Engine Controls - System/component Tests Chevrolet Beretta I

INTRODUCTION

Before testing separate components or systems, it is highly recommended that all procedures listed in the BASIC TESTING article be performed. Since many computer controlled and monitored components will set a trouble code if they malfunction, it is also recommended that self-diagnosis be performed. Refer to the 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 to isolate wiring harness shorts or opens. 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 (GENERAL HELP INFORMATION) article.

GENERAL MOTORS A & C CHART REFERENCE

GENERAL MOTORS A & C CHART REFERENCE

Ground Circuits

1. Using an ohmmeter, check for continuity to ground on control unit ground terminals. Use appropriate schematic in WIRING DIAGRAMS article to determine ECM ground terminals. Resistance to ground should be zero ohms. If reading is other than zero ohms, 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 one volt or more, check for open, corrosion 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 an 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 the SENSOR RANGE CHARTS article.

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 under MISCELLANEOUS ECM 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 (All 2.3L & 3.1L J & W Bodies)

A malfunction in A/C pressure sensor circuit will set a related trouble code. For testing procedures, refer to the appropriate G - TESTS W/ CODES article. 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 next step. 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 under MISCELLANEOUS ECM 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 C)

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

Camshaft Sensor (Cadillac Except Brougham)

1. Turn ignition off. Disconnect 3-terminal connector at distributor. Turn ignition on and verify presence of 12 volts on Pink wire of 3-terminal connector. Turn ignition off. Reconnect 3-wire connector.
2. Disconnect 6-terminal, 5-wire connector at HEI distributor. Connect a DVOM to terminal "E" (pos.) and terminal "D" (neg.) on distributor side of connector. (Scheme 32)below under HALL EFFECT CAMSHAFT SENSOR (CADILLAC EXCEPT BROUGHAM).
3. Turn ignition on and note DVOM reading. Using ignition key, bump starter to rotate distributor. Note DVOM reading. Repeat procedure several times. Depending upon distributor/Hall Effect switch window position, DVOM should read either zero or 12 volts. Reading should NOT remain constant as engine is bumped.
4. Crank engine and note voltage reading. Depending on voltmeter, reading may fluctuate rapidly between zero and 12 volts or average about 6 volts. If voltmeter readings are correct, Hall Effect switch is okay. If voltmeter readings are not correct, replace Hall Effect switch.

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. See CTS RESISTANCE VALUES table.

CTS RESISTANCE VALUES

Crankshaft Position Sensor (2.3L 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 crank sensor. Also, check if sensor is still magnetized. Replace as necessary.

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.

Dual Crank (Combination) Sensor (3.3L C(3)I System)

This test should only be performed if vehicle will not start, injectors will not pulse and spark plugs will not fire. This simulates a dual crank sensor signal. If spark and injector pulse occur, dual crank sensor or sensor connections are bad.

Scheme 30

1. Turn ignition off. Disconnect No. 6 spark plug wire from coil tower. Install Spark Tester (ST-125) to coil tower. Install spark plug wire to spark tester.
2. Connect injector test light to any injector connector. Connect jumper wire across dual crank sensor connector terminals "A" and "B". (Scheme 30)and (Scheme 31). Turn ignition on, engine off (DO NOT crank engine).
3. Using a test light connected to ground, momentarily touch dual crank sensor terminal "A". Note injector test light and spark tester. Test light should blink and spark should be present at spark tester. NOTE: Repeatedly grounding terminal "A" at dual crank sensor or terminals "C" and "B" at C(3)I module may cause engine to flooded.
4. If spark was present and test light came on, check for poor connection at dual crank sensor terminal. If connections are okay, replace faulty dual crank sensor. If there was no spark and test light did not come on, check for poor connection at C(3)I module or replace faulty C(3)I module.

Scheme 31

Engine Oil Temperature Sensor (Corvette)

If engine oil temperature sensor circuit malfunctions, a related code will be set. However, if sensor is out of calibration, this will not set a code. To check calibration, use the following method. Disconnect engine oil temperature sensor connector. Using an ohmmeter, measure resistance between sensor terminals. Resistance should be as specified. See ENGINE OIL TEMPERATURE SENSOR RESISTANCE table. Also see Code 52 and Code 62 in appropriate G - TESTS W/ CODES article.

