Engine Controls - Basic Diagnostic Procedures - 4.3l Chevrolet Caprice Impala SS

MODEL IDENTIFICATION

MODEL IDENTIFICATION

INTRODUCTION

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

The first step in diagnosing any driveability problem is verifying the customer's complaint by test driving vehicle under the conditions in 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.

Note. Unless otherwise instructed in test procedures, perform all voltage tests using a Digital Volt-Ohmmeter (DVOM) with a minimum 10-megohm input impedance.

Note. Following table provides location of commonly used diagnostic information. These former "A" and "C" charts are now written in text and inserted into appropriate location of engine performance workflow. To learn about engine performance workflow, see HOW TO USE THE ENGINE PERFORMANCE SECTION in this section.

"A" & "C" CHART REFERENCE

VISUAL INSPECTION

Visually inspect all electrical wiring. Look 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. If necessary, see VACUUM DIAGRAMS - 4.3L article to verify routing and connections. Inspect air induction system for possible vacuum leaks.

Compression

Check engine mechanical condition using a compression gauge, vacuum gauge or engine analyzer capable of performing a relative compression test. If using engine analyzer, see engine analyzer instruction manual for availability and description of relative compression feature.

Exhaust System Backpressure

Before replacing any components, check exhaust system for restrictions. Use a vacuum gauge or a low-pressure (1-5 psi) pressure gauge to check exhaust system.

If using a vacuum gauge, connect gauge to intake manifold vacuum, and start engine. Observe gauge while holding throttle steady at 2000-2500 RPM. If vacuum gauge reading slowly drops after stabilizing, exhaust system may be restricted.

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 before installation.

Diagnosis

1. Start engine and bring to operating temperature. Increase engine speed to 2000-2500 RPM. Note gauge. Exhaust system is restricted if reading exceeds 1.25 psi (.09 kg/cm 2 ).
2. Check exhaust system for collapsed pipe, heat distress and possible internal muffler failure. If none of these conditions exists, check for restricted catalytic converter. Replace as required.

DEFINITION

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

Note. Before performing following tests, check battery condition, engine cranking speed and fuel supply.

General Inspection

1. Ensure proper starting procedure is being used.
2. Visually check vacuum hoses for splits, kinks and improper connections. See underhood emission control information label. Check ignition wires for cracks, hardness, and improper connections at distributor cap and spark plugs.
3. Remove spark plugs. Check and replace if necessary.
4. Remove distributor cap and check for moisture, dust, cracks, burns, and arcing to ground through coil mounting screws or rotor.
5. Try to turn distributor shaft by hand. Drive gear pin may be broken.
6. If vehicle has been exposed to very cold temperatures, ensure oil is of proper viscosity and not contaminated with gasoline.

