Engine Controls - Basic Testing Chevrolet Caprice Impala SS

INTRODUCTION

The following diagnostic steps will help prevent overlooking a simple problem. This is also where to begin diagnosis for a no-start condition.

The first step in diagnosing any driveability problem is verifying the customer's complaint with a test drive under the conditions during which the problem reportedly occurred.

Before entering self-diagnostics, perform a careful and complete visual inspection. Most engine control problems result from mechanical breakdowns, poor electrical connections or damaged/misrouted vacuum hoses. Before condemning the computerized system, perform each test listed in this article.

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 in the ENGINE PERFORMANCE section.

GENERAL MOTORS A & C CHART REFERENCE

Note. Perform all voltage tests with a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance, unless stated otherwise in test procedure.

VISUAL INSPECTION

Visually inspect all electrical wiring, looking for chafed, stretched, cut or pinched wiring. Ensure electrical connectors fit tightly and are not corroded. Ensure vacuum hoses are properly routed and not pinched or cut. See VACUUM DIAGRAMS article in the ENGINE PERFORMANCE section to verify routing and connections (if necessary). Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition with a compression gauge, vacuum gauge, or an engine analyzer. See engine analyzer manual for specific instructions. For compression specifications, see C - SERVICE & ADJUSTMENT SPECIFICATIONS article in the ENGINE PERFORMANCE section.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. The exhaust system can be checked with a vacuum gauge or a low pressure (0-5 psi) pressure gauge.

If a vacuum gauge is used, connect vacuum gauge hose to intake manifold vacuum port and start engine. Observe vacuum gauge. Open throttle part way and hold steady. If vacuum gauge reading slowly drops after stabilizing, exhaust system should be checked for a restriction. If using a low pressure gauge, connect gauge in one of the following manners

1. Check At AIR Pipe

Remove rubber hose at exhaust manifold AIR pipe check valve and remove check valve. Install pressure gauge to hose and nipple via Propane Enrichment Device(J-26911). Nipple should be inserted into exhaust manifold AIR pipe.

1. Check At Oxygen Sensor

Remove oxygen sensor. Install backpressure tester in place of oxygen sensor. After test is completed, coat oxygen sensor threads with anti-seize compound.

Diagnosis

1. Start engine and bring to operating temperature. Increase engine speed to 2000-2500 RPM and note gauge. Reading should not exceed 1.25 psi (.09 kg/cm 2 ). Exhaust system is restricted if specification is exceeded.
2. Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exist, check for restricted catalytic converter. Replace as required.

PCM REPLACEMENT CHECK

Many test procedures and test charts in ENGINE PERFORMANCE lead to conclusion that PCM is faulty. Before replacing PCM, check driven circuits and PCM Quad-Drivers (QDR) using following procedures and accompanying flow chart. See QDR Check Flow Chart below.

1. Using a DVOM, backprobe PCM drive circuit in question and measure pin voltage with ignition on. If system voltage is present, go to next step. If system voltage is not present, turn ignition off. Disconnect PCM connector. Measure voltage on connector terminal for suspect circuit. If low voltage is measured, check circuit for an open or short to ground. Repair as necessary. If system voltage is measured, PCM is defective.
2. With DVOM still backprobing PCM at suspect circuit, ground DLC "test" terminal. Monitor voltage drop on circuit for at least 2 minutes. Voltage drop should not be greater than .5 volt. Some circuits may have low voltage until DLC "test" terminal is grounded. A "switching" operation should occur. Some Torque Converter Clutch (TCC) systems have pressure switches in series with TCC solenoid. On these applications, complete this circuit by disconnecting transmission/transaxle connector and using a test light in series between terminals "A" and "D" of transmission/transaxle connector.
3. If voltage drops and then floats back up, this occurs when QDR circuit is going into current limiting. Check circuit current flow (.75 amp maximum, except as noted in QDR check chart). See QDR Check Flow Chart below.

Note. For PCM-equipped vehicles, transmission/transaxle functions can only be energized through use of TECH 1 scan tester. Shift solenoids on these models will not energize when DLC is grounded.

QDR Check Flow Chart. Scheme 211

NO START DIAGNOSIS (A-3)

Note. Some vehicles are equipped with anti-theft systems (VATS or PASS-Key(R)) which will not allow vehicle to be started if improper starting procedures or improperly coded ignition keys are used. Both fuel injection and cranking systems will be disabled. Loss of fuel enable signal from anti-theft decoder module should set a trouble code in PCM memory.