ENGINE OIL TEMPERATURE SENSOR RESISTANCE

Hall Effect Camshaft Sensor (Cadillac Except Brougham)

1. Turn ignition off. Disconnect 3-terminal connector at distributor. Turn ignition on and verify presence of 12 volts on Pink wire of 3-terminal connector. Turn ignition off. Reconnect 3-wire connector.
2. Disconnect 6-terminal, 5-wire connector at HEI distributor. Connect a DVOM to terminal "E" (pos.) and terminal "D" (neg.) on distributor side of connector. (Scheme 32)
3. Turn ignition on and note DVOM reading. Using ignition key, bump starter to rotate distributor. Note DVOM reading. Repeat procedure several times. Depending upon distributor/Hall Effect switch window position, DVOM should read either zero or 12 volts. Reading should NOT remain constant as engine is bumped.
4. Crank engine and note voltage reading. Depending on voltmeter, reading may fluctuate rapidly between zero and 12 volts or average about 6 volts. If voltmeter readings are correct, Hall Effect switch is okay. If voltmeter readings are not correct, replace Hall Effect switch.

Scheme 32

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 and Code 43 in the appropriate G - TESTS W/ CODES article.

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. 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, 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 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 33

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.

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 table. With vehicle sitting overnight, MAT sensor and coolant sensor should have close to the same resistance reading.

MAT SENSOR RESISTANCE

Mass Airflow Sensor (3.3L & 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.

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

1. Disconnect P/N switch harness connector. Connect ohmmeter between the P/N switch terminals. (Scheme 34) 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 34

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 35) 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 the appropriate article below

1. D - ADJUSTMENTS - 4-CYL
2. D - ADJUSTMENTS - V6
3. D - ADJUSTMENTS - V8

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

Scheme 35

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 SELF-DIAGNOSTICS article for diagnosis.

Electric Air Pump (3.4L Man. Trans.)

Locate 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 in this article.

A/C Clutch Relays

See MISCELLANEOUS CONTROLS in this article.

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

See EMISSION SYSTEMS & SUB-SYSTEMS in this article.

Fuel Pump Relay

See FUEL SYSTEM in this article.

SOLENOIDS

Note. All ECM-controlled solenoids should have at least 20 ohms of resistance.

Air Injection Solenoids

See EMISSION SYSTEMS & SUB-SYSTEMS in this article.

Canister Purge Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS in this article.

EGR Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS in this article.

FUEL DELIVERY

Note. For fuel system pressure testing, refer to article in BASIC TESTING .

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 unsatisfactory, 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 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 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 in the WIRING DIAGRAMS 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.

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. 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 at end of this article. (Scheme 117)- (Scheme 145).
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 36

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 FUEL INJECTOR RESISTANCE table.

FUEL INJECTOR RESISTANCE

Note. If injectors are dirty, they should be cleaned using approved injector cleaning procedure before performing 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 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 the TESTS W/O CODES article.

Oxygen Sensor

See ENGINE SENSORS & SWITCHES in this article.

Idle Air Control (IAC) Motor

1. Disconnect harness connector to motor. Check resistance across IAC coil terminals "A" and "B" (B coil) and "C" and "D" (A coil). Resistance should be 40-80 ohms. (Scheme 37) 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 37

Idle Speed Control (ISC) Motor (Cadillac Except Brougham)

A malfunction in the ISC circuit will set a trouble code. For testing procedures, see appropriate SELF-DIAGNOSTICS article. Also, see ISC minimum and maximum adjustment procedures in ON-VEHICLE ADJUSTMENTS article.

Idle Switch (Cadillac Except Brougham)

The ISC motor is equipped with an internal idle switch (also called a throttle or nose switch) which informs the ECM when it should be controlling idle.

Disconnect ISC connector. Connect an ohmmeter across Pink and Black/White wire terminals ("A" and "B") of ISC motor connector. (Scheme 38) With throttle closed, continuity should exist. With throttle open enough to relieve tension from the ISC plunger, continuity should not exist.

Scheme 38

IGNITION SYSTEM (EXC. C3I)

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

Scheme 39

Scheme 40

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.

Electronic Spark Control (ESC) Retard System, Without 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 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, refer to KNOCK SENSOR under ENGINE SENSORS & SWITCHES in this article.

Electronic Spark Control (ESC) Retard System, With 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 under ENGINE SENSORS & SWITCHES in this article.