Ignition System

1. Disconnect tachometer wire at distributor tachometer terminal (if equipped). A shorted tachometer or tachometer circuit prevents vehicle from starting. Ensure TP sensor and coolant sensor codes are not present, and sensors are not out of calibration.
2. Check for battery voltage at "+" terminal of ignition coil with ignition on. (Scheme 283)- (Scheme 286). Repair as necessary.
3. Connect Spark Tester (ST-125) to end of one plug wire. Crank engine. If spark is present, check fuel delivery.
4. If spark does not occur, disconnect 4-wire EST connector at distributor and check for spark at ignition coil tower using Spark Tester (ST-125). If spark now occurs, check cap and rotor for damage or wear. Check for open in coil wires.
5. If spark does not occur, reconnect EST connector. Leave spark tester connected to coil tower for remainder of test. Disconnect ignition coil Black 2-wire connector, and check voltage on "C" and "+" terminals with ignition on.
6. If voltage on both terminals is 10 volts or greater, go to next step. If voltage on both terminals is less than 10 volts, repair wire from module "+" terminal to "B" terminal on coil Black 2-wire connector. If voltage is less than 10 volts on "C" terminal only, check for open or short to ground in circuit from "C" terminal of ignition module to ignition coil. If circuit is okay, problem is faulty coil or coil connections.
7. Connect voltmeter between ground and tachometer terminal at coil with ignition on. Tachometer terminal may be taped back against harness. If voltage is greater than 10 volts, go to step 9).
8. If voltage is 1-10 volts, replace ignition module and check for spark at coil tower using Spark Tester (ST-125). If voltage is less than one volt, repair open in tachometer lead or tachometer connector. After repairs, recheck voltage at tachometer terminal.
9. Connect test light between ground and tachometer terminal. Crank engine. If test light remains on (steady), go to next step. If test light flashes, replace ignition coil with a known-good unit and recheck for spark. If spark does not occur, reinstall original ignition coil and replace ignition module.
10. Remove distributor cap. Unplug ignition module pick-up coil connector. Connect voltmeter between ground and tachometer terminal. Turn ignition on. Using jumper wires, connect positive end of a known-good 1.5-volt battery to terminal "P" of module. Observe voltmeter at tachometer terminal as negative end of test battery is momentarily grounded to distributor housing.
11. If voltage at tachometer terminal does not drop, check ignition module ground. Check for open in wires from ignition coil to module. If all is okay, replace ignition module.
12. If voltage at tachometer terminal drops, check for spark at spark tester as jumper is removed from terminal "P". If spark does not occur, go to next step. If spark occurs, check pick-up coil connections. Check for 500-1500 ohms resistance at pick-up coil leads. Ensure leads are not shorted to ground. Repair as necessary.
13. Test ignition module with module tester. If module tests okay, check ignition coil wire. If module tester is not available, replace ignition coil, and touch terminal "P" again. If spark occurs, system is okay. If spark does not occur, reinstall original ignition coil, and check coil wire from distributor cap. If no problem is found, replace ignition module.

Fuel System (TBI)

1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure (9-13 psi) and capacity (one pint in 30 seconds).
2. Crank engine, and watch for injector spray. If injector spray occurs, go to step 5). If no spray occurs, disconnect injector harness. Check for battery voltage at harness with ignition on. Battery voltage should be present on one injector terminal. If battery voltage is not present, check for blown injector power fuse. If battery voltage is present on both terminals, check for wires shorted together.
3. If battery voltage is present on only one terminal, connect injector test light to injector harness. Crank engine, and note light. If light flashes, check for stored PCM codes. See «DIAGNOSTIC CIRCUIT CHECK»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) . If no codes are present, refer to HARD START symptom in «TESTS W/O CODES - GASOLINE»(ref-20057) article in this section. If light does not flash, momentarily touch test light from battery voltage to PCM RPM reference terminal (circuit No. 430 - Purple/White wire).
4. Each time test light is removed from PCM RPM reference terminal, injector test light should flash. If test light does not flash, check for open in RPM reference wire or injector drive (ground) circuit. If wiring is okay, replace faulty PCM. Before replacing PCM, check PCM power and ground circuits.
5. Disconnect injector harness, and crank engine. If injector spray or leakage occurs, a no-start condition may be caused by excessive fuel being delivered during cranking. Repair faulty injector or injector seal. If no spray or leakage occurs, refer to HARD START symptom in «TESTS W/O CODES - GASOLINE»(ref-20057) article in this section.

Fuel System (CPI 4.3L VIN W)

Before checking fuel system for a no-start condition, check ignition for adequate spark. For fuel system testing, see FUEL SYSTEM PRESSURE TEST under A-7, BASIC FUEL SYSTEM CHECKS.

Scheme 283

Scheme 284

Scheme 285

Scheme 286

1. Ensure proper starting procedure is being used.
2. Visually check vacuum hoses for splits, kinks and improper connections. See underhood emission control information label. Check ignition wires for cracks, hardness, and improper connections at distributor cap and spark plugs.
3. Remove spark plugs. Check and replace if necessary.
4. Remove distributor cap and check for moisture, dust, cracks or burns. Try to turn distributor shaft by hand. Drive gear pin may be broken.
5. If vehicle has been exposed to very cold temperatures, ensure oil is of proper viscosity and not contaminated with gasoline.

