Engine Controls - System/component Tests Buick Regal III

INTRODUCTION

Before testing separate components or systems, perform all procedures listed in BASIC TESTING article in this section. Since many computer-controlled and monitored components will set a trouble code if they malfunction, it is also recommended that self-diagnosis be performed. See appropriate G - TESTS W/ CODES article in this section.

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 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 the HOW TO USE THE ENGINE PERFORMANCE SECTION article in this section.

GENERAL MOTORS A & C CHART REFERENCE

Ground Circuits

1. Using an ohmmeter, check for continuity to ground on control unit ground terminals. Use appropriate wiring diagram at the end of this article to determine ECM ground terminals. Resistance to ground should be zero ohms. If reading is other than zero ohms, repair open to circuit 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 one volt or more, check for open, corroded or loose connection on ground lead.

Power Circuits

1. Using a voltmeter, check for battery voltage between control unit constant battery power terminals and ground. If battery voltage is not present, check control unit power supply fuse. If fuse is okay, check for open in power supply or control unit wiring.
2. Turn ignition switch to the ON position. Using a voltmeter, check for battery voltage between control unit ignition power terminals and ground. If battery voltage is not present, check power supply fuse(s). If fuse is okay, check for an open in wiring between fuse and control unit, or check for a defective ignition switch.
3. Connect voltmeter between ground and control unit start (crank) signal terminal. Turn ignition switch to the START position. Battery voltage should be present between control unit start terminal and ground ONLY when ignition switch is in the START position.
4. If voltage is not present, check fuse(s). If fuse is okay, check for an open in wiring between fuse and control unit, or check for a defective ignition switch.

ENGINE SENSORS & SWITCHES

Note. For additional sensor testing specifications, see SENSOR RANGE CHARTS article in this section.

A/C ON (A/C Request) Switch Test

1. Start engine and allow to idle. If a "Scan" tester is available, scan A/C request parameter. Move A/C mode selector back and forth between ON and OFF positions. Scan status should change.
2. If "Scan" tester is not available or scan status does not change, measure voltage between ground and ECM A/C request terminal. For wiring schematics, see mini-schematics in «A/C CLUTCH»(/buick/regal/iii-1988-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__ac-clutch-relay) under MISCELLANEOUS CONTROLS.
3. With A/C mode selector in the ON position, 12 volts should be present. If 12 volts are not present, check for open between A/C mode select switch and ECM A/C request terminal, low A/C refrigerant level causing low pressure switch to open, bad A/C fuse or bad A/C mode select switch.

A/C Pressure Sensor

A malfunction in A/C pressure sensor circuit will set a related trouble code. For testing procedures, see appropriate G - TESTS W/ CODES article in this section. For wiring schematics, see mini-schematics in A/C CLUTCH under MISCELLANEOUS ECM CONTROLS.

A/C Pressure Switch

1. Connect A/C pressure gauges to system and start engine. Note high and low pressure readings. If pressures are normal, go to step 2). If pressures are less than normal, check system for leaks. Evacuate and recharge as necessary. If pressures are high, check for system overcharge, overheating or mechanical failure in freon delivery system.
2. Disconnect high and low pressure switches. Jumper across each switch harness connector to allow A/C system to function normally. Using an ohmmeter, check continuity between pressure switch terminals.
3. Continuity should be present on both high and low switches (if equipped). If continuity is not present, replace A/C pressure switch. For wiring schematics, see mini-schematics in «A/C CLUTCH»(/buick/regal/iii-1988-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests__ac-clutch-relay) under MISCELLANEOUS CONTROLS.

Brake Switch

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

Camshaft Sensor (3.8L VIN L)

A malfunction in the camshaft sensor circuit will set a related trouble code. For testing procedures, see appropriate G - TESTS W/ CODES article in this section.

Coolant Temperature Sensor (CTS)

If a coolant sensor-related code is present, see appropriate G - TESTS W/ CODES article in this section. 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. See CTS RESISTANCE VALUES .

CTS RESISTANCE VALUES

Crankshaft Position Sensor (DIS)

1. If a "Scan" tester is available, scan RPM parameter while cranking engine. If RPM is indicated, crankshaft position sensor is operating properly.
2. If "Scan" tester is not available, disconnect crankshaft sensor harness connector. Set ohmmeter to 2-k/ohm position, measure resistance across sensor terminals. Resistance should be 900-1200 ohms.
3. Set voltmeter on the 2-volt AC scale. Crank engine and measure voltage across sensor terminals. Voltmeter reading should be .1 volt or greater. If resistance reading is not as specified or sensor does not produce a voltage reading, repair faulty wiring or crank sensor.

Crankshaft Position Sensor (IDI)

1. If a "Scan" tester is available, scan RPM parameter while cranking engine. If RPM is indicated, crankshaft position sensor is operating properly.
2. If "Scan" tester is not available, disconnect crankshaft sensor connector, located above oil filter. Set ohmmeter to 2-k/ohm position, measure resistance across sensor terminals. Resistance should be 500-900 ohms.
3. Set voltmeter on the 2-volt AC scale. Crank engine and measure voltage across sensor terminals. Voltmeter reading should be .1 volt or greater. If resistance reading is not as specified or sensor does not produce a voltage reading, replace faulty crankshaft sensor. Also, check if sensor is still magnetized. Replace as necessary.

Intake Air Temperature Sensor

A malfunction in the intake air temperature sensor will set a related trouble code. For testing procedures, see appropriate G - TESTS W/ CODES article in this section.

Knock Sensor

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 DVOM between sensor terminal and ground. Set voltmeter to 2-volt AC scale. 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 under IGNITION SYSTEM. Also see Code 43 in appropriate G - TESTS W/ CODES article in this section.

Manifold Absolute Pressure (MAP) Sensor (C-1)

1. A malfunction in the MAP sensor circuit should set a related trouble code in ECM memory. If a code is present, see appropriate G - TESTS W/ CODES article in this section. 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. (Scheme 44) If driveability condition improves, 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»(/buick/regal/iii-1988-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) table. If MAP sensor voltage is as specified, go to step 3). 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 about 1.2-2.3 volts less than as specified in table. If voltage is not as specified or voltage reading does not immediately follow vacuum change, MAP sensor is faulty.

