Engine Control System & Engine Component Tests - 4.3l Vins [W] & [X] Chevrolet Blazer S15

MODEL IDENTIFICATION

Vehicle series 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 table.

MODEL IDENTIFICATION

INTRODUCTION

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

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

A & C CHART REFERENCE TABLE

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 this 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 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 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 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, 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.

A/C ON SWITCH/SYSTEM TEST

1. Turn ignition switch to RUN position. Move mode selector switch to any position other than OFF position. With A/C control assembly connected, measure voltage between mode selector switch Light Green wire and ground. For wiring schematics, see appropriate WIRING DIAGRAMS article.
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 mode 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 switch Dark Green/White wire or Dark Green 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 TESTS W/CODES 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 TESTS W/CODES 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 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 (C-4 & C-5)»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) under IGNITION SYSTEM and TESTS W/CODES 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 TESTS W/CODES 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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) table.
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 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 TESTS W/CODES 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 34)
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 article. Also see TP SENSOR ADJUSTMENT in ADJUSTMENTS article.

Scheme 34

TRANSMISSION RANGE SWITCH

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

VEHICLE SPEED SENSOR (PM GENERATOR)

Disconnect vehicle speed sensor harness connector (located in transmission). 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 article.

A/C Relays

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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x). Apply battery voltage and ground to fuel pump relay winding terminals (control and ground). (Scheme 35)
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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x).

Scheme 35

SOLENOIDS

Note. All 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 PCM/VCM CONTROLS .

FUEL DELIVERY

Note. For fuel system pressure testing, see BASIC TESTING 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 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 CONTROL

Note. To check Central Sequential Port Injectors (CSI), see tests A-7 and A-8.

See ENGINE SENSORS & SWITCHES .

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

A-7, Central Sequential Port Injector Balance Test (CSI)

Note. Central Sequential Port Injector Balance Test has been revised due to Technical Service Bulletin (TSB) 61-63-16, dated February, 1997. See SMU - SECTION 6E - REVISED 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. All injector should have the same pressure drop of 1.5 psi (.11 kg/cm 2 ).

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. Perform on-board diagnostics. Check for Diagnostic Trouble Code (DTC). If DTC(s) are present, diagnose DTC first. If DTC(s) are not present, go to next step.
2. Turn ignition off. Connect Fuel Pressure Gauge (J-34730-1A) to pressure tap. Unplug injector harness connector. Connect Adapter Harness Connector (J-39021-30) and Injector Switch Box (J-39021-210). Turn ignition on. Monitor fuel gauge. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm 2 ). If pressure if as specified, go to step 3). If pressure is not as specified, check fuel pump and fuel lines for restriction.
3. Run fuel pump for at least 2 seconds after ignition is turned on. Energize injectors one at a time. Note fuel pressure for each injector. Fuel pressure should drop about 1.5 psi (.11 kg/cm 2 ) on each injector. If fuel pressure drop is as specified, go to next step. If fuel pressure drop is not as specified, perform injector coil test. See A8, INJECTOR COIL TEST (CSI).
4. Remove upper manifold assembly. Connect Poppet Nozzle Tester (J-34730-230). Connect a fused jumper wire (10-amp) to fuel pump test connector. Energize each injector one at a time, fuel should spray from each poppet nozzle tester. If fuel did not spray from an injector, go to next step.
5. Replace faulty injector and poppet nozzle assembly. After replacing injector and poppet nozzle assembly, go to next step.
6. Install Tech 1 scan tool. Select DTC, CLEAR INFO. Start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and dies, repeat step 2).
7. Warm engine to normal operating temperature. Select DTC, FAILED THIS IGN. If additional DTC(s) are displayed, diagnose DTC. If DTC(s) are not present, go to next step.
8. Using scan tool, select CAPTURE INFO, REVIEW INFO. If additional DTC(s) are present, diagnose DTC. If DTC(s) are not present at this time, system is okay.