Scheme 287

1. Check if Code P1221 is stored. If code is present, diagnose code first. Check TP sensor voltage. If TP sensor voltage is greater than 2.5 volts at closed throttle, see Code P0118. Check ECT sensor reading. If ECT sensor reading is -30°C, see Code P0118. See «TESTS W/CODES - 4.3L»(ref-20077) article.
2. Turn ignition off for 10 seconds. Turn ignition on. Listen for fuel pump operation. Fuel pump should operate for 2 seconds. If fuel pump operates, go to next step. If fuel pump does not operate, check fuel pump relay and circuit. See the appropriate «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article.
3. Disconnect injector connector at intake manifold. Connect Injector Test Light (J 34730) to wiring harness connector. Crank engine. If injector test light flashes, go to next step. If injector test light does not flash or stays on steady, go to step 6).
4. Disconnect one spark plug wire. Install Spark Tester (ST-125). Crank engine and check for spark. If spark is present, go to next step. If spark is not present, proceed to the «C-4, BASIC IGNITION SYSTEM CHECKS»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l).
5. Reconnect spark plug wire. Turn ignition off. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l__fuel-system-pressure-relief). Connect Fuel Pressure Gauge (J 34730-1). Turn ignition on. Fuel pressure should be 55-61 psi (3.9-4.3 kg/cm 2 ) when fuel pump stops. Pressure should hold steady. If fuel pressure is as specified and holds steady, check injectors. See «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article. If fuel pressure is not as specified, go to «A-7, BASIC FUEL SYSTEM CHECKS»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l).
6. If injector test light does not illuminate, go to next step. If injector test light stayed on steady, check for short to ground in injector circuit. (Scheme 287) If circuit is okay, check injector resistance. Injector resistance should be greater than 1.3 ohms. If resistance is as specified, replace VCM-A. If resistance is not as specified, replace faulty injector.
7. Turn ignition off. Disconnect coil high voltage wire. Disconnect3-pin crank sensor harness connector. Turn ignition on. Using test light connected to battery, momentarily touch test light to Yellow wire crank sensor harness connector terminal. If injector test light flashed, check for faulty crank sensor harness connection or faulty crank sensor. If injector test light does not flash, go to next step.
8. Turn ignition off. Check for open or grounded Yellow wire to crank sensor or faulty VCM-A connector. If wire or connector is okay, remove injector test light.
9. Turn ignition on. Using test light connected to ground, probe engine harness connector. If test light illuminates on one of the terminals, go to next step. If test light illuminates on both terminals, repair short to voltage in Dark Blue wire between injector and VCM-A. If test light does not illuminate on either terminal, repair open between ignition coil (Driver module) Pink wire to VCM-A.
10. Turn ignition off. Reconnect injector connector. Disconnect Red VCM-A connector. Turn ignition on. Using test light connected to ground, probe VCM-A terminal No. RE3. If test light illuminates, check for faulty VCM-A connector or faulty VCM-A. If test light does not illuminate, check for faulty VCM-A connector or open in circuit.

Fuel System (CPI)

Before checking fuel system for a no-start condition, check ignition for adequate spark. For fuel system testing, see the FUEL SYSTEM PRESSURE TEST under A-7, BASIC FUEL SYSTEM CHECKS.

A-4 INJECTOR CIRCUIT DIAGNOSIS

Note. This test only applies to vehicles equipped with a VCM.