Scheme 44

MAP SENSOR VOLTAGE RANGE

Manifold Air Temperature (MAT) Sensor

On some models, MAT sensor may also be referred to as a Intake Air Temperature (IAT) sensor. If a MAT sensor related code is present, see appropriate G - TESTS W/ CODES article in this section. 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 . After vehicle has sat overnight, MAT sensor and coolant sensor should have close to the same resistance reading.

MAT SENSOR RESISTANCE

Mass Airflow (MAF) Sensor (3.8L)

A malfunction in the MAF sensor circuit will set a related trouble code. For testing procedures, see Code 34 in appropriate SELF-DIAGNOSTICS article. If driveability problems exist, MAF sensor failure is suspected and no Code 34 is present, disconnect MAF sensor connector. If driveability improves, replace MAF sensor.

Oxygen (O2) Sensor

1. Start engine and warm to operating temperature. Disconnect oxygen sensor. Connect a DVOM between lead of oxygen sensor and ground. Place meter on the 2-volt scale. Voltmeter reading should increase to greater than .8 volt.
2. Using another DVOM on the 20-volt scale. Connect voltmeter in series between the O2 wire from the ECM and the positive post of battery. Reading on voltmeter connected to oxygen sensor should decrease to a low voltage (less than .3 volt).
3. If a second DVOM is not available, install short jumper in O2 wire from the ECM. Hold jumper in one hand and touch positive post of battery with other hand. This should cause oxygen sensor to produce less than .3 volt. For additional testing procedures, see appropriate G - TESTS W/ CODES article in this section.

Park/Neutral (P/N) Switch (C-1)

1. Disconnect P/N switch harness connector. Connect ohmmeter between the P/N switch terminals. (Scheme 45) 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.
2. With park/neutral switch connector disconnected, turn ignition on. Check for 12 volts on the Orange/Black wire of park/neutral switch harness. If 12 volts are not present, check for open or short to ground between switch harness connector and ECM.

Scheme 45

Power Steering (P/S) Pressure Switch (C-1)

1. If "Scan" tester is available, scan power steering pressure switch status. Note status with engine running and wheels in straight-ahead position. Turn steering wheel to full left or right position and again note status. If status changed, power steering pressure switch is okay. If status did not change or "Scan" tester is not available, go to next step.
2. Turn ignition off. 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 left or right position. Continuity should now be present. If readings are not as specified, replace P/S pressure switch.
3. With P/S pressure switch connector disconnected and ignition on, check for 12 volts on switch harness from ECM. If 12 volts are not present, check for open or short to ground in harness between switch connector and ECM.

Throttle Position Sensor (TPS)

Install jumper wires to enable connection of a DVOM in parallel between TPS harness connectors. Connect DVOM positive lead to Dark Blue TPS signal wire terminal. Connect negative lead to Black sensor ground wire terminal. (Scheme 46) 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 in this section.

A malfunction in the TPS circuit should set a related trouble code. For further information, see appropriate G - TESTS W/ CODES article in this section.

Scheme 46

Vehicle Speed Sensor (PM Generator Type)

Disconnect vehicle speed sensor harness connector (located in 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 vary from 0.1-0.5 volt AC as the wheel is turned. If reading is not as specified, replace vehicle speed sensor.

Vehicle Speed Sensor (LED Type)

A speed sensor or buffer malfunction should set a related code in ECM memory. If a code is set, refer to appropriate G - TESTS W/ CODES article in this section for diagnosis.

AIR Electric Air Pump (3.4L Man. Trans.)

Locate AIR electric air pump in right front of engine compartment. Disconnect Orange and Black wire connector. Apply ground to the Black wire terminal of air pump. Apply battery voltage with a fused jumper wire to Orange wire terminal of pump. Pump should run. If pump does not run, replace pump.

Idle Air Control (IAC) Motor & Idle Speed Control (ISC) Motor

See IDLE CONTROL SYSTEM .

A/C Clutch Relays

See MISCELLANEOUS CONTROLS .

Electric Air Pump Relay (3.4L Man. Trans.)

See EMISSION SYSTEMS & SUB-SYSTEMS .

Fuel Pump Relay

See FUEL SYSTEM .

SOLENOIDS

Note. All ECM-controlled solenoids should have at least 20 ohms of resistance when checked with positive ohmmeter lead connected to power supply terminal of solenoid and negative ohmmeter lead connected to ground terminal of solenoid. Some solenoids are equipped with internal diodes. On these solenoids, resistance values will differ if ohmmeter test leads are reversed.

Air Injection Solenoids

See EMISSION SYSTEMS & SUB-SYSTEMS .

Canister Purge Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS .

EGR Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS .

FUEL DELIVERY

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

Fuel Pressure Regulator (PFI)

1. Install fuel pressure gauge to fuel rail fuel pressure test fitting. Remove vacuum hose from fuel pressure regulator. Turn ignition on and note fuel pressure on gauge.
2. Start engine. Check for manifold vacuum at pressure regulator vacuum hose. Repair as necessary. Reconnect vacuum hose to pressure regulator and note fuel pressure on gauge. Compare first and second reading. Fuel pressure reading should be 4-7 psi (.28-.49 kg/cm 2 ) less with vacuum hose installed. Fuel pressure should decrease as vacuum increases. If results are not as specified, 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 fuel filter, fuel pump pressure and volume. If fuel pressure is too high, check for restricted fuel tank return line. If no faults are found and pressure is too high or too low, replace fuel pressure regulator.

Fuel Pump Relay (A-5)

1. Disconnect fuel pump relay connector. Refer to «COMPONENT LOCATIONS»(/buick/regal/iii-1988-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) at the end of this article to locate fuel pump relay. Apply battery voltage and ground to fuel pump relay winding terminals. To identify fuel pump relay terminals, see appropriate wiring diagram at the end of this article.
2. Using an ohmmeter, check for continuity between fuel pump relay power supply terminal and fuel pump drive terminal. Continuity should exist ONLY with relay energized. If relay does not test as indicated, replace relay.
3. To by-pass fuel pump relay (to test fuel pump and wiring when fuel pump is not energizing), see FUEL PUMP RELAY BY-PASS PROCEDURE below.

