Engine Controls - System/component Tests GMC Jimmy Typhoon

MODEL IDENTIFICATION

Vehicle model can be 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

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 appropriate G - 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 article in this section 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 AA - USING THIS SECTION article in this section.

GENERAL MOTORS A & C CHART REFERENCE

TURBOCHARGER (4.3L PFI)

Note. No turbocharger testing information is available from manufacturer. For symptom diagnosis see TESTS W/O CODES article.

Charge Air Cooler Pump

Remove charge air cooler radiator and charge air cooler pump. See REMOVAL, OVERHAUL & INSTALLATION article. Apply battery voltage and ground to charge air cooler pump electrical terminals. If pump does not activate, replace pump.

Wastegate Diaphragm

Disconnect hose from wastegate diaphragm (actuator). Connect a radiator pressure tester to wastegate diaphragm fitting. Actuator should start to move when approximately 5 lbs. of pressure have been applied to diaphragm and reach full travel at 15 lbs. of pressure. Internal diaphragm return spring should return diaphragm when pressure bleeds off.

Wastegate Solenoid

Disconnect electrical connector from solenoid. Apply battery voltage and ground to solenoid electrical terminals. With solenoid energized, apply vacuum to one solenoid vacuum fitting with a hand-held vacuum pump. Vacuum should hold only while solenoid is energized.

CONTROL UNIT

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

Ground Circuits

1. Using an ohmmeter, check for continuity to ground on ECM 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 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 terminal and power source.
2. Turn ignition switch on. Using a voltmeter, check for battery voltage between ECM 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 terminal. If okay, check for a defective ignition switch.
3. Connect voltmeter between ground and ECM 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 the START position.
4. If voltage is not present, check CRANK fuse or fusible link between ignition switch and ECM terminal. If fuse or fusible link is okay, check for an open in wire between ignition switch and ECM terminal, or check for a defective ignition switch.

Quad-Driver Check (Except 3.1L & 4.3L Turbo)

1. Remove ECM from vehicle. Using DVOM on 100/200 k/ohm scale, measure resistance between ECM case and each ECM quad-driver terminal. Touch negative DVOM lead to case and positive lead to ECM terminal. See appropriate ECM QUAD-DRIVER TERMINAL IDENTIFICATION table.
2. Each terminal should have at least 50 k/ohms resistance. If all quad-driver terminals have greater than 50 k/ohms resistance, go to step 4). If any terminal has less than 50 k/ohms resistance, locate driven component(s) for the quad-driver with the lowest resistance. Disconnect component from circuit and check circuit for short to voltage.
3. If circuit is not shorted to voltage, replace component before replacing ECM to prevent recurring ECM failure. See appropriate ECM QUAD-DRIVER TERMINAL IDENTIFICATION table.
4. Using a fused ammeter capable of measuring at least 2 amps, turn ignition on with engine off. Connect one lead of ohmmeter to chassis ground. Connect remaining lead to each ECM harness quad-driver circuit. Measure each circuit for sustained current flow for at least 2 minutes. NOTE: TCC solenoid cannot be easily tested for current draw, since completed circuit depends upon internal transmission oil pressure switches.
5. If no circuit has more than .75 amp sustained current flow, replace ECM. If any circuit has more than .75 amp sustained current flow, check for short to voltage in that circuit. If short to voltage is not present, replace related solenoid or relay.

ECM QUAD-DRIVER TERMINAL IDENTIFICATION (2.5L)

ECM QUAD-DRIVER TERMINAL ID (2.8L W/O 4L80-E TRANS)

ECM QUAD-DRIVER TERMINAL ID (3.8L W/ 4T60-3 TRANS)

ECM QUAD-DRIVER TERMINAL ID (4.3L CPI W/O 4L80-E TRANS)

ECM QUAD-DRIVER TERMINAL ID (4.3L TBI & V8 W/O 4L80-E TRANS)

ECM QUAD-DRIVER TERMINAL ID (ALL WITH 4L80-E TRANS)

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 (Dark Green/White on "L", "M" & "U" Series) and ground. For wiring schematics, see mini-schematics in A/C CLUTCH under «MISCELLANEOUS ECM CONTROLS»(/gmc/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__miscellaneous-ecm-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 was 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.

COOLANT TEMPERATURE SENSOR (CTS)

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

CTS RESISTANCE VALUES

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 in this article and appropriate G - TESTS W/ CODES article.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR

1. A malfunction in the MAP sensor circuit should set a related code in ECM memory. (Scheme 18) If a code is present, see appropriate G - 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 tester connected to the ALDL connector. Voltage should be as specified in «MAP SENSOR VOLTAGE RANGE»(/gmc/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests). 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.
3. 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

Scheme 18

MANIFOLD AIR TEMPERATURE (MAT) SENSOR

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

MAT SENSOR RESISTANCE

OXYGEN (O2) SENSOR

1. Start engine and warm to operating temperature. Disconnect oxygen sensor. Connect a DVOM between Purple lead of oxygen sensor and ground. Place meter on the 2-volt scale.
2. Using another DVOM on the 20-volt scale, connect voltmeter in series between Purple wire from the ECM and the positive post of battery. This will simulate a rich condition, causing ECM to respond by leaning mixture. Reading on voltmeter connected to oxygen sensor should decrease to less than .3 volt.
3. Move voltmeter lead from battery positive post to battery negative post. This will simulate a lean condition, causing ECM to respond by richening mixture. Reading on voltmeter connected to oxygen sensor should increase to greater than .8 volt. If reading does not change as specified, replace O2 sensor.
4. If a second DVOM is not available, install a jumper in Purple wire from the ECM. 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 appropriate G - TESTS W/ CODES article.