1. «ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l__on-board-diagnostic-obd-system-check) . After performing OBD system check, go to next step.
2. Disconnect injector connector at intake manifold. Turn ignition on. Using test light connected to ground, probe ignition feed circuits. See WIRING DIAGRAMS article. If test light illuminates, go to next step. If test light does not illuminate, go to step 7).
3. Install injector test light to each injector circuit, one at a time. Crank engine. If injector test light flashes on each injector, check fuel injector(s). See FUEL SYSTEM in I -SYSTEM/COMPONENT TESTS article. If injector test light did not flash on an injector circuit, go to next step.
4. If injector test light was on steady on an injector circuit, go to next step. If injector test light did not illuminate on an injector circuit, go to step 6).
5. Install injector test to affected injector. Turn ignition off. Disconnect Blue and Red VCM connectors. Turn ignition on, with engine off. If injector test light turns off, go to step 11). If injector test light stays on, go to step 8).
6. Check for open or short in battery circuit to affected injector. If circuit is open or shorted, go to next step. If circuit is okay, go to step 9).
7. Repair circuit as necessary. After repairs, go to step 12).
8. Repair short to ground in injector control circuit. After repairs, go to step 12).
9. Check terminal contact at VCM connector. If terminal contact is faulty, go to next step. If terminal contact is okay, go to step 11).
10. Repair terminal contact. After repairs, go to step 12).
11. Replace VCM. Reprogram replacement VCM. After replacing VCM, go to next step.
12. Using scan tool, select DTC, CLEAR INFO mode. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and stalls/dies, go to step 14).
13. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN mode. If other DTC(s) are present, diagnose affected DTC(s). If no other DTC(s) are present, go to next step.
14. Using scan tool, select CAPTURE INFO, REVIEW INFO. If additional undiagnosed DTC(s) are present, diagnose affected DTC(s). If DTC(s) are not present, system is okay.

FUEL SYSTEM PRESSURE RELIEF

CPI (4.3L VIN W)

Fuel system is under pressure, which must be relieved before servicing fuel system. Fuel pressure may be relieved by one of the following methods.

1. Disconnect fuel pump at rear body connector. Start and allow engine to run until it stalls. Crank starter 3 seconds to remove remaining fuel from fuel lines. 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. Place gauge bleed hose in container. Open bleed valve to relieve fuel pressure.

TBI Unit

Fuel pressure is relieved and drops to zero when ignition is turned off. To minimize risk of fire and personal injury, cover area to be disconnected with a shop rag.

FUEL SYSTEM PRESSURE TEST

1. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l__fuel-system-pressure-relief) . Connect Fuel Pressure Gauge (J-34730-1) to fuel pressure connector. Turn ignition on. With ignition on and engine off, pressure should be 54-64 psi (3.8-4.5 kg/cm 2 ) and should hold steady.
2. If pressure is within specification and holds, go to next step. If pressure is within specification but does not hold, go to step 4). If fuel pressure is not present, go to step 7). If pressure is greater than specification, go to step 9). If pressure is less than specification, go to step 5).
3. Start engine. With engine at operating temperature, open throttle quickly and note fuel pressure. If fuel pressure is not close to64 psi (4.5 kg/cm 2 ), replace CPI assembly. If fuel pressure is close to 64 psi (4.5 kg/cm 2 ), problem does not exist.
4. Check for leaking injectors or fittings by pinching off fuel return line. If pressure does not hold, replace CPI assembly. If pressure holds, check for faulty in-tank fuel pump or pump connections.
5. 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 «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article.
6. Gradually pinch off flexible fuel return line between CPI assembly and fuel tank. If fuel pressure increases to within specification, pressure regulator is faulty. Replace CPI assembly. If fuel pressure does not increase with line pinched, check for faulty in-tank fuel pump, loose pump connections or partially blocked fuel strainer.
7. Use a 10-amp fused jumper wire to apply battery voltage to fuel pump test connector. For location of fuel pump test connector, see COMPONENT LOCATIONS in «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article. 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.
8. If fuel pressure is present with voltage applied to test connector, test fuel pump relay and voltage supply to relay. See «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article.
9. Relieve fuel pressure. Disconnect fuel return line. Attach a 5/16" flexible fuel line to CPI side of fuel line. Route hose into appropriate container. Turn ignition on, and note fuel pressure. If fuel pressure is within specification, locate and repair restriction in fuel return line. If fuel pressure is still greater than specification, replace CPI assembly.