CHART C-4-1 (MISFIRE AT IDLE 3.3L VIN N)

1. If engine misfires under load only, see C-4-2 below. With egine idling at normal operating temperature, disconnect IAC. While observing engine RPM, temporarily disconnect each injector connector. All injector(s) should result in a drop in RPM. If it does, see ROUGH, UNSTABLE OR INCORRECT IDLE, STALLING in TESTS W/O CODES article. If it doesn't, turn ignition OFF, and install injector tester J 34730-2 or equivalent on injector connector which did not result in an RPM drop. Crank engine while observing injector test light. Light should blink. If it doesn't, see NO START (PFI W/C3I). If it does, then proceed to step 2.
2. With ignition OFF, install spark tester (ST-125) J 26792 or equivalent on plug lead(s) which did not result RPM drop (1,3,5 at plug and 2,4,6 at coil). Spark should jump tester gap while cranking engine. If it does, check for faulty, worn or cracked spark plug(s), plug fouling due to engine mechanical fault. If spark plugs are OK, then see CUTS OUT MISSES in TESTS W/O CODES article. If it doesn't, then check resistance of plug wire which did not fire spark tester. Wire resistance should be less than 30,000 ohms each and wires shouldn't be grounded. Are wires are OK. If Not, then replace faulty wire(s) and retest. If OK go to step 3.
3. First remove coil that didn't fire tester from module, then inspect coil, plug wires & plug wire nipples. They should be free of carbon tracking. If NOT, replace faulty component. If OK got to step 4.
4. Check secondary coil resistance. It should be 5-8K Ohms resistance. If NOT, replace coil. If YES, then go to next step.
5. Install a known good coil. Spark should jump tester gap at problem cylinder with engine idling. If it did not, then there is a faulty ignition module. If it did, then original ignition coil is faulty.

Chart C-4-1 Schematic, Misfire At Idle (3.3L VIN N). Scheme 41

CHART C-4-2 (MISFIRE UNDER LOAD 3.3L VIN N)

1. If engine misfires at idle, see C-4-1 above. With ignition OFF install spark tester (ST-125) J 26792 or equivalent to #1 plug wire at plug end. Remove 15 amp ECM fuse from fuse block. Crank engine, spark should jump tester gap while cranking. Repeat above steps for each plug wire, recording any cylinder(s) that did not fire. Reconnect each plug wire to plug before moving on to next cylinder. Spark should jump tester gap at all plugs while cranking.

If OK

1. Check for faulty, cracked, fouled or worn spark plug(s).
2. Perform injector balance test, see Chart C-2A.
3. Refer to CUTS OUT MISSES in TESTS W/O CODES article.

Cylinder Pair Not Firing (1-4, 2-5 or 3-6)

1. Check for an open plug wire for cylinder pair that Did Not fire (plug wires should measure under 30Kohm). If Ok, swap problem coil with one for cylinder pair that does fire and retest. If problem follows coil, replace coil. If not, replace ignition module.

Single Cylinder or Unpaired Cylinders Not Firing

1. Check for grounded plug wire(s) for cylinder(s) that did not fire and replace as necessary. If Ok, swap problem coil for one that operates properly. If problem follows coil, replace it. If Not, replace plug wire for cylinder(s) that did not fire.

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 back. Replace valve if airflow is allowed against the direction of flow.

Deceleration Valve (3.1L "F" Body)

1. Remove air cleaner. Remove and plug vacuum hose to air cleaner. Connect tachometer to engine. With engine at idle, remove deceleration valve signal hose from vacuum port.
2. Reconnect signal hose to deceleration valve while listening for air flow through ventilation pipe into deceleration valve. Engine speed should drop when hose is reconnected.
3. If airflow lasts less than one second or engine speed does not drop, check for defective hose(s) or deceleration valve.

Electric Air Divert Valve 3.1L, 3.4L & 5.0L & 5.7L "B" & "D" Bodies)

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.

Air Management System - Pressure Operated Electric Divert/Electric Switching (V8 "F" & "Y" Bodies)

1. When engine is cold, port solenoid should be energized, allowing airflow to exhaust ports.
2. When engine is warmed up, port switch is de-energized (off) and converter solenoid should be energized, forcing airflow past the converter valve to the catalytic converter.
3. On the divert mode, both solenoids are de-energized, which opens the converter valve, allowing air out to divert/relief tube to atmosphere. If valves are not operating as specified, check circuit to solenoids. Repair or replace components as necessary.

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 table.

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 EGR CONTROL SOLENOID in this article.

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. 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, replace solenoid. Solenoid should have at least 20 ohms of resistance.

Digital EGR Valve (2.3L "W" Body)

1. If an EGR-related code is set, go to appropriate SELF-DIAGNOSTICS article for diagnosis. Start and allow engine to idle. With engine at normal operating temperature, disconnect digital EGR valve solenoid harness connector. Using a 12-volt power source and a fused jumper wire, very quickly energize EGR solenoid No. 1. RPM should drop slightly. (Scheme 42)
2. Next, energize EGR solenoid No. 2. RPM should drop slightly more than step 1). If RPM drops as indicated, EGR is okay. If not, check for plugged EGR passages or defective digital EGR valve. Check EGR solenoid windings for opens or shorts.