OXYGEN SENSOR HEATING ELEMENT (4.3L TURBO PFI)

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.

PARK/NEUTRAL (P/N) SWITCH

Disconnect P/N switch (located on transmission) harness connector. Connect ohmmeter between P/N switch terminals. (Scheme 19) Continuity should be present only when gear shift selector is in Park or Neutral. If continuity is not present, check P/N switch adjustment or replace defective P/N switch.

Scheme 19

POWER STEERING PRESSURE SWITCH (2.5L)

Disconnect P/S pressure switch harness connector. Connect ohmmeter between P/S pressure switch terminals. Start engine. With no load on power steering, continuity should not be present. Turn steering wheel to full stop, continuity should now be present. If readings are not as specified, replace defective P/S pressure switch.

THROTTLE POSITION SENSOR (TPS)

1. Install jumper wires to enable connection of a DVOM in parallel between TPS harness connectors. Connect DVOM positive lead to Dark Blue wire terminal. Connect negative lead to Black wire terminal. (Scheme 20)
2. Turn ignition on, engine off. 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, adjust or replace TPS. See ADJUSTMENTS article.
3. A malfunction in the TPS circuit should set a related trouble code. For further information, see appropriate G - TESTS W/ CODES article. Also see appropriate TPS ADJUSTMENT in ADJUSTMENTS article.

Scheme 20

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 be varying from 0.1-0.5 volt AC as the wheel is turned. If reading is not as specified, replace vehicle speed sensor. If a code is set, refer to appropriate G - TESTS W/ CODES article.

A/C Relays

See MISCELLANEOUS ECM CONTROLS.

Cold Advance Relay (6.2L Diesel)

See DIESEL COLD ADVANCE SYSTEM CHECK under FUEL SYSTEM (DIESEL).

Fuel Pump Relay

1. If a prolonged crank is required to start vehicle, fuel pump relay may be faulty. To verify this, 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. (Scheme 21) Apply battery voltage and ground to fuel pump relay winding terminals. To identify fuel pump relay terminals, see appropriate WIRING DIAGRAMS article in this section.
3. Using an ohmmeter, check continuity between fuel pump relay power and fuel pump relay drive 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/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests).

Scheme 21

SOLENOIDS

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

Canister Purge Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS.

Electronic Air Control (EAC) Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS.

EGR Control Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS.

Torque Converter Clutch (TCC) Solenoid

See MISCELLANEOUS ECM CONTROLS.

Wastegate Solenoid (4.3L Turbo PFI)

See AIR INDUCTION SYSTEMS.

Charge Air Cooler Pump (4.3L Turbo)

See AIR INDUCTION SYSTEMS.

Idle Air Control (IAC) Motor

See IDLE CONTROL SYSTEM under FUEL SYSTEM (GASOLINE).

FUEL DELIVERY

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

Fuel Pressure Regulator (PFI)

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 occurred, replace fuel pressure regulator.

Fuel Pressure Regulator (TBI)

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 table.

FUEL INJECTOR RESISTANCE

Oxygen Sensor

See ENGINE SENSORS & SWITCHES.

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

PFI Injector Balance Test (C-2)

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 down to avoid irregular readings due to "hot soak" fuel boiling. To prevent flooding, the PFI 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 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.
7. If injectors are all okay, plug in harness connectors and review SYMPTOMS in TESTS W/O CODES article.

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" to "B" and "C" to "D". (Scheme 22) Resistance should be 40-80 ohms. If okay, go to next step. If resistance is not as specified, replace IAC motor.
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 motor

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

Scheme 22

DESCRIPTION

The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards it's seat, reducing bypass air flow. A Tech 1 "Scan" tool will read the ECM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher idle). Lower counts indicate less air is allowed to bypass (lower idle).

TEST DESCRIPTION

Number(s) below refer to circled number(s) on the diagnostic chart.

1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
2. This step checks the quality of the IAC movement in Step 1). Between 700 RPM and about 1500 RPM the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash Red and Green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash Red and Green, check the circuits for faults, beginning with poor terminal contacts.

IAC VALVE RESET PROCEDURE

Ignition "OFF" for 10 seconds. Start and run engine for 5 seconds. Ignition "OFF" for 10 seconds.

IDLE AIR CONTROL SYSTEM CHECK CHART

Note. If a repair has been made refer to IAC RESET PROCEDURE before retesting.

Idle Air Control System Check Chart. Scheme 23

Diagnostic Aids

A slow, unstable, or fast idle may be caused by a non-lAC system problem that cannot be overcome by the IAC valve. Out of control range, IAC Tech I "Scan" tool counts will be above 60 if idle is too low, and zero counts if idle is too high The following checks should be made to repair a non-lAC system problem.

Vacuum Leak (High Idle)

If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including PCV system. Also, check for binding of throttle blade or linkage.

System Too Rich (Low Air/Fuel Ratio)

The idle speed will be too low. Tech 1 "Scan" tool IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust.

1. Tech 1 "Scan" tool O2 voltage will be fixed above 800 mV (.8 volt).
2. Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensors "Scan" voltage will be slow to respond.

Throttle Body

Remove IAC valve and inspect bore for foreign material.

IAC Valve Electrical Connections

IAC valve connections should be carefully checked for proper contact.