Fuel Pump Relay By-Pass Procedure

1. If fuel pump will not energize, relay may be by-passed to test fuel pump and related wiring. (Scheme 47) Turn ignition off. Disconnect fuel pump relay connector. Using a fused jumper wire, apply battery voltage to fuel pump test connector (located in engine compartment). For fuel pump test connector location, refer to «COMPONENT LOCATIONS»(/buick/regal/iii-1988-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) at end of this article. (Scheme 84)- (Scheme 86).
2. If fuel pump runs and relay tests okay, check for faulty connections at relay. If fuel pump does not run, check for faulty wiring between relay and fuel pump or replace defective fuel pump.

Scheme 47

Oil Pressure Switch Fuel Pump Back-Up

With engine idling, disconnect fuel pump relay. Engine should continue to run through oil pressure switch back-up circuit. If engine stalls, check oil pressure switch and related wiring.

Fuel Injector(s)

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

INJECTOR RESISTANCE SPECIFICATIONS (1)

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 in this section.

Oxygen Sensor

See ENGINE SENSORS & SWITCHES.

Idle Air Control (IAC) Motor

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" and "B" (coil "B") and "C" and "D" (coil "A"). (Scheme 48) Resistance should be 40-80 ohms. If resistance is as specified, 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. Additional testing of Idle Air Control (IAC) motor requires an IAC motor actuator and node light, or a "Scan" tester capable of cycling ECM output devices (General Motors Tech 1).

Scheme 48

IGNITION SYSTEM

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

Electronic Spark Timing (EST) Advance System

1. A malfunction in the EST circuit should set a related trouble code. Start engine and warm to operating temperature. On vehicles equipped with a manual transmission, increase engine speed to about 2000 RPM. On vehicle equipped with an automatic transmission, slightly increase idle speed.
2. On all vehicles, ground "test" terminal "B" of ALDL. A noticeable change in engine speed should occur. If no change occurs, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section.

Electronic Spark Control (ESC) System W/O ESC Controller(C-5)

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. Also, see KNOCK SENSOR in ENGINE SENSORS & SWITCHES.

Electronic Spark Control (ESC) Sys W/ESC Controller (C-5)

1. An open or short circuit on the ESC wire to the ECM will cause a loss of the 12-volt ESC controller signal. This will cause the ECM to fully retard ignition timing.
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 a DVOM to the ECM ESC signal terminal. With engine idling, 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 ESC controller. On vehicles equipped with automatic transmission, it may be necessary to place transmission in Drive for timing change to occur. Also, see KNOCK SENSOR in ENGINE SENSORS & SWITCHES.

Air Pump (Belt-Driven)

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 hoses, pump belt tension, leaky valves or defective air injection pump.

Check Valve

Detach check valve and blow through valve in direction of check valve flow (to cylinder head). Attempt to suck air back. Replace valve if airflow is allowed against the direction of flow.

Electric Air Divert Valve (3.1L & 3.4L)

With engine at idle, check for at least 10 in. Hg to valve. Run engine at part throttle (less than 2000 RPM). Air should go into exhaust ports until system goes into closed loop, then divert the air to atmosphere. If this does not occur, check terminal harness connector to valve, check for short to ground in air solenoid harness or replace defective valve. Regardless of open or closed loop operation, air should always divert to atmosphere on heavy deceleration.

1. Start engine. With engine fuel system operating in open loop, electric air pump should run and air should be routed through the EADV to the exhaust manifolds. Air pump should also run with ALDL test terminal "B" grounded with engine running.
2. If electric air pump is operating, allow engine to idle for at least 3 minutes. With vehicle in closed loop (or approximately 3 minutes after start), ECM should de-energize electric air pump relay and electric air pump should stop. If this does not occur, go to step 3). If air pump functions as described, relay is functioning properly.
3. Disconnect relay connector. Relay is located in right side electrical center. Connect ohmmeter across Orange wire terminals of relay. Apply battery voltage to Brown/White wire terminal of relay. Apply ground to Pink/Black wire terminal of relay.
4. Continuity should exist between Orange wire terminals of relay ONLY with relay energized. If relay does not test as indicated, replace relay.

EXHAUST GAS RECIRCULATION (C-7)

There are 3 types of EGR systems used: pulse width modulated backpressure EGR (positive and negative) with a control solenoid, pulse width modulated backpressure (positive and negative) EGR without a control solenoid, and digital EGR. To determine EGR system usage, see EGR SYSTEM IDENTIFICATION .

EGR SYSTEM IDENTIFICATION

System Test (Vacuum Operated)

Start and run engine to normal operating temperature. With engine at idle, push up on underside of EGR diaphragm. RPM should drop as EGR valve is opened. If RPM does not drop, remove EGR valve and check for blocked EGR passages. If RPM drops as diaphragm is lifted and EGR vacuum supply is regulated by an ECM-controlled solenoid, verify vacuum is available to solenoid at 2000 RPM and check solenoid using appropriate procedure. See appropriate EGR CONTROL SOLENOID procedure below.

EGR Control Solenoid (Normally Closed)

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 12-volt power supply. Vacuum should now be present or registered at vacuum gauge. If vacuum is not present, check EGR solenoid resistance. Solenoid should have at least 20 ohms of resistance.

EGR Control Solenoid (Normally Open)

1. Disconnect solenoid harness connector. Install vacuum pump to vacuum source side of solenoid. Apply vacuum to solenoid. Vacuum should pass through when solenoid connector is disconnected.
2. Apply battery voltage and ground to solenoid terminals. With solenoid energized, apply vacuum to solenoid. Vacuum should not pass through solenoid. If results are not as specified, check EGR solenoid resistance. Solenoid should have at least 20 ohms of resistance.