Scheme 42

Scheme 43

1. If an EGR-related code is set, go to appropriate SELF-DIAGNOSTICS article 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. RPM should drop slightly. (Scheme 43) 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 the appropriate G - TESTS W/ CODES article.

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)

Note. On 2.0L, 2.2L & 2.5L engines, no canister purge solenoid is used. Canister is purged by ported vacuum when engine is running above idle. On all other models, one of 2 types of solenoid is used: normally open or normally closed. See CANISTER PURGE SOLENOID IDENTIFICATION table.

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 tank pressure control valve. The diaphragm should hold vacuum for at least 20 seconds. If not, 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, 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 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 not, 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.

Damper Door - Wax Pellet Check ("B" Body)

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

MISCELLANEOUS CONTROLS

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

A/C CLUTCH (C-10)

Note. A/C clutch control is also covered in greater detail in The ENGINE COOLING FAN 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. (Scheme 44)- (Scheme 68).
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.

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ELECTRIC COOLING FAN (C-12)

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

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 69)- (Scheme 91). 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 69)- (Scheme 91). 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.

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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 the appropriate SENSOR RANGE CHARTS article.

The "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.

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.

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 92)- (Scheme 112).

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 in this article.

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 92)- (Scheme 112). Accelerate vehicle to 45 MPH and note test light. If test light does not illuminate, problem is a faulty ECM connector or ECM

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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.
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 as described in the BASIC TESTING article.
4. If shift light does not come on, ground Tan/Black wire at ECM terminal B7 (A12 on Corvette or GF4 on 2.3L Beretta, Cutlass Calais & Grand Am) using a jumper wire. 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 B7 (or A12 or GF4) with a jumper wire, problem is a bad ECM connection or bad ECM.

Scheme 113

Scheme 114

1-4 Shift System Check (Man. Trans. Corvette)

1. With engine off, depress clutch and place transmission in 1st gear. Shift transmission into 2nd or 3rd gear. If transmission cannot be shifted into 2nd or 3rd gear, check for short to voltage on circuit between 1-4 shift relay and 1-4 shift solenoid, defective 1-4 shift solenoid, or internal mechanical transmission problem. (Scheme 116)
2. Turn ignition on with engine not running. Ground ALDL "test" terminal "B". Shift transmission into 2nd or 3rd gear. If transmission cannot be shifted into 2nd or 3rd gear, system is functioning correctly.
3. If transmission can be shifted into 2nd and 3rd gear, turn ignition off. Disconnect 1-4 shift relay connector. Turn ignition on and check for voltage from ground to harness terminals "D" and "E". If voltage is not present on both harness terminals, check for blown fuses or open circuit to terminal which did not illuminate test light.
4. If test light illuminated when touched to both terminals, ground ALDL "test" terminal "B" and connect test light from battery voltage to terminal "F" of 1-4 shift relay harness connector.
5. If test light does not illuminate, check for open in circuit No. 108 between relay and ECM, poor ECM terminal contact or defective ECM. If test light did illuminate, perform 1-4 SHIFT RELAY test procedures.

1-4 Shift Light (Man. Trans. Corvette)

1. This testing procedures assumes that a problem exists with the 1-4 shift light. Use this procedure only if the light will 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.
3. With ignition on and engine off, ground "test" terminal "B" of ALDL connector. The "SERVICE ENGINE SOON" light should start to flash and 1-4 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 the BASIC TESTING article.
4. If 1-4 shift light does not come on, ground ECM terminal C13 using a jumper wire. If light still does not come on, check for blown GAUGES fuse, blown bulb or open circuit between fuse and ECM. If light comes on when grounding terminal C3 with a jumper wire, problem is a bad ECM connection or bad ECM.

Scheme 115

1-4 Shift Relay (Man. Trans. Corvette)

1. Turn ignition off. Disconnect 1-4 shift relay connector. Check for continuity between terminals "D" and "F" of relay. If continuity does not exist, replace relay. If continuity does exist, check for continuity between relay terminals "C" and "A". Continuity should also exist when relay is not energized. If not, replace relay.
2. Energize relay by applying battery voltage to terminal "D" of relay and ground terminal "F". Check continuity between terminals "A" and "E". Continuity should exist while relay is energized. Replace relay if it does not test as described.

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See also:
• BASIC TESTING
• DIS MISFIRE - 3.1L (VIN T) "W" BODY - C4F Chart