PCV Valve

An incorrect or faulty PCV valve may result in an incorrect idle speed. Refer to BASIC TESTING article in this section section.

A/C Compressor Or Relay Failure

Refer to A/C diagnosis if circuit is shorted to ground. If the relay is faulty, idle problem may exist. Refer to BASIC TESTING article in this section section.

Note. If intermittent poor drivability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance or replace IAC.

FUEL SYSTEM (DIESEL)

Note. The 6.2L light duty emissions engine (LH6) uses the Diesel Electronic Control (DEC) system. For complete system testing and diagnosis of DEC system, see G - DIESEL - TESTS W/ CODES article.

NORMAL GLOW PLUG CIRCUIT OPERATION

A normal functioning system should operate as follows

1. With ignition switch in the ON position, engine not running and at room temperature, glow plugs are on for 4-6 seconds, then off for about 4.5 seconds. Following this on-and-off cycle, glow plugs are on for about 1.5 seconds and off for about 4.5 seconds. The glow plugs continue to cycle on and off in this time sequence for about 20 seconds.
2. If engine is cranked during or after the above sequence, glow plugs will cycle on and off for a total duration of 25 seconds after the ignition switch is returned from the crank position, whether engine starts or not. Engine does not have to be running to terminate glow plug cycling.

Note. Glow plug on-and-off times vary with engine temperature, system voltage and/or ambient temperature. Lower temperatures cause longer duration of cycling.

PRELIMINARY GLOW PLUG DIAGNOSIS

1. If system does not operate as described in NORMAL GLOW PLUG CIRCUIT OPERATION, ensure glow plug system is correctly installed. Ensure all connectors are properly attached, clean and tight. Inspect engine harness ground connection. Ensure nut securing 4-wire connector at controller is tightened to 96 INCH lbs. (11 N.m), with connector fully seated and latched.
2. Ensure controller copper stud upper nuts are tightened to 96 INCH lbs (11 N.m). DO NOT tighten lower nuts. Ensure temperature switch connector in water crossover near front of engine is tightened to 48 INCH lbs. (5.4 N.m). Inspect WAIT light on instrument panel for tight connection and operation.

INHIBIT SWITCH (BLACK CAP)

1. Remove connector from inhibit switch when engine temperature is less than 100°F (38°C). Inhibit switch is located in water crossover near front of engine. Set ohmmeter on low range. Test continuity across switch terminals. Switch should be closed (a reading of less than 1.0 ohm on meter.)
2. Test terminals for continuity to ground with a test light or ohmmeter on high scale. Light should be off. Meter should show greater than 1.0 megohm. Replace switch if results are not as specified.
3. Disconnect plug from switch terminals when engine temperature is greater than 125°F (52°C). Change ohmmeter setting to highest scale or use a self-powered test light. Test continuity across switch terminals. Test continuity from each terminal to ground. Switch should be open (test light off or high ohm reading of greater than 1.0 megohm on meter). Replace switch if it is closed. When installing replacement switch, tighten to 17 ft. lbs. (23 N.m).

Electrical Check

Turn ignition off. Disconnect all glow plug connectors. Using an ohmmeter, check resistance between each glow plug terminal and ground. Glow plug resistance should be greater than 2 ohms. Replace glow plug if resistance is not as specified.

NO START - COLD

Perform these diagnostic procedures if engine does not start when cold; GLOW PLUG light may or may not come on. Before proceeding, check fuel system to ensure it is okay. Ensure battery voltage is 12.4 volts or more with turned ignition off. Ensure cranking speed is at least 100 RPM.

1. With ignition off, measure voltage at battery stud (single wire) on glow plug. (Scheme 24) If voltage is present, go to next step. If voltage is not present, repair battery-to-glow plug controller circuit.
2. If voltage was present in step 1), turn ignition off and measure voltage at glow plug feed stud (twin lead) on glow plug controller. If battery voltage is present, go to next step. If battery voltage is not present, relay contacts are shorted. Replace controller and all glow plugs.
3. If voltage is present in step 2), disconnect harness from all glow plugs. Using an ohmmeter, measure resistance between glow plug terminals and engine block. Replace glow plug if resistance is greater than 2 ohms. Reconnect all glow plugs before continuing with diagnosis.
4. With all glow plugs reconnected, place ignition switch in RUN position. Remove controller connector and check voltage at harness connector terminal "D". (Scheme 24)
5. If no voltage is present, repair open in ignition feed circuit to controller. If voltage is present, measure resistance between terminal "E" of connector and engine block (ground). If measurement is greater than one ohm, repair ground circuit to controller.
6. With ground circuit working properly in step 4), measure resistance between terminals "C" and "E" of connector. If reading is greater than 2 ohms, go to next step. If reading is less than 2 ohms, go to step 8).
7. If reading in step 6) is greater than 2 ohms, check for excessive resistance in voltage sense circuit to controller. Repair as necessary.
8. If reading in step 5) was less than 2 ohms, reconnect controller harness connector and ensure complete engagement. Connector locking latch should click over controller locking tab. With controller connector harness correctly connected, measure voltage at glow plug feed stud (twin lead) on glow plug controller, while turning ignition switch from OFF position to RUN position.
9. If no voltage is present, replace glow plug controller. If battery voltage is present, measure voltage at any one glow plug harness connector when turning ignition switch from OFF to RUN position. Test both right and left banks.
10. If no voltage was present in step 9), repair glow plug feed circuit to glow plugs. If battery voltage is present and instrument panel GLOW PLUG light does not come on, locate and repair bulb or circuit.

