seating of terminals at spark plugs, distributor and ignition coil before proceeding with test. Check spark at plug with J-26792 spark tester or equivalent while cranking. (If there is no spark on one wire, check a second wire). A few sparks then nothing is considered no spark. If adequate spark is present, go to HARD START diagnosis. If adequate spark is not present, go to step 3).
3. Remove coil wire from distributor cap. Insert J-26792 spark tester into the coil wire and clamp the tester onto a ground. Crank engine. If adequate spark is present, go to step 13). If adequate spark is not present, go to step 4).
4. Measure coil wire resistance using a DVOM. The resistance should be approximately 1000 ohms/inch. If resistance is within specified value, go to step 5). If resistance is not within specified value, go to step 32).
5. Disconnect ignition coil harness connector. Probe ignition coil connector terminal C with a test lamp connected to battery positive terminal. Crank engine. If test lamp is flashing while cranking engine, go to step 7). If test lamp is not flashing while cranking engine, go to step 6).
6. Turn OFF ignition. Probe ignition coil harness connector terminal C with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 28). If not, go to step 9).
7. Probe ignition coil harness connector terminal B with a test lamp connected to battery positive. If test lamp is illuminated, go to step 29). If not, go to step 8).
8. Ignition ON, engine OFF. Measure terminal A voltage with a DVOM connected to a ground. If DVOM measures above 10V, go to step 21). If not, go to step 18).
9. Turn OFF ignition. Disconnect ignition control module harness connector. Check for an open circuit between ignition coil harness connector terminal C and ignition control module harness connector terminal D. If circuit is open, go to step 27). If circuit is not open, go to next step.
10. Disconnect ignition control module harness connector. Ignition ON, engine OFF. Measure voltage on terminal A at ignition control module harness connector with DVOM connected to ground. If DVOM measures above 10V, go to next step. If not, go to step 19).
11. Probe ignition control module hamess connector terminal C with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to next step. If not, go to step 23).
12. Disconnect ignition control module harness connector. Probe ignition control module harness connector terminal B with DVOM set to AC scale connected to ground. Crank engine. Observe voltage while engine is being cranked. Voltage should be between 1-4 VAC. If voltage is as specified, go to step 20). If voltage is not as specified, go to step 15).
13. Remove distributor cap. Check cap for cracks, moisture, carbon tracks or physical damage. If any of these conditions are present, go to step 34). If cap is okay, go to next step.
14. Crank engine. Observe distributor rotor while engine is being cranked. If rotor did not turn, refer to appropriate REMOVE/INSTALL/OVERHAUL article. If rotor did turn, go to step 33).
15. Turn OFF ignition. Disconnect VCM C3 connector. Check ignition timing signal circuit for an open between the VCM C3 harness connector and ignition control module harness connector terminal B. If circuit is open, go to step 24). If circuit is okay, go to next step.
16. Probe ignition control (IC) circuit at VCM C3 connector with a test lamp connected to battery positive terminal. If test lamp is illuminated, go to step 25). If not, go to next step.
17. Ignition ON, engine OFF. Probe ignition control (IC) circuit at VCM C3 connector with a test lamp connected to ground. If test lamp is illuminated, go to step 26). If not, go to step 22).
18. Check for an open or shorted to ground ignition positive voltage circuit at terminal A of ignition coil. Repair as necessary. Go to step 37).
19. Check for an open or shorted to ground ignition positive voltage circuit at terminal A of the ignition control module. Repair as necessary. Go to step 37).
20. Check for poor ignition control module connection. If problem was found, go to step 30). If problem was not found, go to step 35).
21. Check for a poor coil connection. If problem was found, go to step 30). If problem was not found, go to step 31).
22. Check for a poor VCM connection. If problem was found, go to step 30). If problem was not found, go to step 36).
23. Repair open ignition control module ground circuit. Go to step 37).
24. Repair open IC circuit between VCM and ignition control module. Go to step 37).
25. Repair grounded IC circuit between VCM and ignition control module. Go to step 37).
26. Repair short to voltage in the IC circuit between the VCM and the ignition control module. Go to step 37).
27. Repair open in tachometer signal circuit between ignition coil and ignition control module. Go to step 37).
28. Repair short to ground in tachometer signal circuit between ignition coil and ignition control module. Go to step 37).
29. Repair short to ground in tachometer input circuit between ignition coil and instrument cluster. Go to step 37).
30. Repair circuit as necessary. Go to step 37).
31. Replace ignition coil. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37).
32. Replace coil wire. Go to step 37).
33. Replace distributor rotor. Go to step 37).
34. Replace distributor cap. Go to step 37).
35. Replace ignition control module. Refer to appropriate REMOVE/INSTALL/OVERHAUL article. Go to step 37).
36. Replace VCM. Go to step 37).
37. Operate vehicle within conditions under which original symptom was noted. If system is not functioning properly, repeat testing from step 1).

Ignition Control Circuit

An open or short to ground in Ignition Control (IC) or by-pass circuit will cause PCM/VCM to turn on Malfunction Indicator Light (MIL) and confirm fault by setting a related trouble code. Refer to the TESTS W/CODES - 5.7L article.

Knock Sensor Circuit (Models Using External Spark Controller Module)

1. An open or short circuit on IC module control wire to PCM/VCM will cause a loss of 12-volt IC module signal. This will cause PCM/VCM to fully retard ignition timing.
2. If a scan tool 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 tool.
3. If a scan tool is not available, backprobe 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 PCM/VCM terminal should drop to zero volts, and return to original voltage when knock signal ceases.
4. If voltage signal does not respond as described, check knock sensor-to-module-signal. On vehicles equipped with automatic transmission, it may be necessary to place transmission in Drive for timing change to occur. See KNOCK SENSOR under «ENGINE SENSORS & SWITCHES»(ref-20719-S02403356452001010300000) .