Digital EGR Valve (3.1L 3.4L & 3.8L)

1. If an EGR-related code is set, go to appropriate G - TESTS W/ CODES article in this section for diagnosis. Start and allow engine to idle. With engine at normal operating temperature, disconnect digital EGR valve solenoid harness connector.
2. Using a 12-volt power source and a fused jumper wire, very quickly energize EGR solenoid No. 1 Blue wire terminal on EGR valve. (Scheme 49) RPM should drop slightly. Next, energize EGR solenoid No. 2 Brown wire terminal on EGR valve. RPM should drop slightly more than step 1). Energize EGR solenoid No. 3 Red wire terminal on EGR valve.
3. RPM should drop more than step 1) or 2). If RPM drops as indicated, EGR is okay. If RPM drop is not as indicated, check for plugged EGR passages or defective digital EGR valve. Check EGR solenoid resistance. See DIGITAL EGR SOLENOID RESISTANCE table.

Note. For additional testing procedures, see appropriate G - TESTS W/ CODES article in this section.

Scheme 49

DIGITAL EGR SOLENOID RESISTANCE (1)

Positive Backpressure EGR Valve

1. Place transmission in Park or Neutral. Set parking brake and block drive wheels. Connect tachometer. With engine running at normal operating temperature, run engine at 2000 RPM.
2. Disconnect vacuum hose from EGR valve and plug hose. EGR valve diaphragm should move down and engine RPM should increase. NOTE: On some engines with ECM-controlled solenoid, EGR vacuum is locked out in Park/Neutral and solenoid must be by-passed with vacuum supply hose.
3. Reconnect vacuum hose. EGR diaphragm should move up and engine RPM should decrease. A slight vibration of diaphragm plate may be noticed in backpressure EGR valves.
4. If engine RPM did not change and EGR diaphragm moved, the EGR valve is functioning properly. If engine RPM did not change and diaphragm did not move, remove EGR valve and apply 10 in. Hg to EGR vacuum signal port. EGR valve should not open.
5. If EGR valve opens, replace EGR valve. With vacuum still applied, direct a stream of air (15 psi maximum) into valve seat. EGR valve should open completely.
6. If air is not available, connect a short piece of hose over EGR valve seat. Connect vacuum pump to signal port. With thumb plugging intake port of EGR valve, operate vacuum pump while alternately blowing and pausing through hose.
7. With vacuum present at signal port, EGR valve should open while pressure is applied and should close when no vacuum is present.

Negative Backpressure EGR Valve

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 20 seconds. If valve does not operate as indicated, replace EGR valve.

FUEL EVAPORATION CONTROL (C-3)

CANISTER PURGE SOLENOID IDENTIFICATION

Canister Purge Solenoid (Normally Closed)

1. Disconnect canister purge solenoid harness connector and vacuum hose. Apply 10 in. Hg to ported intake manifold vacuum side of solenoid valve. If vacuum holds, go to next step. If vacuum does not hold, replace canister purge solenoid.
2. Using a 12-volt power source, energize canister purge solenoid. Vacuum should release. If vacuum does not release, replace canister purge solenoid. Solenoid resistance should be at least 20 ohms.

Canister Purge Solenoid (Normally Open)

1. Turn ignition ON. Apply vacuum to throttle body vacuum side of solenoid valve. If vacuum holds, go to next step. If vacuum does not hold, go to step 3).
2. Ground diagnostic terminal. If vacuum then drops, go to step 5). If vacuum still holds, go to step 6).
3. Disconnect solenoid electrical connector. Connect test light between connector terminals. If test light does not light, go to next step. If test light lights, check for poor connection. If connection is okay, replace defective solenoid.
4. Probe each terminal with a test light to ground. If test light does not light on either terminal, repair open in wire between fuse and solenoid. See WIRING DIAGRAMS article. If test light lights on one terminal only, repair open in wire between solenoid and solenoid driver terminal of ECM. See WIRING DIAGRAMS article. If test light lights on both terminals, repair short to voltage in wire between solenoid and solenoid driver terminal of ECM. See WIRING DIAGRAMS article.
5. Verify at least 10 Inch Hg (34kPa) of vacuum is available to purge solenoid at idle. If vacuum is as specified, no problem found at this time. If vacuum is not as specified, repair vacuum line or engine mechanical malfunction.
6. Disconnect solenoid electrical connector. If vacuum now drops, repair short to ground in wire between solenoid and solenoid driver terminal to ECM. See WIRING DIAGRAMS . If vacuum still holds, check hoses. If hoses are not restricted or blocked, replace purge solenoid.

Fuel Tank Pressure Control Valve

Apply approximately 15 in. Hg to fuel tank pressure control valve. The diaphragm should hold vacuum for at least 20 seconds. If fuel tank pressure control valve does not hold vacuum, replace tank pressure control valve.

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 or plugged or broken hoses BEFORE adjusting idle. Check 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. If a clear rattle is not heard, replace PCV valve.
3. Visually inspect valve for varnish or deposits which may make PCV valve operation sticky or restricted, or cause incomplete seating of valve. 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. An engine operating without any crankcase ventilation can be damaged, so 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.

Temperature Sensor (Vacuum Motor Type)

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, replace defective thermostatic air cleaner temperature sensor.

Vacuum Motor Diaphragm

1. Turn engine off. Disconnect vacuum hose to vacuum motor. Apply 7 in. Hg to vacuum motor. Damper door should close. If door does not close, check if linkage is properly hooked up.
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.

MISCELLANEOUS CONTROLS

Note. Although some of the controlled devices listed here are not technically engine performance components, they can affect driveability if they malfunction.

HOT LIGHT OR COOLANT TEMPERATURE LIGHT

Note. These checks assume vehicle is not overheating. Verify proper operation of cooling system prior to diagnosing hot light. The coolant temperature sensor, in rare cases, may fail to indicate the correct coolant temperature without setting a malfunction code (Code 14 or 15). This could result in turning on the hot light without having an overheating condition. It could also result in engine overheating without turning on the hot light. Check coolant sensor temperature-to-resistance values in SENSOR RANGE CHARTS article in this section.

Hot light is powered by the 10-amp INDIC or GAGES fuse. Light will turn on when ECM provides a ground for the circuit. If circuit grounds between light and ECM, light will illuminate any time the ignition is turned on.