TBI

1. Turn ignition off for 10 seconds. Turn ignition on, and listen at fuel tank for fuel pump operation. Pump should run 2 seconds (20 seconds on models with fuel module). If fuel pump runs, go to next step. If fuel pump does not run, go to step 7).
2. If fuel pump runs, turn ignition off. Verify fuel tank has fuel. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l__fuel-system-pressure-relief) . Remove air cleaner, and plug air cleaner vacuum ports (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658-D) and Adapter (J-2968A-85) between steel fuel line and flexible hose, on outlet side of fuel filter.
3. Turn ignition on, and note reading on pressure gauge. If fuel pressure is 9-13 psi (.63-.91 kg/cm 2 ) or 26-32 psi (1.8-2.4 kg/cm 2 ) on 7.4L engines, no problem exists. If pressure is less than either of the low readings, go to step 5). If pressure is greater than either of the higher readings, turn ignition off, and bleed fuel pressure. Disconnect fuel return line downstream of pressure gauge. Insert return line into a gasoline container.
4. Turn ignition on. If pressure is now as specified, correct restriction in fuel return line between disconnected point and fuel tank. If fuel pressure is greater than either of the higher readings, check for restricted return line (including fuel filter) downstream of pressure gauge. If no restrictions are present, replace fuel pressure regulator assembly.
5. Check for restricted fuel line between in-tank fuel pump and pressure regulator. If fuel line is okay, disconnect injector connector. Turn ignition on. Gradually pinch fuel pressure gauge outlet hose. Note pressure. CAUTION: DO NOT pinch off fuel return line completely. DO NOT allow fuel pressure build-up to exceed specification, as damage to fuel pressure regulator may occur.
6. If pressure is greater than 13 psi (.91 kg/cm 2 ) or 32 psi (2.2 kg/cm 2 ) on 7.4L engines, replace fuel pressure regulator. If pressure is less than 9 psi (.63 kg/cm 2 ) or 26 psi (1.8L kg/cm 2 ) on 7.4L engines, check for faulty fuel pump or incorrect part. Check fuel pump coupling hose and pump inlet filter in fuel tank. On models with dual fuel tanks, check for faulty fuel tank selector valve and meter switch.
7. Apply 12 volts to fuel pump test connector. For fuel pump test connector location, see underhood engine component views under COMPONENT LOCATIONS in «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article. If fuel pump now runs, repair open in fuel pump relay drive or power circuit or repair faulty fuel pump relay. To test relay, see «SYSTEM/COMPONENT TESTS - 4.3L»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-systemcomponent-tests-43l-2) article.
8. If fuel pump does not run with 12 volts applied to fuel pump test connector, repair open in fuel pump power or ground circuit, or repair faulty fuel pump.

FIELD SERVICE MODE CHECK

Note. Assembly Line Data Link (ALDL) connector may also be referred to as Data Link Connector (DLC). SERVICE ENGINE SOON (SES) light may also be referred to as Malfunction Indicator Light (MIL).

Note. Oxygen sensor may cool off while engine idles, causing system to enter open loop. To restore closed loop mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

This test confirms proper fuel system operation and verifies closed loop operation. Clear codes and perform this test after any repair is completed. When performing this test, 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 diagnostic test terminal "B" of DLC. (Scheme 291)- (Scheme 294). In closed loop mode, Malfunction Indicator Light (MIL) flashes once per second.
2. In open loop, light flashes 2.5 times per second. A lean exhaust is indicated if light is usually off. A rich exhaust is indicated if light is usually on.

A-8, INJECTOR COIL TEST

1. Perform on-board diagnostic test. Diagnose any Diagnostic Trouble Code (DTC) present. If DTC is not present, go to next step.
2. Turn ignition off. Relieve fuel pressure. Connect Fuel Injector Tester (J-39021) to battery positive. Connect Injector Switch Box (J-39021-210). Set amperage on fuel injector tester to 2.5-amp. Connect DVOM to fuel injector tester. Connect Tech 1 scan tool and monitor engine coolant temperature. If coolant temperature is 50-95°F (10-35°C), go to step 4). If coolant temperature is not as specified, go to next step.
3. Allow engine to warm or cool as necessary. If engine coolant temperature is now 50-95°F (10-35°C), go to next step.
4. Using fuel injector tester, press PUSH TO START TEST button and monitor DVOM reading. Perform this test to each injector. Voltage reading should be 5.44-7.53 volts. If voltage reading is as specified, go to next step. If voltage reading is not as specified, perform injector balance test. See A-7, CENTRAL SEQUENTIAL PORT INJECTOR BALANCE TEST (CSI).
5. Replace faulty injector nozzle and poppet assembly and then proceed to next step.
6. Using Tech 1 scan tool, select DTC, CLEAR INFO. Start engine. If engine starts and continues to run, go to next step. If engine does not start or starts and dies, repeat step 2).
7. Warm engine to normal operating temperature. Using scan tool, select DTC, FAILED THIS IGN. If DTC(s) are present, diagnose DTC(s). If DTC(s) are not present, go to next step.
8. Using scan tool, select CAPTURE INFO, REVIEW INFO. If additional DTC(s) are present, diagnose DTC(s). If DTC(s) are not present, system is okay.