Scheme 24

DIESEL COLD ADVANCE SYSTEM CHECK

With ignition on voltage is supplied to the cold advance relay through the 20-amp GAGES fuse. When coolant temperature is less than 80°F (27°C), as sensed by the coolant temperature sensor, the ECM provides a ground at terminal C6. (Scheme 25) Grounding terminal C6 energizes the cold advance relay, closing the relay contacts and supplying 12 volts to the cold advance solenoid (in the injection pump), and the glow plug controller. The cold advance solenoid is now energized, causing injection pump timing to be advanced about 4 degrees.

Cold Advance Relay

1. Disconnect harness connector to cold advance relay. Using an ohmmeter, check resistance between relay terminals "D" and "F". (Scheme 25) Continuity should be present or a resistance of 20 ohms. If not, replace cold advance relay.
2. Using an ohmmeter, check continuity between relay terminals "A" and "E". Continuity should not be present. If continuity is present, replace grounded cold advance relay.

Scheme 25

GLOW PLUG CONTROLLER

1. Ensure coolant temperature is less than 80°F (27°C) before beginning test. Place ignition switch in RUN position and allow glow plugs to cycle. After 2 minutes, crank engine for one second. It is NOT important that the engine starts. Return ignition switch to RUN position. Glow plugs should cycle on at least once.
2. If glow plugs do not cycle, disconnect controller (controller is located on top, rear of valve cover). Connect a 12-volt test light between Purple/White wire terminal of harness connector (terminal "B") and ground. (Scheme 24) With ignition switch in RUN position, test light should be off. Test light should glow when engine is cranked.
3. If test light does not glow as specified, repair short or open in engine harness Purple/White wire. If test light operates properly but glow plugs did not cycle, replace controller.

INJECTION PUMP HOUSING LEAKAGE TEST

1. Remove injection pump and drain all fuel. Connect an air supply line to fuel inlet fitting. Ensure air supply is clean and dry. Seal off return line fitting. Completely immerse pump assembly in a container of clean test oil.
2. Apply 20 psi (1.4 kg/cm 2 ) to pump. Leave pump immersed for 10 minutes to allow trapped air to escape. Watch for leaks after 10 minutes. If no leaks are observed after 10 minutes, reduce air pressure to 2 psi (.14 kg/cm 2 ) for 30 seconds.
3. If there are still no leaks, increase pressure to 20 psi (1.4 kg/cm 2 ) again. If no leaks are observed, pump is ready for use. If leaks are noticed, pump must be replaced.

Test Preparation

1. Remove injector nozzles from engine. Clean carbon from tip area of nozzle with soft brass wire brush. DO NOT use steel brush or motorized brush to clean nozzle tip. Damage to nozzle tip may result. CAUTION: Use Nozzle Socket (J-36142) to remove and replace injection nozzle. Attach socket to 30mm hex portion of nozzle. Failure to do so will result in damage to the injector nozzle.
2. Connect injector nozzle to injection nozzle tester. Place clear plastic tubing on both fuel return fittings to prevent bleed-off from being confused with leaks. Close tester shutoff valve to pressure gauge.
3. Fill tester with test fluid. Fill and flush nozzle assembly with test fluid by operating tester lever briskly and repeatedly. This purges air from injector nozzle and coats all parts with test fluid.

Opening Pressure Test

1. Open tester shutoff valve 1/4 turn from closed position. Slowly depress tester lever and observe gauge. Note pressure at which needle of pressure gauge stops. Some injector nozzles may pop while other injector nozzles may drip (this is not a leak).
2. Injector opening pressure should not fall to less than 1500 psi (105 kg/cm 2 ). Replace any injector nozzle that does not meet lowest acceptable pressure. Release tester pressure.

Leakage Test

1. Open tester shutoff valve 3/4 to 1 3/4 turns from closed position. Blow dry injector nozzle tip. Slowly depress tester lever until pressure gauge reads 1400 psi (98 kg/cm 2 ). Maintain pressure for 10 seconds and observe injector nozzle tip.
2. A drop may form on end of injector nozzle, but should not fall off within a 10 second period. Replace injector nozzle if a drop of test fluid falls from tip during the 10 second period. (Scheme 26) Release tester pressure.

Scheme 26

Chatter Test

Chatter for new and used injector nozzles may vary. With some used injector nozzles, chatter is difficult to detect during slow actuation of tester lever. Some injector nozzles may chatter louder than others. As long as there is chatter, the injector nozzle is acceptable.

1. Close tester shutoff lever at pressure gauge. Slowly depress lever and note whether chatter can be heard. If no chatter is heard, increase speed of lever movement until it reaches a point at which injector nozzle chatters.
2. At fast lever movement, injector nozzle may emit a "hissing" or "squealing" sound rather than normal chatter. This is acceptable. These sounds indicate that injector nozzle needle moves freely and injector nozzle seat, guide and pintle have no mechanical defects. Replace any injector nozzle assembly that does not chatter.

Spray Pattern Test

The injector nozzles used with this system have several features that make pattern testing difficult. These features include longer nozzle overlap, greater pintle-to-body clearances and an internal wave washer between injector nozzle nut and injector. Typical injector nozzle tester cannot deliver fuel with sufficient velocity to obtain proper spray patterns. DO NOT replace injector nozzle(s) based on spray pattern.

IGNITION SYSTEM

Note. For basic ignition system checks, see BASIC TESTING article.