DEFINITION

No start is defined as engine cranks properly, but does not start. Engine may fire a few times.

General Inspection

1. Ensure proper starting procedure is being used. Visually check vacuum hoses for splits, kinks and proper connections, as shown on Vehicle Emission Control Information label. Check ignition wires for cracking, hardness and proper connections at both coil pack and spark plugs.
2. Remove spark plugs. Check and replace as necessary. In very cold temperatures, ensure oil is proper viscosity and not contaminated with gasoline.

Ignition System

1. Check for stored trouble codes. If PCM Codes 16, 18, 41, 42, 46 or 48 are set, use those code charts first. See appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section. If engine has not been started for at least 8 hours, compare scanned ECT sensor temperature with actual coolant temperature.
2. If scanned value are not close, replace Engine Coolant Temperature (ECT) sensor. Ensure TP sensor scan is greater than 2.5 volts at closed throttle. If not, see PCM Code 21 chart in appropriate TESTS W/CODES article in the ENGINE PERFORMANCE section.
3. Using Tech 1 scan tester, scan engine RPM during cranking. If RPM is indicated on scan tester, go to step 6). If RPM is not indicated on scan tester, turn ignition off. Disconnect distributor harness connector.
4. Turn ignition on. Probe terminal "C" (Red wire) of distributor harness connector (PCM side) with test light connected to ground. If test light illuminates, go to next step. If test light does not illuminate, check for faulty PCM connection, open or grounded ignition feed circuit (Pink wire), or a faulty PCM.
5. Turn ignition off. Check for continuity between ground and distributor harness connector terminal "D" (Pink/Black wire). (Scheme 212) If continuity exists, problem is faulty distributor connector connection or faulty distributor. If continuity does not exist, problem is faulty PCM connection, open ground circuit (Pink/Black wire) between PCM and distributor harness connector or faulty PCM.
6. Install Spark Tester (ST-125). Crank engine and check for spark on at least 2 spark plug wires. If spark is not present, go to step 8). If spark is present, check injector fuses. If fuses are okay, go to next step. If fuses are faulty, check injector(s) or injector circuits for short to ground.
7. Using scan tester, enable fuel pump. If fuel pump does not operate, go to FUEL SYSTEM test. If fuel pump operates, turn ignition off. Install fuel pressure gauge. Turn ignition on. Fuel pressure should be 41-47 psi (2.9-3.3 kg/cm 2 ). If fuel pressure is not as specified, go to BASIC FUEL SYSTEM CHECKS. If fuel pressure is as specified, check for sticking EGR valve, faulty MAF or leaks between throttle body and MAF.
8. Check spark at coil wire using Spark Tester (ST-125). Leave spark tester connected to coil for remaining steps. If spark does not occur, go to next step. If spark occurs, inspect distributor cap for cranks. Inspect secondary wiring (plug wires) for opens, shorts or poor connections. If cap and wires are okay, replace distributor.
9. Remove coil wire. Check coil wire resistance. Coil wire resistance should be less than 5000 ohms. If coil wire resistance is not as specified, replace coil wire. If coil wire is okay, go to next step.
10. Disconnect ignition coil module connector. Turn ignition on. Using DVOM, check voltage between terminals "A" and "D" and ground. If voltage on one terminal is less than 10 volts, check for open or short or faulty coil connection on that circuit. If circuit and connection are okay, coil is faulty.
11. If voltage on both terminals is less than 10 volts, check for faulty power feed circuit to coil or grounded external coil circuit. If external circuits and power feed are okay, coil is faulty.
12. If voltage on both terminals is 10 volts or greater, turn ignition off. Reconnect ignition coil module connector. On "B" and "F" Bodies, go to step 14) On "D" and "Y" Bodies, disconnect ignition coil connectors. Using a fused jumper wire, jumper between Gray connector terminal "B" and Black connector terminal "B". Connect test light between battery voltage and ignition coil Gray connector terminal "A".
13. Crank engine and observe test light. Test light should flash. If test light flashes, ignition coil is faulty. If test light does not flash, check for faulty ignition coil module connection or faulty ignition coil module.
14. Connect DVOM set on AC scale to ground and terminal "B" of coil driver harness. Crank engine. If voltage is not 1-4 volts, go to next step. If voltage is 1-4 volts, probe coil driver connector terminal "C" with a test light connected to battery voltage. If test light does not illuminate, coil driver ground circuit is open. If test light illuminates, coil driver connection or coil driver is faulty.
15. Crank engine and monitor low resolution signal on scan tester. If low resolution signal is present, problem is faulty ignition control connection at PCM or open ignition control circuit, or faulty PCM. If low resolution signal is not present, turn ignition off. Disconnect distributor electrical connector.
16. Turn ignition on. Probe terminal "C" of distributor harness with a test light connected to ground. If test light illuminates, go to next step. If test light does not illuminate, problem is faulty PCM connection, open in ignition feed circuit between PCM and distributor harness connector terminal "C" or faulty PCM.
17. Turn ignition off. Check for continuity between ground and distributor harness connector terminal "D". If continuity exists, go to next step. If continuity does not exist, open ground circuit between PCM and distributor harness connector terminal "D", faulty PCM connection or faulty PCM.
18. Turn ignition on. Using DVOM set on AC scale, measure voltage on terminal "A" of distributor harness connector. If about 5 volts are present, go to next step. If voltage is not as specified, problem is open or short in low resolution circuit between PCM and distributor harness connector terminal "A", faulty PCM connection or faulty PCM.
19. Measure voltage on terminal "B" of distributor harness connector. If about 5 volts are present, distributor connection or distributor is faulty. If about 5 volts are not present, problem is open or short in high resolution circuit between PCM and distributor harness connector terminal "B", faulty PCM connection or faulty PCM.