Scheme 288

ENHANCED/HIGH VALUE IGNITION SYSTEM (OBD-II)

Note. This system is used on "S" and "T" Series with 4.3L engine equipped with VCM-A controller. Timing is not adjustable on this system.

Spark

1. If equipped with factory tachometer, disconnect tachometer terminal before performing tests. To disconnect tachometer lead, it is necessary to disconnect the Gray ignition coil connector from ignition coil. Using a fused jumper wire, jumper connector terminal "B" of ignition coil and Terminal Test Adapter Kit (J 35616-A).
2. Using Spark Tester (ST-125), crank engine and check for spark at coil wire and at each spark plug wire. If spark is not present or intermittent, go to next step. If spark is present, check spark plugs and fuel system.
3. Check coil wire or spark plug wire resistance on suspect wire. Resistance should be about 1000 ohms per inch.

Ignition Coil Power Source

1. Turn ignition off. Disconnect ignition coil driver module connector. Turn ignition on. Using DVOM, check voltage at ignition coil driver module terminals "A" and "D". (Scheme 287) Both terminals should read 10 volts or greater. If voltage is as specified on both terminals, go to next step. If voltage is 10 volts or less on either or both terminals, go to step 6).
2. Place DVOM on AC scale. Connect DVOM to ignition coil module terminal "B" and ground. Crank engine. Voltage reading should be 1-4 volts. If voltage reading is as specified, go to next step. If voltage reading is not as specified, go to step 4).
3. Connect test light to battery voltage and probe coil driver module connector terminal "C". (Scheme 287) If test light illuminates, check for faulty ignition coil driver module connection or faulty ignition coil module. If test light does not illuminate, check for open ignition coil driver module ground.
4. Turn ignition off. Disconnect Gray VCM-A harness connector. Check for continuity between VCM-A ignition control connector terminal and ignition driver module connector terminal "B". If continuity is present, go to next step. If continuity is not present, check for open IC circuit between VCM-A and ignition coil driver module.
5. Connect test light to battery voltage and probe VCM-A ignition control harness terminal. If test light illuminates, check for grounded IC circuit between VCM-A and ignition coil driver module. If test light does not illuminate, check for faulty VCM-A connection or VCM-A.
6. If voltage is less than 10 volts on both terminals, check for faulty ignition feed circuit to ignition coil, grounded external coil circuit or faulty ignition coil. If voltage is less than 10 volts on only one of the terminals, check circuit from affected terminal or check for faulty coil connection or faulty coil.

HEI-EST DISTRIBUTOR

Note. Only basic ignition timing is adjustable in HEI/EST ignition system.

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

Turn ignition on. Use voltmeter to check voltage between terminal "B" of ignition coil and ground. If battery voltage does not exist, check for open circuit, blown ignition fuse or defective ignition switch.

Ignition Coil Resistance

1. Remove coil connectors and secondary coil wire. In test "A", use high ohmmeter scale. (Scheme 289) Resistance value should be very high (infinite). If reading is not infinite, replace coil.
2. In test "B", use low ohmmeter scale. Reading should be very low (near zero ohms). If reading is not near zero ohms, replace coil. In test "C", use high ohmmeter scale. Continuity should exist. If no continuity exists, replace coil.