1. Turn ignition on with engine off (bulb test position). If hot light illuminates, go to step 3). If hot light does not illuminate, check the following: 10-amp INDIC OR GAGES fuse. Faulty instrument cluster bulb. Open circuit between fuse and hot light.
2. Backprobe ECM hot light driver terminal with a test light to battery voltage. Turn ignition on. If test light does not illuminate, ECM terminal connection is bad or ECM is faulty. If test light illuminates, turn ignition off. Disconnect ECM connectors. Jumper ECM hot light driver harness terminal to ground. Turn ignition on. If hot light does not illuminate, check for open circuit between hot light and ECM. If light does not illuminate and all circuits are intact and power is available to light, instrument cluster must be replaced.
3. Start engine. If test light goes off, no problem is evident. If test light is on, turn ignition off. Disconnect ECM connector. Probe ECM hot light driver harness terminal with a test light to battery voltage. If light is off, replace ECM. If light is on, repair short to ground in hot light driver circuit. If no short is present, replace instrument cluster.

TRANSMISSION (C-8)

Note. ECM transmission controls are also covered in greater detail in appropriate TRANSMISSION article in the TRANSMISSION SERVICING Section for domestic vehicles.

Converter Clutch Solenoid

Disconnect harness connector to TCC solenoid. Measure resistance between TCC solenoid terminals "A" and "D". Solenoid resistance should be greater than 20 ohms. (Scheme 50)- (Scheme 54).

Note. Some solenoids have an internal pressure switch in series with the solenoid winding and will not show continuity until that pressure switch is applied by transmission hydraulic pressure. (Scheme 50)- (Scheme 54).

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. 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. On some vehicles this is terminal "F" of the ALDL connector. (Scheme 50)- (Scheme 54). Accelerate vehicle to 45 MPH and note test light. If test light does not illuminate, problem is a faulty ECM connector or ECM.

Scheme 50

Scheme 51

Scheme 52

Scheme 53

Scheme 54

Shift Light (Man. Trans.)

1. These tests assume a shift light problem exists. Use this procedure only if the light will not illuminate, or illuminates all of 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 the ECM, or a bad ECM. (Scheme 55)
3. With ignition on and engine off, ground test terminal 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 in BASIC TESTING article in this section.
4. If shift light does not come on, ground Tan/Black wire at appropriate ECM terminal using a jumper wire. See SHIFT LIGHT CIRCUIT IDENTIFICATION table. If light still does not come on, check for blown GAUGES fuse (AIRBAG fuse on Corvette), blown bulb or open circuit between fuse and ECM. If light comes on when grounding terminal with a jumper wire, problem is a bad ECM connection or bad ECM.

SHIFT LIGHT CIRCUIT IDENTIFICATION

Scheme 55

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

Note. For additional information on electric cooling fans, see ELECTRIC COOLING FANS article in the 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. 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 of relay. Continuity should now exist between clutch drive circuit terminals of relay. Replace A/C clutch relay if readings are not as specified.

Cooling Fan System & Quad-Driver Check

1. Connect a test light to battery voltage. Touch test light probe to the cooling fan control driver terminal of the ECM. (Scheme 56)- (Scheme 83). Disconnect coolant temperature sensor. This should set a code, causing ECM to engage cooling fan through relay. On some models it may be necessary to jumper the coolant temperature sensor harness connectors. On some models, grounding the ALDL with the ignition on and engine off will cause the ECM to activate the cooling fan control driver (ground circuit).
2. If test light illuminates and cooling fan does not come on, check cooling fan relay, power circuits, cooling fan motor, and relay and fan motor ground circuits. If test light does not illuminate, problem is a faulty ECM connector or ECM. Clear trouble code(s) from ECM memory after testing.
3. If cooling fan functions normally during testing but fails to operate under normal conditions, check ECM monitored inputs which affect cooling fan operation. These include the following: coolant temperature sensor, A/C request signal from A/C control switch and A/C pressure sensor or pressure/temperature switch signals (if equipped).

Cooling Fan Relay

1. Disconnect cooling fan relay harness connector. Using an ohmmeter, check continuity of relay winding. (Scheme 56)- (Scheme 83). Continuity should exist. Check continuity across power delivery terminals of relay. With relay not energized, no continuity should exist.
2. With ohmmeter still attached to power delivery terminals of relay, apply battery voltage and ground to energize relay winding. Continuity should now be present between cooling fan relay power delivery terminals. Replace cooling fan relay if readings are not as specified.

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 in this section.

CHART C-12, 2.5L (1 OF 2), COOLING FAN CKT DIAGNOSIS

Primary (No. 1) and secondary (No. 2) cooling fans are ECM controlled based on inputs from coolant temperature sensor, A/C control switches, vehicle speed and state of A/C compressor/engine cooling fan temperature switch. ECM controls cooling fans by grounding circuits No. 335 and/or No. 473, which energizes cooling fan relay(s). When cooling fans are energized, battery voltage is supplied to fan motors.

ECM grounds circuit No. 335 when coolant temperature is greater than 223°F (106°C) or when A/C has been requested. A/C compressor/engine cooling fan temperature switch opens with high A/C pressure, approximately 250 psi (17.6 kg/cm 2 ). ECM then grounds circuit No. 473 to energize secondary (No. 2) cooling fan relay. When ECM energizes either relay, ECM will keep fan relay on at least 60 seconds or until vehicle speed exceeds 70 MPH for fan No. 1 or 40 MPH for secondary(No. 2) fan. If Code 14 or 15 sets, ECM is in back-up mode: fan No. 1 will run at all times.

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

1. With diagnostic terminal grounded, cooling fan control driver(s) will close, which should energize fan relay(s).
2. If A/C compressor/engine cooling fan temperature switch or circuit is open, both fans would run whenever A/C is requested.
3. When A/C compressor/engine cooling fan temperature switch opens, secondary (No. 2) cooling fan relay is energized by ECM. Switch opens when A/C line pressure exceeds approximately 250 psi (17.6 kg/cm 2 ).

Diagnostic Aids

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

If vehicle is overheating and gauge or light indicates so but cooling fan is not coming on, check coolant sensor temperature using a "Scan" tester. Sensor may have shifted out of calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system.