Fuel System

1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure and capacity. See BASIC FUEL SYSTEM CHECKS.
2. Check fuel pump fuse. If fuel pump fuse is okay, go to step 4). If fuse is faulty, turn ignition off. Remove fuse. Disconnect fuel pump wiring harness. Using a test light connected to battery voltage, probe fuel pump harness Gray wire connector. If test light illuminates, Gray wire is shorted. If test light does not illuminate, go to next step.
3. Reconnect fuel pump harness connector. Replace fuel pump fuse. Turn ignition on and recheck fuse. If fuse is okay, problem is intermittent short to ground in Gray wire. If fuse blows again, fuel pump harness connector is grounded or fuel pump is faulty.
4. Remove fuel pump relay. Turn ignition on. Using a test light connected to ground, probe fuel pump relay harness battery feed circuit. If test light illuminates, go to next step. If test light does not illuminate, check for open in battery feed circuit.
5. Connect test light to battery voltage. Probe fuel pump relay harness ground circuit. If test light illuminates, go to next step. If test light does not illuminate, check for open in relay ground circuit.
6. Connect DVOM between ground and fuel pump relay harness connector Dark Green/White wire terminal. Using Tech 1 scan tester, enable fuel pump. If battery voltage is present, go to next step. If battery voltage is not present, check for open in Dark Green/White wire, faulty PCM connector or faulty PCM.
7. Using a fused jumper wire, jumper fuel pump relay connector Gray and Orange wire terminals. If fuel pump does not operate, go to next step. If fuel pump operates, check for faulty relay connection of faulty relay.
8. Leave jumper connected. Disconnect fuel pump harness connector. Using a test light, probe fuel pump relay connector Gray wire terminal. If test light illuminates, go to next step. If test light does not illuminate, check for open in circuit from fuel pump relay to fuel pump.
9. Using test light, probe fuel pump harness connector Gray wire terminal. If test light illuminates, check for open in fuel pump harness connector or faulty fuel pump. If test does not illuminate, check for open in fuel pump ground circuit.

Scheme 212

Scheme 213

BASIC FUEL SYSTEM CHECKS (A-7)

Fuel Pressure Relief

Fuel system is under pressure. Pressure must be relieved prior to servicing fuel system. Fuel pressure may be relieved by using one of the following methods.

1. On all models, disconnect fuel pump at rear body connector. Start engine and run engine until it stalls. Crank starter for 3 seconds to remove remaining fuel from fuel lines. Turn ignition off. Reconnect rear body connector.
2. Install Fuel Pressure Gauge (J-34730-1) on fuel pressure connector of fuel rail. Wrap shop towel around pressure connection when installing fuel pressure gauge to absorb fuel leakage. Install gauge bleed hose in container. Open bleed valve to bleed fuel pressure.