A-8, Injector Coil Test (CSI)

1. Perform on-board diagnostic test. Diagnose any Diagnostic Trouble Code (DTC) present. If DTC is not present, go to next step.
2. Turn ignition off. Relieve fuel pressure. Connect Fuel Injector Tester (J-39021) to battery positive. Connect Injector Switch Box (J-39021-210). Set amperage on fuel injector tester to 0.5-amp. Connect DVOM to fuel injector tester. Connect Tech 1 scan tool and monitor engine coolant temperature. If coolant temperature is 50-95°F (10-35°C), go to step 4). If coolant temperature is not as specified, go to next step.
3. Allow engine to warm or cool as necessary. If engine coolant temperature is now 50-95°F (10-35°C), go to next step.
4. Using fuel injector tester, press PUSH TO START TEST button and monitor DVOM reading. Perform this test to each injector. Voltage reading should be 5.44-7.53 volts. If voltage reading is as specified, go to next step. If voltage reading is not as specified, perform injector balance test. See A-7, CENTRAL SEQUENTIAL PORT INJECTOR BALANCE TEST (CSI).
5. Replace faulty injector nozzle and poppet assembly and then proceed to next step.
6. Using Tech 1 scan tool, select DTC, CLEAR INFO. Start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and dies, repeat step 2).
7. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN. If DTC(s) are present, diagnose DTC(s). If DTC(s) are not present, go to next step.
8. Using scan tool, select CAPTURE INFO, REVIEW INFO. If additional DTC(s) are present, diagnose DTC(s). If DTC(s) are not present, system is okay.

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" to "B" and "C" to "D". (Scheme 36) 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 articles may refer to Tech 1 tester, General Motors' bidirectional scan tool.

Scheme 36

CIRCUIT DESCRIPTION

System includes a distributor, ignition coil, ignition control module, secondary wires, spark plugs, VCM and crank sensor. Ignition system is controlled by Vehicle Control Module (VCM). The VCM monitors the information from various engine sensors, computes the desired spark timing and controls the dwell and firing of the ignition coil via an (IC) line to the ignition control module.

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).

Diagnostic Aids

Note. Numbers below refer to the step numbers in diagnostic test.

Note. Battery should be fully charged prior to any tests.

1. Perform the OBD system check before proceeding unless it was already performed.
2. Checks for proper output from the enhanced ignition system. The spark tester requires a minimum of 25,000 volts to operate. This check can be used in case of an ignition miss, because the system may provicle enough voltage to run the engine but not enough to operate a spark plug under heavy load.
3. This test separates distributor cap, rotor and ignition wires from ignition coil in order to help identify a secondary ignition system problem. NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 3) to step 5). No test procedures have been omitted.
4. This test checks ignition control module, connections, and wiring. NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 5) to step 12). No test procedures have been omitted.
5. This test begins to determine if the VCM is providing a signal to the ignition control module. If the VCM is not providing a signal to the ignition control module, the problem exists between the ignition control module and the VCM. NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 12) to step 14). No test procedures have been omitted.
6. This test checks for a basic engine mechanical problem.

C-4, 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 TESTS W/CODES article.

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

1. An open or short circuit on ESC wire to PCM/VCM will cause a loss of 12-volt ESC controller 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 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 under «ENGINE SENSORS & SWITCHES»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) .

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 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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) .

Linear EGR Valve (Digital Valve)

1. Install scan tool. Ensure P/N switch is operating properly. See «ENGINE SENSORS & SWITCHES»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) . 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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x__control-unit) under COMPUTERIZED ENGINE CONTROLS. If circuits are okay, replace EGR valve.

Enhanced EVAP Control System

A problem in the enhanced EVAP control system will set a Diagnostic Trouble Code (DTC). See TESTS W/CODES article.

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 1valve 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 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 37) Solenoid resistance should be greater than 20 ohms.