C-4, Electronic Spark Timing (Gasoline Engines)

An open or short to ground in the Electronic Spark Timing (EST) or by-pass circuits will cause ECM to turn on SERVICE ENGINE SOON light and confirm fault causing Code 42 is present. Refer to G - GAS - TESTS W/ CODES article.

C-5, Electronic Spark Control (ESC) Circuit (Gas Engines Except 2.5L, 4.3L Turbo & Vehicles With 4L80-E Transmission)

1. An open or short circuit on ESC wire to ECM will cause a loss of 12-volt ESC controller signal. This will cause ECM to fully retard ignition timing.
2. If a scan tester is available, connect it to the ALDL connector. 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 ESC 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 terminal should drop to zero volts, and return when knock signal ceases.
4. If signal does not respond as described, check knock sensor signal 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/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__knock-sensor) under ENGINE SENSORS & SWITCHES.

C-5, Electronic Spark Control (ESC) Circuit (4.3L Turbo & Vehicles With 4L80-E Transmission)

1. An open or short circuit on the ESC wire to the ECM will set a related trouble code. A false detonation signal will not cause ECM to set a code.
2. If a scan tester is available, connect it to the ALDL connector. 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 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/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__knock-sensor) under ENGINE SENSORS & SWITCHES.

Air Pump

Accelerate engine to approximately 1500 RPM and observe airflow from hoses. If airflow increases as engine is accelerated, pump is working properly. If airflow does not increase, check system for plugged or restricted hoses, proper pump belt tension, leaky valves or defective air injection pump.

Check Valve

Allow engine to cool. Remove check valve from engine and blow through valve in direction of check valve flow (to cylinder head). Attempt to suck back. Replace valve if airflow is allowed against the direction of flow.

1. Turn ignition on, engine off. Disconnect EAC solenoid connector. Connect test light between solenoid connector terminals. (Scheme 27) If test light comes on, check for grounded wire from solenoid to ECM. If wire is not grounded, replace ECM. If test light does not turn on, go to next step.
2. Ground ALDL diagnostic connector. If test light comes on, check for faulty EAC solenoid connector. If connector is okay, replace EAC valve. If test light does not turn on, connect test light between harness terminal "A" and ground. (Scheme 27) If test light still did not turn on, check for open in fuse or wire to ignition.
3. If test light comes on, check for open in wire from solenoid to ECM. If wire is okay, check EAC solenoid resistance. If resistance is less than 20 ohms, replace EAC solenoid and perform ECM quad-driver check. See «CONTROL UNIT»(/gmc/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__control-unit) under COMPUTERIZED ENGINE CONTROLS. If EAC solenoid resistance is greater than 20 ohms, replace ECM.

Scheme 27

System Test

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 (4.3L CPI)

1. Install scan tester. Ensure P/N switch is operating properly. See «ENGINE SENSORS & SWITCHES»(/gmc/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) . With engine at normal operating temperature, command EGR pintle position to 0%. Increase engine speed to 2000 RPM. If scan tester reads actual EGR pintle position at greater than 3%, EGR valve is stuck open. Replace EGR valve.
2. If scan tester reads actual EGR pintle position at 3% or less, command a 25% position step increase (i.e. 0-25%, 25-50%, 50-75%, etc.). Observe MAP reading and actual EGR pintle position for 3 seconds. EGR should increase by about 25% position and MAP reading should also increase.
3. If actual EGR pintle position is stable and within +/-10% position 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%. 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. See «CONTROL UNIT»(/gmc/jimmy-typhoon/1992-1993/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__control-unit) under COMPUTERIZED ENGINE CONTROLS. If circuits are okay, replace EGR valve.

Vapor Canister Purge Valve ("P" Series & 4.3L CPI)

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 (C, G, K & M 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 (Except 2.5L)

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 operation sticky, restricted or incompletely seated. Replace if necessary.
4. An engine must be sealed for the PCV system to function as designed. If leakage, sludging or dilution of oil is noted and the 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.

Note. The 2.5L engine does not use a conventional PCV valve. Valve consists of a fixed restricted orifice. To test valve, simply check for presence of vacuum with crankcase vent tube inlet end removed from rocker cover and engine running.

Temperature Sensor Vacuum Motor Type (2.8L)

1. Air cleaner temperature should be less than 86°F (30°C). Place thermometer as close as possible to sensor inside air cleaner. Start and idle engine. Damper door should close off outside air immediately.
2. When damper door starts to open snorkel passage, remove air cleaner cover and read thermometer temperature. Thermometer should read about 131°F (55°C).
3. If damper door does not open to outside air at the specified temperature, check vacuum motor diaphragm. If okay, replace defective thermostatic air cleaner temperature sensor.

Vacuum Motor Diaphragm (2.8L)

1. Turn engine off. Disconnect vacuum hose to vacuum motor. Apply 7 in. Hg to vacuum motor. Damper door should close. If not, check if linkage is properly connected.
2. With vacuum still applied, trap vacuum in vacuum diaphragm motor by bending hose. Damper door should remain closed. If damper door does not remain closed, replace vacuum diaphragm motor assembly.

Damper Door Wax Pellet Check (Except 2.8L)

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.

Crankcase Depression Regulator (CDR)

To test CDR valve, connect one hose of a water manometer to engine oil dipstick tube. Leave other hose of manometer open to atmosphere. Install air cleaner and run engine. CDR valve specification is one inch of water pressure at idle to 3-4 inches at full load. Add amount of distance water travels down one side of gauge to distance water travels up other side of gauge to obtain reading.