Scheme 56

Scheme 57

CHART C-12, 2.5L (2 OF 2), COOLING FAN CKT DIAGNOSIS

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

1. B+ should be available to each relay when ignition is on.
2. Checks ability of ECM to ground circuits No. 335 and 473. SERVICE ENGINE SOON light should also be flashing at this point. If SERVICE ENGINE SOON light is not flashing, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section.
3. If fan does not turn on at this point, circuit No. 702 and/or No. 532 is open or cooling fan motors are faulty.
4. A short to ground anywhere in battery feed circuit up to and including fan motor(s) may cause a blown fusible element.

Scheme 58

CHART C-10, 2.5L, A/C CLUTCH CTRL CKT DIAGNOSIS

A/C clutch control is ECM controlled. ECM delays A/C clutch engagement approximately .4 second after A/C is turned on. This allows Idle Air Control (IAC) to adjust engine RPM before clutch engages. ECM also disengages A/C clutch during Wide Open Throttle (WOT), when high power steering pressure is present, or if engine is overheating. A/C clutch relay is energized when ECM provides a ground for circuit No. 459. A/C low pressure switch will open if A/C pressure is less than 40 psi (2.8 kg/cm 2 ). A/C high pressure switch will open if A/C pressure exceeds 440 psi (30.9 kg/cm 2 ). A/C compressor/engine cooling fan temperature switch (if equipped) opens when A/C pressure exceeds approximately 250 psi (17.6 kg/cm 2 ). Cooling fan No. 2 will then turn on.

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

1. ECM will only energize A/C relay when engine is running. Determines if relay or circuit No. 459 is faulty.
2. For clutch to properly engage, low pressure switch must be closed to provide battery voltage to relay. High pressure switch must be closed, so A/C request (B+) will be present at ECM.
3. Determines if signal is reaching ECM from circuit No. 66 from A/C control panel. Signal should only be present when A/C mode or defrost mode is selected.
4. Short to ground in any part of A/C request circuit could cause a blown fuse.
5. With engine idling and A/C on, ECM should ground circuit No. 459, which should cause test light to glow.

If complaint is insufficient cooling, problem may be an inoperative cooling fan No. 1 or 2 or A/C compressor/engine cooling fan temperature switch. Cooling fan No. 2 should engage when A/C pressure exceeds a value to open switch, which causes ECM to energize cooling fan relay No. 2. See CHART C-12 for cooling fan diagnosis. If fan operates correctly, check for a basic A/C problem.

Scheme 59

Scheme 60

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

Cooling fan is ECM controlled based on inputs from coolant temperature sensor, A/C control switches, A/C pressure sensor and Vehicle Speed Sensor (VSS). ECM grounds circuits No. 335 and/or 473, energizing cooling fan relay(s) and turning on cooling fan(s). ECM grounds circuits No. 335 and/or 473 when coolant temperature is more than 223°F (106°C) or when A/C has been requested and A/C pressure is about 200 psi (14.1 kg/cm 2 ). When cooling fan is turned on, ECM will keep fan energized at least 60 seconds or until vehicle speed exceeds 70 MPH for fan No. 1 or 40 MPH for secondary (No. 2) fan. If Code 14 or 15 sets, ECM is in back-up mode; fan No. 1 will run continuously.

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

1. With diagnostic terminal grounded, cooling fan control driver(s) will close, which should energize cooling fan relay(s).
2. If A/C fan control switch or circuit is open, fan will run whenever A/C is requested.
3. With A/C clutch engaged, A/C fan control switch should open when A/C pressure exceeds approximately 200 psi (14.1 kg/cm 2 ). This signal should cause ECM to energize cooling fan relay(s). See CHART C-10.
4. Determines if A/C pressure sensor is faulty or if ECM or circuitry is faulty.

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

If vehicle is overheating and gauge or light indicates so but cooling fan is not coming on, check coolant sensor temperature using a Tech 1 scan tester. Sensor may have shifted out of calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system.

Scheme 61

Scheme 62

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

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

1. B+ should be available to circuit No. 250 and terminal No. 2 of fan relay No. 1 when ignition is on.
2. Checks ability of ECM to ground circuits No. 335 and 473. SERVICE ENGINE SOON light should also be flashing at this point. If SERVICE ENGINE SOON light is not flashing, see DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section.
3. If cooling fan does not run at this point, circuit No. 702 or 150 is open or cooling fan motor is faulty.

Scheme 63

CHART C-10, 3.1L (1 OF 2), A/C CLUTCH CONTROL CKT DIAGNOSIS

A/C relay is ECM controlled to delay A/C clutch engagement .4 second after A/C is turned on. This allows IAC to adjust engine RPM for additional load.

The ECM will temporarily disengage the A/C clutch relay for calibrated times for one or more of the following

1. Hot engine restart.
2. Wide Open Throttle (WOT) and TPS angle at 90 percent.
3. Power steering pressure high (open PSPS).
4. Engine speed greater than 6000 RPM.
5. During IAC reset.

The A/C clutch is energized when ECM provides a ground path for circuit No. 459. The A/C pressure sensor is used to determine high and low pressure in the system and also turns on the cooling fan, when needed.

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

1. ECM will only energize A/C relay when engine is running. This test will determine if relay or circuit No. 459 is faulty.
2. This test determines if signal is reaching ECM on circuit No. 66 from A/C control panel. Signal should only be present when A/C or defrost mode has been selected.
3. If ECM sees a high power steering pressure signal, ECM will disengage A/C clutch.

If complaint was insufficient cooling, problem may be caused by an inoperative cooling fan. See CHART C-12 for cooling fan diagnosis. If fan operates correctly, check for basic A/C system problem. If A/C pressure is out of range (43-428 psi), ECM will disable compressor. Using Tech 1 scan tester, observe A/C pressure for 2 minutes with engine at idle and A/C on. Pressure should be within 20 psi of actual reading. If pressure goes out of range, check for basic A/C system problem. If pressure was okay, check circuit using Code 66 chart or replace A/C pressure sensor.