Scheme 37

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 37) 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. (Scheme 37) 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. On some models, lock-up signal may be checked at Data Link Connector (DLC) terminal "F" instead of at 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 PCM/VCM or for bad PCM/VCM.
3. Turn ignition on, with engine off, ground test terminal "B" 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 BASIC TESTING 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»(/chevrolet/blazer/s15-1994-1998/remont/testing-diagnostics/#engine-control-system-engine-component-tests-43l-vins-w-x) table. (Scheme 38)
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 38

Warning Light Diagnosis

1. Perform On-Board Diagnostics (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in BASIC TESTING article. After performing OBD system check, go to next step.
2. Check instrument cluster. See INSTRUMENT PANEL article in ACCESSORIES/SAFETY EQUIPMENT section. If instrument panel is okay, go to next step.
3. Turn ignition off. Disconnect PCM connector. Turn ignition on. Using a DVOM, check voltage between affected PCM output circuit at PCM harness connector terminal and ground. See appropriate WIRING DIAGRAMS article. Battery voltage should be present. If battery voltage is present, go to next step. If battery voltage was not present, go to step 7).
4. Set DVOM to 10-amp scale. Check current between affected PCM output circuit and ground. Monitor reading for about 2 minutes. DVOM should read .05-1.50 amps. If reading is as specified, go to step 12). If reading is not as specified, go to next step.
5. Disconnect instrument cluster, leaving PCM connector disconnected. Using DVOM, check voltage between affected PCM output circuit and ground. DVOM should read zero volts. If voltage reading is as specified, go to step 15). If voltage reading is not as specified, go to next step.
6. Locate and repair short to voltage in affected PCM output circuit. After repairs, go to step 17).
7. Check ignition feed fuse for instrument cluster indicator lights. If fuse is blown, go to next step. If fuse is okay, go to step 9)
8. Locate and repair short to ground in ignition feed circuit for instrument cluster indicator lights. Replace fuse and go to step 17).
9. Disconnect instrument cluster connector. Turn ignition on. Check voltage between ignition feed circuit for instrument cluster indicator lights and ground. Battery voltage should be present. If battery voltage is present, go to next step. If battery voltage was not present, go to step 14).
10. Check affected PCM output circuit for an open or shorted circuit to ground. If circuit is open or shorted, go to step 17). If circuit was okay, go to next step.
11. Check affected PCM output circuit and ignition feed circuit for poor connection at instrument cluster and at PCM. If problem is present, repair as necessary and then go to step 17). If problem was not found, go to step 15).
12. Turn ignition off. Reconnect PCM harness connector. Disconnect instrument cluster connector. Turn ignition on. Connect a test light between affected PCM output circuit and ignition feed circuit at instrument panel harness connector. Using a scan tool, perform OUTPUTS TEST function to cycle affected warning light on and off. If test light flashes on and off, check for shorted component or circuit in the output driver circuit. Check for faulty instrument cluster. If test light does not flash on and off, go to next step.
13. Check affected PCM output circuit for poor connection to PCM. Replace defective terminal and then go to step 17). If terminal is okay, go to step 16).
14. Repair open in ignition feed circuit to instrument cluster indicator lights. After repairs, go to step 17).
15. Replace instrument cluster and then go to step 17).
16. Replace PCM and then go to next step.
17. Using scan tool, operate affected warning light. If warning light does not operate properly, go to step 3).