EGR Valve Check

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

EXHAUST PRESSURE REGULATION (EPR) SYSTEM

Note. For testing of the EPR system, see appropriate chart in G - DIESEL - TESTS W/ CODES article.

VACUUM PUMP

Connect vacuum gauge to vacuum pump inlet (small fitting). DO NOT plug or disconnect outlet fitting. With engine idling, vacuum should be 18 in. Hg one minute after start. If not, check for belt slippage, vacuum leaks or other obvious defects. If no defects are present, replace vacuum pump.

MISCELLANEOUS ECM 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 "A" and "D". (Scheme 28)- (Scheme 31). Solenoid resistance should be greater than 20 ohms.

Scheme 28

Scheme 29

Scheme 30

Scheme 31

Scheme 32

Note. Some solenoids have an internal pressure switch in series with the 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 terminals "A" and "D" of converter clutch harness. (Scheme 28)- (Scheme 31). 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. If harness is okay, see CONVERTER LOCK-UP SIGNAL FROM ECM.

Converter Lock-Up Signal From ECM

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 28)- (Scheme 31). Accelerate vehicle to 45 MPH and note test light. If test light does not illuminate, problem is a faulty ECM connector or ECM. On some models, lock-up signal may be checked at ALDL connector terminal "F" instead of at ECM 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 light is on, check for a short to ground between the bulb and ECM or a for bad ECM.
3. With ignition on and engine off, ground test terminal "B" of ALDL connector. SERVICE ENGINE SOON light should start to flash and shift light should come on. If light comes on, go to next step. If SERVICE ENGINE SOON light does not flash, perform DIAGNOSTIC CIRCUIT CHECK as described in BASIC TESTING article.
4. If shift light does not come on, ground Tan/Black light driver wire at ECM terminal using a jumper wire. (Scheme 33) If light still does not come on, check for blown GAGES fuse, blown bulb or open circuit between fuse and ECM. If light comes on when grounding ECM terminal with a jumper wire, problem is a bad ECM connection or bad ECM.

Scheme 33

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

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

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 34)- (Scheme 37). 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 ELECTRIC COOLING FANS article in ENGINE COOLING section.

Cooling Fan Relay

1. Disconnect cooling fan relay harness connector. Using an ohmmeter, check continuity of relay winding. (Scheme 39)- (Scheme 43). 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 G - TESTS W/ CODES article.

CHART C-10, A/C CLUTCH CONTROL CKT DIAGNOSIS 2.5/2.8L (1 Of 2)

The ECM controls the A/C clutch engagement to improve idle quality and performance by delaying A/C clutch engagement until idle speed is increased, disengages clutch when idle speed is too low, and smooths cycling of compressor by providing additional fuel the instant A/C clutch is engaged.

Note. Test numbers refer to test numbers on diagnostic charts.

1. Checks for low A/C refrigerant charge, causing A/C clutch not to engage.
2. This and the following tests checks for faulty A/C control relay.

Scheme 34

Scheme 35

CHART C-10, A/C CLUTCH CONTROL CKT DIAGNOSIS 2.5/2.8L (2 Of 2)

The ECM controls the A/C clutch engagement to improve idle quality and performance by delaying clutch engagement until idle speed is increased, disengages 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 switch supplies battery voltage to clutch control relay terminal "B" and ECM terminal B8. After a delay of about .5 seconds, the ECM will ground ECM terminal A4, closing the control relay and engages A/C compressor clutch.

Note. Test numbers refer to test numbers on diagnostic charts.

1. 3) Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using "drivers". Each driver is a part of a group of 4, called quad-drivers. Failure of one driver can damage any other driver in the set. Solenoid and relay coil resistance should be greater than 20 ohms.

Check resistance of each ECM controlled relay or solenoid coil before replacing ECM. See CONTROL UNIT under COMPUTERIZED ENGINE CONTROLS. Replace any relay or solenoid with a resistance of less than 20 ohms.

Scheme 36

CHART C-10, A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS (FRONT A/C ONLY) 3.1L (1 of 2)

A/C clutch control circuit is ECM controlled to delay A/C clutch engagement approximately 8 seconds after engine is started. This allows the Idle Air Control Valve (IAC) to adjust engine RPM before A/C clutch engages. ECM also causes relay to disengage A/C clutch during Wide Open Throttle (WOT) operation or if engine is overheating.

A/C clutch control relay is energized when ECM provides a ground path for circuit No. 459. Low pressure switch will open if A/C pressure is less than 40 psi (2.8 kg/cm 2 ); high pressure switch will open if A/C pressure is approximately 440 psi (30.9 kg/cm 2 ).

Vehicles equipped with rear A/C option use a pressure cycling switch instead of a low pressure switch. A/C pressure cycling switch closes at pressure greater than 43-49 psi (3.0-3.4 kg/cm 2 ) and opens at pressure of less than 23-25 psi (1.6-1.8 kg/cm 2 ). An A/C refrigerant fan request switch opens to request fan(s) on, when A/C refrigerant pressure exceeds approximately 200 psi (14.1 kg/cm 2 ).

Note. Test numbers refer to test numbers on diagnostic charts.