Fuel Pressure Check

1. Relieve fuel pressure as previously described in FUEL PRESSURE RELIEF. Connect Fuel Pressure Gauge (J-34730-1) to fuel pressure fitting on fuel rail. With gauge installed, turn ignition on. With ignition on and engine off, fuel pressure should read within specification. See «FUEL PRESSURE»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-testing) table. If no fuel pressure is present, go to step 5).
2. Start engine. Pressure should drop 3-10 psi (.2-.7 kg/cm 2 ). Turn ignition off. Pressure should hold. If pressure does not hold, check for leaking injectors or fittings. If injectors or fittings are not leaking, replace pressure regulator.
3. If pressure is present but less than specification, check for restricted delivery line or fuel filter. Repair as necessary. If no restriction is evident, apply battery voltage to fuel pump test connector using a 10-amp fused jumper wire. For location of fuel pump test connector, see COMPONENT LOCATIONS in appropriate I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section.
4. Gradually pinch off fuel return line between gauge and fuel tank. If fuel pressure increases to within specification, replace fuel pressure regulator. If fuel pressure does not increase with line pinched, check for faulty in-tank fuel pump or partially blocked fuel strainer.
5. Apply battery voltage to fuel pump test connector using a 10-amp fused jumper wire. For location of fuel pump test connector, see COMPONENT LOCATIONS in I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section. Observe fuel pressure reading. If fuel pressure is still not present, check wiring between test connector and fuel pump. If wiring is okay, replace fuel pump.
6. If fuel pressure is present with voltage applied to test connector, test fuel pump relay and voltage supply to relay. See I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section.

FUEL PRESSURE

Fuel Pump Relay

See MOTORS, RELAYS & SOLENOIDS in appropriate I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section.

Fuel Pump Relay By-Pass Procedure

See FUEL DELIVERY in I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section.

FIELD SERVICE MODE CHECK

Note. Field service mode check can only be performed on vehicles equipped with 12-pin DLC with a wire present in test terminal "B". See DATA LINK CONNECTOR (DLC) TEST TERMINALS table.

Note. Oxygen sensor may cool off while engine is idling. This causes system to go into "open loop". To restore "closed loop" mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

Field service mode check confirms proper fuel system operation and verifies "closed loop" operation. Clear codes and perform this test after any repair is completed. When performing this check, always engage parking brake and block DRIVE wheels. Parking brake on FWD models does NOT hold drive wheels.

1. Start engine. With engine running, ground DLC terminal "B". (Scheme 214) In "closed loop" mode, Malfunction Indicator Light (MIL) will flash once a second.
2. In "open loop", MIL will flash 2.5 times a second. If MIL is off most of the time, a lean exhaust is indicated. If MIL is on most of the time, a rich exhaust is indicated.

DATA LINK CONNECTOR (DLC) TEST TERMINALS

Scheme 214

Scheme 215

HEI-EST DISTRIBUTOR

Note. The only adjustments that can be made to HEI/EST ignition system are basic ignition timing (on distributor-type ignitions) and spark plug gap.

Spark

1. If factory tachometer is connected at coil tachometer terminal, disconnect it before performing tests. When removing spark plug wire from spark plug, twist and pull on boot (not on wire).
2. Using Spark Tester (ST-125), check for spark at coil wire (if applicable) and at each spark plug wire using spark tester. Check spark plug wire resistance on suspect wires. Resistance should be less than 30,000 ohms.

Ignition Coil Power Source

1. Turn ignition on. Using voltmeter, check voltage between terminal "+" of ignition coil and ground on models with remote-mounted coil.
2. On models equipped with integral ignition coil, check voltage between BAT terminal and ground at distributor. Battery voltage should exist. If battery voltage does not exist, check for open circuit, blown ignition fuse or defective ignition switch.

Ignition Coil Resistance

1. Turn ignition off. Remove distributor cap and coil assembly. Invert cap. (Scheme 216) Set ohmmeter to low scale. Connect leads to coil BAT and TACH terminals. Resistance should be zero or nearly zero. If resistance is not zero or nearly zero ohms, replace ignition coil.
2. Set ohmmeter on high scale. Connect one lead to coil secondary terminal and other lead to ground terminal. If resistance reading is infinite, replace ignition coil.