Tachometer Control Circuit Diagnosis

1. Diagnose instrument cluster. See INSTRUMENT PANEL article in the ACCESSORIES/SAFETY EQUIPMENT section. After diagnosis, go to next step.
2. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in BASIC TESTING article. After performing OBD system check, go to next step.
3. Turn ignition off. Disconnect PCM connector. Turn ignition on. Using a DVOM, check voltage between tachometer control circuit at PCM harness connector terminal and ground. See appropriate WIRING DIAGRAMS article. Battery voltage should be present. If battery voltage is present, go to next step. If battery voltage was not present, go to step 7).
4. Set DVOM to 10-amp scale. Check current between tachometer control circuit and ground. Monitor reading for about 2 minutes. DVOM should read .05-1.50 amps. If reading is not as specified, go to next step. If reading is as specified, go to step 12).
5. Disconnect instrument cluster, leaving PCM connector disconnected. Using DVOM, check voltage between tachometer control circuit and ground. DVOM should read zero volts. If voltage reading is as specified, go to step 15). If voltage reading is not as specified, go to next step.
6. Locate and repair short to voltage in tachometer control circuit. After repairs, go to step 17).
7. Check ignition feed fuse for instrument cluster. If fuse is blown, go to next step. If fuse is okay, go to step 9).
8. Locate and repair short to ground in ignition feed circuit to instrument cluster. Replace fuse and go to step 17).
9. Disconnect instrument cluster connector. Turn ignition on. Check voltage between ignition feed circuit for instrument cluster and ground. Battery voltage should be present. If battery voltage was not present, go to step 14). If battery voltage is present, go to next step.
10. Check tachometer control circuit for an open or shorted circuit to ground. If circuit is open or shorted, go to step 17). If circuit was okay, go to next step.
11. Check tachometer control circuit and ignition feed circuit for poor connection at instrument cluster and at PCM. If problem is present, repair as necessary and then go to step 17). If problem was not found, go to step 15).
12. Turn ignition off. Reconnect PCM harness connector. Disconnect instrument cluster connector. Turn ignition on. Connect a test light between tachometer control circuit and ignition feed circuit at instrument panel harness connector. Using a scan tool, perform OUTPUTS TEST function to cycle tachometer control output light on and off. If test light flashes on and off, check for shorted component or circuit in the output driver circuit. Check for faulty instrument cluster. If test light does flash on and off, go to next step.
13. Check tachometer control circuit for poor connection to PCM. Replace defective terminal and then go to step 17). If terminal is okay, go to step 16).
14. Repair open in ignition feed circuit to instrument cluster. After repairs, go to step 17).
15. Replace instrument cluster and then go to step 17).
16. Replace PCM and then go to next step.
17. Start engine and observe tachometer. If tachometer does not operate properly, diagnose instrument cluster. See INSTRUMENT PANEL article in the ACCESSORIES/SAFETY EQUIPMENT section.

A/C CLUTCH (C-10)

Note. For a more specific system testing, refer to the following C-10 diagnostic tests. If any test other than a C-10 test is referenced, see TESTS W/CODES article.

Note. Tests requires use of Tech 1 Scan Tool (TK O-A) and Cartridge Kit (7000001), Tech 1 OBD II Vehicle Interface Module Kit (7000041). Other scan tools are available, refer to tester manufacturer for compatibility to vehicle system being tested.

Description

Vehicle Control Module (VCM) control of A/C clutch improves idle quality and performance by delaying clutch engagement until idle speed is increased, releasing clutch when idle speed is too low, and smooths cycling of compressor by providing additional fuel the instant clutch is applied.

Turning on A/C supplies battery voltage through pressure switches to VCM. When VCM receives voltage on A/C request signal, A/C enable relay circuit will be grounded. A/C compressor clutch will then engage.