Scheme 64

Scheme 65

CHART C-10, 3.1L (2 OF 2), A/C CLUTCH CTRL CKT DIAGNOSIS

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

1. Determines if pressure transducer is out of range, causing compressor clutch to be disengaged.
2. With engine stopped and field service mode activated, the ECM should be grounding circuit No. 459, which should cause test light to illuminate.

If complaint was insufficient cooling, problem may be caused by an inoperative cooling fan or A/C low pressure switch. Engine cooling fan should turn on when A/C pressure exceeds a value to close low pressure switch, which causes ECM to energize cooling fan relay. See CHART C-12 for cooling fan diagnosis. If fan operates correctly, check for basic A/C system problem.

Scheme 66

CHART C-12, 3.4L (1 OF 2), COOLING FAN CKT DIAGNOSIS

Cooling fan(s) are ECM controlled based on inputs from coolant temperature sensor, A/C control switch, Vehicle Speed Sensor (VSS) and A/C pressure sensor. ECM grounds circuits No. 335 and/or 473, energizing cooling fan relay(s) and turning on cooling fan(s). ECM grounds circuits No. 335 and/or 473 when coolant temperature is more than 223°F (106°C) or when A/C has been requested and A/C pressure is approximately 200 psi (14.1 kg/cm 2 ). When cooling fan is turned on, ECM will keep fan energized at least 30 seconds or until vehicle speed exceeds 70 MPH for cooling fan No. 1 (primary cooling fan) or 40 MPH for cooling fan No. 2 (secondary cooling fan). Also, if Code 14 or Code 15 is set or ECM is in fuel back-up mode, fan will run continuously.

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

1. With diagnostic terminal grounded, cooling fan control driver(s) will close, which should energize fan control relay(s).
2. If A/C pressure is more than 300 psi (21.1 kg/cm 2 ) or circuit is open, fan would run whenever A/C is requested.
3. With A/C clutch engaged and A/C pressure sensor functioning properly, fan should turn on when pressure exceeds approximately 200 psi (14.1 kg/cm 2 ). This signal should cause ECM to energize fan control relay(s). See C-10 chart in the appropriate SYSTEM/COMPONENT TESTS article.
4. Determines if A/C pressure sensor is faulty or if ECM or circuitry is faulty.

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

If vehicle is overheating and gauge or light indicates so but cooling fan is not coming on, check coolant sensor temperature using a Tech 1 scan tester. Sensor may have shifted out of calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system.

Scheme 67

Scheme 68

CHART C-12, 3.4L (2 OF 2), COOLING FAN CKT DIAGNOSIS

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

1. B+ should be available to circuits No. 250 and 1444 on relays when ignition is on.
2. Checks ability of ECM to ground circuits No. 335 and 473. SERVICE ENGINE SOON light should also be flashing at this point. If SERVICE ENGINE SOON light is not flashing, refer to DIAGNOSTIC CIRCUIT CHECK in BASIC TESTING article in this section.
3. If fan does not turn on, circuit No. 702, 532 or 150 is open or cooling fan motor(s) are faulty.

Scheme 69

CHART C-10, 3.4L (1 OF 2), A/C CLUTCH CONTROL CKT DIAGNOSIS

The A/C clutch control relay is ECM controlled to delay A/C clutch engagement about .4 second after A/C is turned on. This allows IAC to adjust engine RPM, before clutch engages. The ECM also disengages the A/C clutch during WOT, when high power steering pressure is present, or if engine is overheating. The A/C clutch control relay is energized when the ECM provides a ground path for circuit No. 459. The A/C pressure sensor is used to determine high or low pressure in the system and also turns on the cooling fan when needed.

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. This test determines if signal is reaching ECM on circuit No. 66 from A/C control panel. Signal should only be present when A/C or defrost mode has been selected.
3. If ECM sees a high power steering pressure signal, ECM will disengage A/C clutch.

If complaint is insufficient cooling, problem may be an inoperative cooling fan. See CHART 12 for cooling fan diagnosis. If fan operates correctly, ensure A/C system is functioning properly. If A/C pressure is not 41-426 psi (2.9-30.0 kg/cm 2 ), ECM will disable compressor. Using Tech 1 scan tester, observe A/C pressure for 2 minutes, with engine idling and A/C on. If pressure is out of range, adjust A/C refrigerant charge. Tech 1 scan tester pressure reading should be within 20 psi (1.4 kg/cm 2 ) of actual pressure. If Tech 1 scan tester pressure reading is not within 20 psi (1.4 kg/cm 2 ) of actual pressure, refer to Code 66 chart in appropriate G - TESTS W/ CODES article in this section or replace A/C pressure sensor.

Scheme 70

CHART C-10, 3.4L (2 OF 2), A/C CLUTCH CTRL CKT DIAGNOSIS

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

1. Determines if pressure transducer is out of range, causing compressor clutch to be disengaged.
2. With engine off and field service mode activated, ECM should ground circuit No. 459, which should cause test light to illuminate.

If complaint is insufficient cooling, problem may be an inoperative cooling fan. See CHART 12 for cooling fan diagnosis. If fan operates correctly, ensure A/C system is functioning properly. If A/C pressure is not 41-426 psi (2.9-30.0 kg/cm 2 ), ECM will disable compressor. Using Tech 1 scan tester, observe A/C pressure for 2 minutes with engine idling and A/C on. If pressure is out of range, adjust A/C refrigerant charge.

Scheme 71

CHART C-12A, 3.8L (1 OF 6), COOLING FAN CKT DIAGNOSIS

ECM uses 3 relays to control 2 cooling fans. Both fans run at low speed when ECM energizes cooling fan relay "B" when coolant temperature exceeds 214°F (101°C) or when A/C is requested. With ignition off, ECM will operate cooling fans at low speed if coolant temperature exceeds 230°F (110°F) and intake air temperature exceeds 102°F (39°F) at time of key off. ECM will keep fans energized at low speed with ignition off for up to 2 minutes or until coolant temperature falls to less than 210°F (99°C).

Both fans run at high speed when ECM energizes all 3 relays with coolant temperature exceeding 226°F (108°C) or when A/C refrigerant pressure is more than 200 psi (14.1 kg/cm 2 ). Cooling fans will not run at high speed with ignition off.