Knock Sensor Circuit (Models Using Knock Sensor With Internal Spark Controller Module)

1. An open or short circuit on knock sensor wire to PCM/VCM will set a related trouble code. A false detonation signal will not cause PCM/VCM to set a code.
2. If a scan tool 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 tool.
3. If a scan tool 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 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 under «ENGINE SENSORS & SWITCHES»(ref-20719-S02403356452001010300000) .

Linear EGR Valve (Digital Valve)

1. Install scan tool. Ensure transmission range switch is operating properly. See «ENGINE SENSORS & SWITCHES»(ref-20719-S02403356452001010300000) . With engine at normal operating temperature, command EGR pintle position to zero percent. Increase engine speed to 2000 RPM. If scan tool reads actual EGR pintle position at greater than 3 percent, EGR valve is stuck open. Replace EGR valve.
2. If scan tool 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 percent, 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 PCM/VCM. See «CONTROL UNIT»(ref-20719-S11480392542001010300000) under COMPUTERIZED ENGINE CONTROLS. If circuits are okay, replace EGR valve.

1. Set parking brake and block drive wheels. Warm engine to normal operating temperature. Ensure engine coolant temperature is greater than 113°F (45°C). Check vacuum source to solenoid. Repair if vacuum is not present. Disconnect purge outlet hose from solenoid and connect vacuum gauge. Connect scan tool and select EVAP PURGE SOLENOID CONTROL.
2. Increase engine speed to 2000 RPM. Using scan tool, command evap purge solenoid ON. Vacuum should pass through solenoid when solenoid is energized. If vacuum passes through solenoid, check purge vacuum hose to canister. Repair as necessary. If vacuum is not present, go to next step.
3. Disconnect purge solenoid harness connector. Connect test light between canister purge control terminal and ignition feed circuit harness connector terminal (Pink wire). Command evap purge solenoid on. If test light illuminates, check for poor connection at purge solenoid or for faulty purge solenoid. If test light does not illuminate, go to next step.
4. Connect test light to an alternate ground and probe ignition feed circuit harness connector terminal (Pink wire). If test light illuminates, go to next step. If test light does not illuminate, repair open or short in ignition feed circuit. NOTE: When replacing PCM/VCM, new PCM/VCM needs to be programmed when installed.
5. Connect test light to battery positive. Backprobe canister purge control circuit PCM/VCM harness connector (Dark Green/White wire). Using scan tool, command evap purge solenoid ON. If test light illuminates, repair open in PCM/VCM harness connector circuit. If test light does not illuminate, check for poor connection at PCM/VCM connector or check for faulty PCM/VCM.

Required Service

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

1. Rough Idle
2. Stalling or Low 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 for 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.

MISCELLANEOUS PCM/VCM CONTROLS

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

TRANSMISSION

Note. To perform the following tests, use appropriate wiring diagram in the WIRING DIAGRAMS article.

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. 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 PCM/VCM. If harness is okay, see CONVERTER LOCK-UP SIGNAL FROM PCM/VCM.

Converter Lock-Up Signal From 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. Accelerate vehicle to 45 MPH and note test light. If test light does not illuminate, problem is a faulty PCM/VCM connector or PCM/VCM.

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 PCM/VCM or for bad PCM/VCM.
3. Turn ignition on, with engine off, ground test terminal No. 2 (Purple wire) of Data Link Connector (DLC). Malfunction Indicator Light (MIL) should flash and shift light should come on. If MIL illuminates, go to next step. If MIL does not flash, perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in the «BASIC TESTING - 5.7L»(ref-20660) article.
4. If shift light does not come on, ground Tan/Black light driver wire at PCM/VCM terminal using a jumper wire. For circuit identification, see SHIFT LIGHT CIRCUIT IDENTIFICATION table. (Scheme 27)
5. If shift light still does not come on, check for blown GAGES fuse, blown bulb or open circuit between fuse and PCM/VCM. If shift light illuminates when grounding PCM/VCM terminal with a jumper wire, problem is a bad PCM/VCM connection or bad PCM/VCM.

SHIFT LIGHT CIRCUIT IDENTIFICATION

Scheme 27

A/C COMPRESSOR CLUTCH CONTROLS

The A/C compressor clutch relay is controlled by the PCM. The PCM improves idle quality by delaying A/C compressor clutch engagement until idle speed is increased, or disengages A/C compressor clutch when idle speed is too low. A/C compressor clutch is cycled by PCM. PCM smooths cycling of A/C compressor clutch by adding fuel the instant A/C compressor clutch is applied.

Note. See the WIRING DIAGRAMS article for component location, terminal and wire color identification.

COOLING FAN

The electric cooling fan is used for radiator and A/C condenser cooling. Cooling fan operates when A/C is on and when engine coolant temperature exceeds a specific value. One or more cooling fan relays may be used. Some RWD trucks, vans and motorhomes are equipped with an auxiliary electric cooling fan. The auxiliary electric cooling fan is not controlled by Powertrain Control Module (PCM).

To help save diagnostic time, ALWAYS check for blown fuses or fusible links before proceeding with any testing. If fuses are blown, locate and repair short circuit before replacing fuses. Ensure all related relay and wire harness connections are clean and tight. Repair as necessary. See WIRING DIAGRAM for component, terminal and wire color identification.

SCAN TESTER

A variety of information is transmitted through Data Link Connector (DLC). This data is transmitted at a high frequency which requires a Tech 1 Scan Tester (94-00101A), appropriate cartridge kit and vehicle interface module kit, or other scan tester for interpretation. Several scan testers (scan tools) are available for diagnostic work. Scan testers other than Tech 1 scan tester will function and provide information for diagnostic work.

COOLING FAN RELAY LOCATION