1. ECM will only energize A/C relay when engine is running. This test determines if relay or circuit No. 459 is faulty.
2. A/C compressor clutch should apply if the following conditions are met: On front A/C option only, low and high pressure switch must be closed so A/C request (12 volts) will be present at ECM. On rear A/C option only, A/C mode is requested via high pressure and pressure cycling switches, 12 volts is supplied to relay coil through high pressure switch. As compressor reduces evaporator pressure to approximately 25 psi (1.8 kg/cm 2 ), pressure cycling switch will open and compressor clutch will disengage. As system equalizes and pressure cycling switch closes, compressor clutch will cycle on. This cycling continues and maintains evaporator discharge and temperature at approximately 33°F (1°C).
3. This step determines if signal is reaching ECM on circuit No. 66 from A/C control panel. Signal should only be present when A/C mode or defrost mode is selected.
4. With engine idling and A/C on, ECM should ground circuit No. 459, causing test light to illuminate.

If complaint was insufficient cooling, problem may be caused by inoperative cooling fan(s) or faulty A/C refrigerant fan request switch. Engine cooling fan(s) should turn on when A/C pressure exceeds a value to open switch, causing ECM to energize cooling fan relay(s). See CHART C-12 for cooling fan diagnosis. If fan operates correctly, ensure A/C system refrigerant charge is okay.

Scheme 37

Scheme 38

CHART C-10, A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS (REAR A/C ONLY) (2 OF 2) 3.1L

Note. Test number refers to test numbers on diagnostic charts.

1. Determines if signal is reaching ECM from A/C control panel. Signal should only be present when A/C mode or defrost mode is selected.

If complaint was insufficient cooling, problem may be caused by inoperative cooling fan(s) or A/C refrigerant fan request switch. Engine cooling fan(s) should turn on when A/C pressure exceeds a value to open switch, which causes ECM to energize cooling fan relay(s). See CHART C-12 for cooling fan diagnosis. If fan operates correctly, ensure A/C system refrigerant charge is okay.

Scheme 39

Scheme 40

CHART C-10, A/C CONTROL DIAGNOSIS (A/T) (1 OF 2) 4.3L TURBO PFI

ECM controls operation of A/C clutch to improve idle quality and performance by

1. Delaying A/C clutch application until idle air rate is increased.
2. Releasing A/C clutch when idle speed is too low.
3. Releasing A/C clutch at wide open throttle operation.
4. Smooths cycling of compressor by providing additional fuel the instant A/C clutch is applied.

Turning on A/C supplies battery voltage to clutch relay (circuit No. 59) and ECM terminal BC9. After a time delay of .5 seconds, the ECM grounds terminal GF1 (circuit No. 459), and closes control relay. A/C compressor clutch then engages.

Note. Test numbers refer to test numbers on diagnostic charts.

1. Checks for low refrigerant as cause for no A/C.
2. This step and the steps that follow checks for faulty A/C control relay. QUAD-DRIVER CHECK is located under COMPUTERIZED ENGINE CONTROLS.

Scheme 41

Scheme 42

CHART C-10, A/C CLUTCH CONTROL DIAGNOSIS (A/T) (2 OF 2) 4.3L TURBO PFI

Note. Test numbers refer to test numbers on diagnostic charts.

1. 3) Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using "drivers". Each driver is a part of a group of 4, called quad-drivers. Failure of one driver can damage any other driver in the set. Solenoid and relay coil resistance should be greater than 20 ohms.

Check resistance of each ECM controlled relay or solenoid before replacing ECM. See CONTROL UNIT under COMPUTERIZED ENGINE CONTROLS. Replace any relay or solenoid that measures less than 20 ohms.

Scheme 43

CHART C-12, COOLING FAN CIRCUIT DIAGNOSIS (1 OF 2) 3.1L

ECM controls electric cooling fan(s) based on inputs from coolant temperature sensor, A/C request, refrigerant fan request switch and vehicle speed. ECM energizes fan(s) by grounding circuits No. 335 and No. 473, which closes cooling fan relay(s). Battery voltage is supplied to cooling fan(s) when energized by the ECM. ECM grounds circuit No. 335 (puller fan) when coolant temperature is approximately 223°F (106°C) or when A/C has been requested and refrigerant fan request switch opens with high A/C pressure, approximately 200 psi (14.1 kg/cm 2 ). Pusher fan relay is grounded when coolant temperature is approximately 217°F (103°C) and/or puller fan is energized for A/C pressure.

Note. Test numbers refer to test numbers on diagnostic charts.

1. With ALDL diagnostic terminal grounded, cooling fan driver closes and energizes fan control relay.
2. If A/C fan request switch or circuit is open, cooling fan operates whenever A/C is operating.
3. With A/C requested and 45 seconds after request is removed, A/C refrigerant fan request switch should open when A/C high pressure exceeds approximately 200 psi (14.1 kg/cm 2 ). This signal should cause ECM to energize fan control relays(s).
4. This test checks if cooling puller and pusher fan relay(s) (circuits No. 335 and 473) are shorted to ground. This condition would cause fans to be energized continuously.

If an overheating condition is suspected, verify if it is due to actual boilover. If gauge or light indicates an overheat condition and boilover is not evident, inspect gauge/light circuit for malfunction.

If vehicle is overheating and gauge or light indicates so, but cooling fan is not operating and scan tester indicates normal readings, coolant temperature sensor is out of calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system.

Scheme 44

Scheme 45

CHART C-12, COOLING FAN CIRCUIT DIAGNOSIS (2 OF 2) 3.1L

Note. Test numbers refer to test numbers on diagnostic charts.