Scheme 216

Distributor Pick-Up Coil Short & Resistance Checks

1. Disconnect pick-up coil leads from HEI/EST module terminals "N" and "P". Set ohmmeter to middle scale. Connect one ohmmeter lead to either pick-up coil lead and the other lead to distributor housing. Flex pick-up coil leads by hand to check for intermittent shorts to ground. Reading should be infinity at all times. If resistance is not infinite, replace pick-up coil.
2. Connect ohmmeter between both pick-up coil leads. Check for intermittent open circuit by flexing wires and connectors. Resistance should be 500-1500 ohms. If resistance is not as specified, replace pick-up coil.

Tach Pulse (RPM) Signal

1. Connect a scan tester to the DLC. RPM should be indicated on tester when engine is cranked or running. If scan tester is not available, tach pulse (RPM reference from ignition module) will be indicated as a voltage signal on a DVOM (with a minimum 10-megohm input impedance) when DVOM is touched to PCM terminal circuit No. 430 with engine cranking.
2. A tach pulse signal may be simulated (to test PCM response) by connecting a test light in series between battery and PCM terminal circuit No. 430. Each time test light is touched to and removed from circuit No. 430, PCM will see this as a tach signal. For circuit and terminal reference, see appropriate schematic under NO START - ENGINE CRANKS OKAY.

Hall Effect Switch (4.9L)

1. Disconnect distributor 4-wire and 6-wire connectors. Apply battery voltage to Red wire of 3-wire distributor connector (distributor side). Connect ground to Black wire of 3-wire distributor connector (distributor side).
2. Connect voltmeter positive lead to terminal "E" (Brown/White wire) of 5-wire distributor connector (distributor side). Connect voltmeter negative lead to terminal "D" (Black/Red wire) of 5-wire distributor connector (distributor side).
3. Bump starter and note voltmeter reading. When distributor blade is in window of Hall Effect switch, voltmeter should read within .5 volt of battery voltage. When distributor blade is not in window of Hall Effect switch, voltmeter should read less than .5 volt. Bump starter several times to cause both conditions to occur.
4. If Hall Effect switch is functioning properly and starter is cranked, voltmeter should show an averaged voltage of about 5-6 volts. If Hall Effect switch does not function as described, replace Hall Effect switch.

Disconnect tachometer wire (if equipped). A shorted tachometer will not allow vehicle to start. Check for adequate spark with Spark Tester (ST-125). Check for spark on more than one plug wire. When removing spark plug wire from spark plug, twist and pull on boot. DO NOT pull on wire.

Turn ignition on. Check for battery voltage on Pink/Black wire to ignition coil. If battery voltage is not present, check for blown 10-amp COIL fuse. If fuse is okay, check for open between fuse and ignition coil.

Information is not available from manufacturer.

Tach Pulse (RPM Reference) Signal

Connect scan tester to DLC. Scan low and high resolution reference pulses. Both high and low pulses should be present when engine is cranking or running. If low resolution pulse is not present, engine will not start and PCM will set Code 16 in memory. If high resolution pulse is not present, vehicle will still run; however, PCM will set Code 36 in memory. For circuit and terminal reference, see appropriate schematic in NO START - ENGINE CRANKS OKAY.

IDLE SPEED & IGNITION TIMING

Ensure idle speed and ignition timing are set to specification. For adjustment procedures, see appropriate D - ON-VEHICLE ADJUSTMENTS article in the ENGINE PERFORMANCE section.

DIAGNOSTIC CIRCUIT CHECK

The Diagnostic Circuit Check determines

1. If Malfunction Indicator Light (MIL) works.
2. If PCM is operating and can recognize a fault.
3. If any codes are stored.

After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS, this is the starting point for utilizing the self-diagnostic system for determining computer-related problems. (Scheme 217)and (Scheme 218). After performing necessary tests as described in the diagnostic circuit check, if no codes are indicated and driveability problems still exist, see H - TROUBLE SHOOTING - NO CODES article in the ENGINE PERFORMANCE section and SCAN TESTER USAGE in TESTS W/CODES article in the ENGINE PERFORMANCE section.

Note. On vehicles equipped with 16-pin DLC or 12-pin DLC without test terminal "B", use of Tech 1 scan tester is required to perform diagnostic circuit check. See DATA LINK CONNECTOR (DLC) TEST TERMINALS table.