Compressor Clutch Control Circuit Diagnosis

1. Before performing diagnosis ensure A/C system is adequately charged. If system is not adequately charged, evacuate and recharge system. If system is adequately charged, go to next step.
2. Start and run engine until is reaches normal operating temperature. Run engine at idle. Turn A/C on and off. If A/C clutch engages and disengages within 20 seconds, system is now operating properly. If A/C clutch does not engage and disengage within 20 seconds, go to next step.
3. Connect scan tool. Turn A/C on. Monitor A/C REQUEST in data stream. If display reads YES, go to next step. If display does not read YES, go to step 8).
4. Disconnect compressor clutch connector. Connect voltmeter between A/C clutch signal circuit (Dark Green wire) and ground circuit of A/C compressor clutch harness connector (Black wire). If battery voltage is present, go to next step. If battery voltage is not present, go to step 12).
5. Check for a faulty compressor clutch harness connector. If a repair was made, go to next step. If a repair was not made, go to step 7).
6. Repair A/C compressor clutch harness connector. Go to step 37).
7. Replace A/C compressor clutch. Go to step 37).
8. Turn ignition off. Disconnect Gray VCM connector. Turn ignition on. Connect voltmeter to VCM harness connector terminal GR25 (Dark Green/White wire) and ground. If battery voltage is present, go to next step. If battery voltage is not present, go to step 11).
9. Check for poor connection at Gray VCM harness connector. If a poor connection is present, go to next step. If a poor connection is not present, go to step 36).
10. Repair VCM harness connector. Go to step 37).
11. Repair open or short to ground in Dark Green/White wire to A/C pressure switch and/or A/C control switch. Go to step 37).
12. Connect voltmeter to A/C clutch signal circuit (Dark Green wire) of A/C clutch harness connector and engine ground. If battery voltage is present, go to next step. If battery voltage is not present, go to step 14).
13. Repair ground circuit of A/C clutch harness connector (Black wire). Go to step 37).
14. Install scan tool. Turn ignition on, engine off. Using OUTPUT TEST function of scan tool, command A/C relay on. If A/C relay clicks, go to step 23). If A/C relay does not click, go to next step.
15. Disconnect A/C relay. Connect voltmeter between terminal No. 85 (Pink wire) of relay harness connector and ground. If battery voltage is present, go to step 17). If battery voltage is not present, go to next step.
16. Repair open or short to ground in harness connector terminal No. 85 (Pink wire). Go to step 37).
17. Connect voltmeter between A/C relay harness connector terminals No. 85 (Pink wire) and 86 (Dark Green/White wire). Install scan tool. Using OUTPUT TEST function of scan tool, command A/C relay on. If battery voltage is present, go to next step. If battery voltage is not present, go to step 19).
18. Replace A/C relay. Go to step 37.
19. Check for a faulty connection at Gray VCM harness connector. If a repair was made, go to next step. If a repair was not made, go to step 21).
20. Repair Gray VCM connector. Go to step 37).
21. Check for an open circuit between A/C relay harness connector terminal No. 86 (Dark Green/White wire) and Gray VCM connector terminal RE9 (Dark Green/White wire). If a problem was found, go to next step. If a problem was not found, go to step 36).
22. Repair open in circuit between A/C relay harness connector terminal No. 86 (Dark Green/White wire) and Gray VCM connector terminal RE9 (Dark Green/White wire). Go to step 37).
23. Remove A/C relay. Connect voltmeter between A/C relay harness connector terminal No. 30 (Orange wire) and ground. If battery voltage is present, go to next step. If battery voltage is not present, go to step 29).
24. Reconnect A/C relay. Disconnect A/C high pressure switch. Connect voltmeter between A/C high pressure switch harness connector (Dark Green wire) and ground. If battery voltage is present, go to step 34). If battery voltage is not present, go to next step.
25. Check for an open in circuit between A/C relay harness connector terminal No. 87 (Dark Green wire) and A/C high pressure switch harness connector. If a problem was found, go to next step. If a problem was not found, go to step 27).
26. Repair open in circuit between A/C relay harness connector terminal No. 87 (Dark Green wire) and A/C high pressure switch harness connector. Go to step 37).
27. Replace A/C relay. Go to step 37).
28. Disconnect low pressure switch. Connect voltmeter between low pressure switch harness connector Light Green wire and ground. If about battery voltage is present, go to step 30). If about battery voltage is not present, go to next step.
29. Repair open circuit to low pressure switch harness connector Light Green wire. Go to step 37).
30. Connect a jumper wire between low pressure switch harness connector. Connect voltmeter between A/C relay harness connector terminal No. 30 (Orange wire) and ground. If battery voltage is present, go to next step. If battery voltage is not present, go to step 32).
31. Replace A/C low pressure switch. Go to step 37).
32. Repair open circuit between low pressure switch harness connector and A/C relay harness connector terminal No. 30 (Orange wire). Go to step 37).
33. Connect a jumper wire between high pressure switch connector. Check for battery voltage at A/C compressor clutch connector. If battery voltage is present, go to next step. If battery voltage is not present, go to step 35).
34. Replace high pressure switch. Go to step 37).
35. Repair open in A/C clutch signal circuit (Dark Green/White wire). Go to step 37).
36. Replace VCM. Go to step 37).
37. Using scan tool, select DTC CLEAR INFO. Start and idle engine until it reaches normal operating temperature. Select DTC SPECIFIC, then enter DTC number that was present. Operate vehicle within conditions that may have set this DTC in the first place. If DTC does not reset, go to next step. If DTC resets, go to step 2).
38. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any DTCs are displayed that have not been diagnosed, perform diagnosis for applicable DTC. See TESTS W/CODES article.

Scheme 39

See also:
• FUEL SYSTEM
• MISCELLANEOUS PCM/VCM CONTROLS
• CONTROL UNIT