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

1. Codes 14 or 15 could mean cooling system or sensor is not operating normally; fan(s) operation cannot be checked correctly.
2. Selecting low speed fans under miscellaneous tests, fan control with Tech 1 scan tester causes ECM to energize fan relay "B" and operates fans at low speed.
3. Selecting high speed fans under miscellaneous tests, fan control with Tech 1 scan tester causes ECM to energize fan relays "B", "C", and "D", operating fans at high speed.
4. Cooling fans should run at low speed if an A/C mode is selected and ECM is receiving an A/C request signal.

If cooling fans operate normally but an overheat condition exists, see COOLANT TEMPERATURE-TO-RESISTANCE VALUES table in CODE 15 chart in appropriate G - TESTS W/ CODES article in this section. Replace coolant temperature sensor if mis-scaled. If coolant temperature sensor is okay, check for basic cooling system problem.

If coolant temperature exceeds 230°F (110°C) and manifold air temperature exceeds 102°F (39°C) with ignition off, ECM will energize cooling fans at low speed for up to 2 minutes or until coolant temperature is less than 210°F (99°C).

Scheme 72

Scheme 73

CHART C-12B, 3.8L (2 OF 6), COOLING FAN NO LOW SPEED

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

1. Test light on verifies B+ to harness terminals No. 2 and 3 with ignition on.
2. Test light on verifies wiring for both fans.
3. Test light on verifies circuit No. 702 between fan relay "D" and left side fan and circuit No. 533 between left side fan and fan relay "B".
4. Test light on verifies circuit 532 between fan relay "D" and right side fan and circuit No. 150 between right side fan and ground.

Scheme 74

CHART C-12C-A, 3.8L (3 OF 6), COOLING FAN NO HIGH SPEED RIGHT SIDE COOLING FAN

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

1. Checks B+ and ignition at relay harness connector.
2. Test light on verifies circuit No. 536 to cooling fan relay "C".
3. Jumpering terminals No. 3 to 5 in relay "C" by-passes relay and should cause fan to run if fan, wiring and motor are okay.
4. Checks for B+ and ground to fan motor. Test light on at this point indicates a faulty fan motor connection or motor.

Scheme 75

CHART C-12C-B, 3.8L (4 OF 6), COOLING FAN NO HIGH SPEED LEFT SIDE COOLING FAN

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

1. Checks ignition feed at relay "D" coil.
2. Ensures circuit No. 150 is not open between relay "D" and ground.
3. Isolates malfunction to ECM, wiring or fan relay "D".

Scheme 76

CHART C-12D-A, 3.8L (5 OF 6), COOLING FAN LOW SPEED FAN(S) ON AT ALL TIMES

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

1. Low speed cooling fans may run for up to 2 minutes with ignition off.
2. Verifies proper ECM control of fans.
3. Checks circuit No. 535 shorted to ground or a faulty fan relay.
4. Checks for A/C request signal to ECM which would cause low speed fans operation.

If cooling fans operate normally but overheat condition exists, see COOLANT TEMPERATURE-TO-RESISTANCE VALUES table in CODE 14 chart in appropriate G - TESTS W/ CODES article in this section. Replace coolant temperature sensor if mis-scaled. If coolant temperature sensor is okay, check for basic cooling system problem.

Scheme 77

CHART C-12D-B, 3.8L (6 OF 6), COOLING FAN HIGH SPEED FAN(S) ON AT ALL TIMES

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

1. Isolates problem to ECM control fault, faulty fan or relay or wiring problem.
2. Checks for proper operation of A/C head pressure switch.

If cooling fans operate normally but overheat condition exists, see COOLANT TEMPERATURE-TO-RESISTANCE VALUES table in CODE 14 chart in appropriate G - TESTS W/ CODES article in this section. Replace coolant temperature sensor if mis-scaled. If coolant temperature sensor is okay, check for basic cooling system problem.

Scheme 78

Scheme 79

CHART C-10, 3.8L (1 OF 3), A/C CLUTCH CTRL CKT DIAGNOSIS

ECM controls A/C relay to delay A/C clutch engagement .4 second after A/C is turned on. This allows Idle Air Control (IAC) to adjust engine RPM before A/C clutch engages. ECM may apply A/C clutch during engine cranking to prevent A/C slugging. ECM also causes relay to disengage A/C clutch during Wide Open Throttle (WOT) operation. A/C relay is energized when ECM grounds circuit No. 366.

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

1. Compressor clutch should not engage with A/C off.
2. Checks operation of A/C cycling switch.
3. Checks if A/C request signal is getting to ECM through circuit No. 67. Test light off at this time indicates circuit No. 67 is open between cycling switch and ECM.
4. Checks for open circuits No. 66, No. 257, No. 750 or A/C control switch.

Scheme 80

Scheme 81

CHART C-10, 3.8L (2 OF 3), A/C CLUTCH CTRL CKT DIAGNOSIS

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

1. Checks if ECM is controlling A/C clutch control relay.
2. Checks for battery voltage to A/C compressor relay through circuit No. 339.
3. By-passes relay to determine if problem is in relay, A/C clutch coil, circuit No. 59 or ground circuit.

Scheme 82

CHART C-10, 3.8L (3 OF 3), A/C CLUTCH CTRL CKT DIAGNOSIS

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

1. A/C request on Tech 1 scan tester should not display YES with A/C off.
2. Locates source of false A/C request signal.
3. Isolates A/C clutch from its control circuitry.
4. Determines whether a faulty relay or relay control circuit is causing clutch to not disengage.

Scheme 83

Scheme 84

Scheme 85

Scheme 86

ECM Wiring Diagram (2.5L VIN R - Lumina). Scheme 87

ECM Wiring Diagram (3.1L VIN T - Cutlass Supreme, Grand Prix, Lumina & Regal). Scheme 88

ECM Wiring Diagram (3.4L VIN X - Cutlass Supreme, Grand Prix & Lumina). Scheme 89

ECM Wiring Diagram (3.8L VIN L - Regal). Scheme 90

See also:
• A/C CLUTCH
• CTS RESISTANCE VALUES
• MISCELLANEOUS CONTROLS