1. Check operation of MIL. Turn ignition on with engine off. MIL should be on steady. If MIL illuminates and stays on steady, go to step 3). If MIL does not illuminate, go to A-1, MIL INOPERATIVE. If MIL flashes, go to step 4).
2. On vehicles equipped with 16-pin DLC or 12-pin DLC without test terminal "B", install Tech 1 scan tester and follow scan tester manufacturer's instructions to proceed with test. If MIL does not come on, go to A-1, MIL INOPERATIVE. If MIL comes on, check for diagnostic trouble codes and diagnose codes using TESTS W/CODES article in the ENGINE PERFORMANCE section. On vehicles equipped with 12-pin DLC with test terminal "B", go to next step.
3. Grounding DLC terminal "B" at this time should cause MIL to flash a Code 12, followed by any codes stored in PCM memory. (Scheme 214) MIL going from bright to dim is not considered a code. If MIL dims or remains on and does not flash Code 12, see A-2, MIL ON STEADY OR WON'T FLASH CODE 12. NOTE: On some models, as long as a 30-second pause may occur between code flashes. This is normal and does not indicate necessary component replacement.
4. If MIL begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between DLC terminal "B" and PCM terminal No. 5. If circuit is okay, replace PCM.

Scheme 217

Scheme 218

A-1, MIL INOPERATIVE

1. If Malfunction Indicator Light (MIL) does not illuminate with ignition on and with engine off, attempt to start engine. If engine starts, go to step 3). If engine does not start, check fusible links at battery and PCM fuse. If fusible links or PCM fuse are blown, repair short to ground.
2. If fusible links and PCM fuse are okay, turn ignition on and check power circuits to PCM, including keep alive memory and ignition feed. See appropriate wiring diagram in WIRING DIAGRAMS section for power terminal identification. If power is not available to power terminals of PCM, check for opens in power circuits. If power is available to PCM power terminals, check for poor PCM ground circuits, or replace faulty PCM.
3. If engine starts and MIL does not illuminate, turn ignition off. Disconnect PCM connectors. Turn ignition on and jumper PCM light driver terminal to ground using a test light. See appropriate wiring diagram in WIRING DIAGRAMS section for power terminal identification.
4. If MIL is now on, repair light driver terminal connections at PCM or replace faulty PCM. If MIL stays off when test light is used to ground light driver terminal, check for blown instrument panel fuse, faulty bulb, open in light driver circuit between PCM and bulb, driver circuit shorted to voltage, or an open in ignition feed to MIL.

A-2, MIL ON STEADY OR WON'T FLASH CODE 12

1. Start engine. Check MIL. If MIL is not flashing, go to next step. If MIL is flashing, turn ignition off. Disconnect PCM connector "D". Using test light connected to battery voltage, probe DLC test terminal "B". If test light illuminates, output/field service enable circuit (12-pin DLC terminal "B") is shorted to ground. If test light does not illuminate, replace faulty PCM.
2. Connect DVOM between ground and DLC output/field service enable terminal (12-pin DLC terminal "B"). DVOM should read 5 volts. If reading is as specified, check for faulty PCM connection or faulty PCM. If reading is not as specified, go to next step.
3. Turn ignition off. Disconnect PCM connector"D". Using jumper wire, ground DLC test terminal "B". With test light connected to battery positive, probe PCM harness connector terminal D20 (White/Black wire). Test light should illuminate. If test light illuminated, check for faulty PCM connection or faulty PCM. If test light did not illuminate, DLC test terminal "B" circuit is open.

SUMMARY

If no faults were found while performing BASIC DIAGNOSTIC PROCEDURES, no trouble codes (or only intermittent ones) were found while performing DIAGNOSTIC CIRCUIT CHECK and driveability problems exist, proceed to H - TROUBLE SHOOTING - NO CODES article in the ENGINE PERFORMANCE section for diagnosis by symptom (i.e., ROUGH IDLE, NO-START, etc.) or intermittent diagnostic procedures.

4.3L (VIN W) & 5.7L (VIN P) PCM Wiring Diagram (All Models - 1 Of 3). Scheme 219

4.3L (VIN W) & 5.7L (VIN P) PCM Wiring Diagram (All Models - 2 Of 3). Scheme 220

4.3L (VIN W) & 5.7L (VIN P) PCM Wiring Diagram (All Models - 3 Of 3). Scheme 221

See also:
• FUEL PRESSURE