1. Battery voltage should be available at cooling fan relay terminals "A" and "D" when ignition switch is in the ON position.
2. This checks ECM's ability to ground circuits No. 335 and 473. SERVICE ENGINE SOON light should be flashing at this point. If SERVICE ENGINE SOON light is not flashing, refer to G - TESTS W/ CODES article.
3. This test checks if cooling puller and pusher fan relays (circuits No. 533 and 702) are open. Jumpering relay terminals "A" and "E" by-passes relay, which should cause fans to operate, if motors and wiring are okay.

Scheme 46

CHART C-12A, COOLING FAN CIRCUIT DIAGNOSIS (1 OF 4) 3.8L

Power for fan motor goes through a 40-amp fusible link to terminal "A" of fan relays. Fan relays are energized when a good ground circuit is completed through the Powertrain Control Module (PCM) quad-driver. PCM energizes puller relay through terminal "GC4", when coolant temperature reaches 212°F (100°C) or when A/C is requested. PCM energizes pusher relay if A/C refrigerant pressure reaches 210 psi (99°C) or coolant temperature reaches 226°F (108°C).

Note. Test numbers refer to test numbers on diagnostic charts.

1. Using Tech 1 miscellaneous tests, low speed fan control will cause PCM to ground circuit No. 335. Puller fan should operate.
2. Using Tech 1 miscellaneous tests, high speed fan allows control of circuit No. 473. Pusher fan should operate.
3. When jumpering coolant temperature sensor wires together, scan tester should display coolant temperature of 304°F (151°C). Both fans will operate when ignition is cycled from off to on.
4. Opening A/C pressure switch will cause pusher fan to operate.

An intermittent may be caused by a poor connection, rubbed-through wire insulation or a broken wire inside insulation. Check PCM harness connector for backed out terminals. Connect a DVOM between affected terminal and ground. Wiggle related wires and connectors while watching DVOM for voltage reading change. Ensure coolant temperature sensor is not mis-scaled. See Code 15 in G - TESTS W/ CODES article in this section. Ensure engine cooling system is operating normally.

Scheme 47

Scheme 48

CHART C-12B, COOLING FAN, ON AT ALL TIMES (2 OF 4) 3.8L

Note. Test numbers refer to test numbers on diagnostic charts.

1. Checks if circuit No. 335 is shorted to ground, keeping relay closed at all times.
2. Checks if circuit No. 473 is shorted to ground. An illuminated test light, indicates wire is shorted to ground.
3. If test light is off after disconnecting PCM, ensure circuit No. 335 is not shorted to B+. If circuit is not shorted to B+, PCM is shorted internally.
4. If test light is off after disconnecting PCM, ensure circuit No. 473 is not shorted to B+. If circuit is not shorted to B+, PCM is shorted internally.

Scheme 49

CHART C-12C, COOLING FAN, NO PULLER FAN (3 OF 4) 3.8L

Note. Test numbers refer to test numbers on diagnostic charts.

1. Checks for battery voltage at relay harness connector.
2. Jumpering cooling fan relay terminals "A" and "E" by-passes relay, causing fans to operate, if fan motors and wiring are okay.
3. Grounding test terminals should cause PCM to ground circuit No. 335. At this point, test light should illuminate, if PCM is good and circuit No. 335 is not open.
4. Checks for battery voltage and ground to fan motor. Test light illuminated at this point indicates a faulty fan motor connection or motor.

Scheme 50

CHART C-12D, COOLING FAN, NO PUSHER FAN (4 OF 4) 3.8L

Note. Test numbers refer to test numbers on diagnostic charts.

1. Test light should be illuminated because harness terminal "F" has battery voltage with ignition in the ON position.
2. Jumpering relay harness terminals "A" and "E" by-passes relay. If fan runs, relay is faulty.
3. Checks circuit No. 473 back to PCM. If circuit No. 473 is okay, relay is bad.
4. Checks wiring to cooling fan motor. If wiring is okay, problem is in the connections, motor or motor ground.

Scheme 51

Note. The following Schematic & Flow Chart are Courtesy of General Motors Corp.

Cooling Fan Circuit Schematic & Flow Chart (7.4L G, C & K Series With A/C). Scheme 52

DESCRIPTION & OPERATION

The tuning valve (rotary solenoid device) is electrically controlled by the PCM based on RPM and TP signal inputs, for accurate switching capability at optimum switching points to enable either a "split" or "single" plenum condition.

During the low and high end of the RPM range, the PCM de-energizes the valve and enables a "split" plenum condition that provides for peak torque along with increased peak horsepower.

During the mid RPM range, the PCM energizes the valve, thus enabling a "single" tuning plenum condition that provides for mid RPM range peak torque.

Removal

Disconnect electrical connector. Remove tuning valve attaching screws, tuning valve and O-ring seal.

Note. To avoid breaking tuning valve mounting ears, alternately tighten attaching screws until they engage mounting ear surface, then tighten each screw to specified torque.

Installation

Lubricate new intake manifold tuning valve O-ring seal with clean engine oil. Install intake manifold tuning valve and attaching screws. Tighten screws to 18 inch lbs. (2.0 N.m). Connect electrical connector.

An intermittent TPS output will cause the ECM to receive a false output.

Scheme 53

Scheme 54

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Scheme 73

Scheme 74

See also:
• MODEL IDENTIFICATION
• MISCELLANEOUS ECM CONTROLS
• DIESEL COLD ADVANCE SYSTEM CHECK
• KNOCK SENSOR
• CONTROL UNIT