Scheme 289

Distributor Pick-Up Coil Short & Resistance Checks

1. Disconnect pick-up coil leads from module. Using ohmmeter, check pick-up coil resistance. Connect ohmmeter to distributor housing and either pick-up coil lead (step 1 in illustration). (Scheme 290) Flex pick-up coil leads by hand to check fo r intermittent short to ground. If reading is not always infinite, replace pick-up coil.
2. Connect ohmmeter between pick-up coil leads (step 2 in illustration). (Scheme 290) Check for intermittent open by flexing wire s and connectors. If resistance is not 500-1500 ohms, replace pick-up coil.

Scheme 290

Tach Pulse (RPM) Signal

Connect scan tester to DLC. RPM should be indicated on tester when engine is cranked or running. Tach pulse (RPM reference) will be indicated as a voltage signal when a DVOM connected to battery voltage is used to backprobe circuit No. 430, PCM terminal. (Scheme 285)-5. If tach pulse signal is not present, vehicle will not run.

IDLE SPEED & IGNITION TIMING

Ensure idle speed and ignition timing are set to specifications. For specifications, see SPECIFICATIONS - 4.3L article. For adjustment procedures, see ADJUSTMENTS - 4.3L article.

Diagnostic circuit check determines the following

1. Whether Malfunction Indicator Light (MIL) works.
2. Whether ECM/PCM/VCM/VCM-A is operating and can recognize a fault.
3. Whether any codes are stored.

After performing basic diagnostic procedures listed under PRELIMINARY INSPECTION & ADJUSTMENTS; A-7, BASIC FUEL SYSTEM CHECKS and C-4, BASIC IGNITION SYSTEM CHECKS, use self-diagnostic system for determining computer-related problems. (Scheme 297)

If codes are NOT indicated and driveability problems still exist after performing necessary tests in diagnostic circuit check, see TESTS W/O CODES - GASOLINE article and SCAN TESTER USAGE under SELF-DIAGNOSTIC SYSTEM in TESTS W/CODES - 4.3L article.

1. Check MIL operation. Turn ignition on with scan tester not connected, DLC terminal not grounded and engine not running. MIL should be on steadily. If light glows and stays on steadily, go to next step. If light does not glow, go to A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If light flashes, go to step 3).
2. Ground DLC test terminal "B". MIL should flash Code 12, followed by any codes stored in PCM memory. If light goes from bright to dim or if light remains on and does not flash Code 12, see A-2, MALFUNCTION INDICATOR LIGHT (MIL) ALWAYS ON OR WON'T FLASH CODE 12.
3. Check for short to ground in diagnostic test terminal wire between DLC terminal "B" and PCM terminal. (Scheme 291)- (Scheme 294). If circuit is okay, replace PCM.

Scheme 291

Scheme 292

Scheme 293

Scheme 294

A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE

1. If MIL does not illuminate with ignition on and engine off, attempt to start engine. If engine starts, go to step 3). If engine does not start, check fusible links at battery. Also check 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 the «WIRING DIAGRAMS»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) section at the end of this article for power terminal identification. If power to PCM power terminals is not available, check for open in power circuits. If power to PCM power terminals is available, check for poor PCM ground circuits. If circuits are okay, replace faulty PCM.
3. Turn ignition off. Disconnect PCM connectors. Turn ignition on, and connect PCM MIL driver terminal to ground using a test light. See See appropriate WIRING DIAGRAM in the «WIRING DIAGRAMS»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) section at the end of this article for power terminal identification.
4. If MIL is now on, repair MIL driver terminal connections at PCM, or replace faulty PCM. If MIL stays off when test light is used to ground MIL driver terminal, check for blown instrument panel fuse, faulty bulb, open in MIL driver circuit between PCM and bulb, short in driver circuit to voltage or open in ignition feed to MIL.

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

1. Turn ignition on, engine off. If MIL does not illuminate, see A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If MIL is on, ground DLC test terminals "A" and "B". Code 12 should flash. If Code 12 does not flash, go to next step. If Code 12 now flashes, problem is no serial data. Check serial data circuit for open or short to ground, or short to voltage between PCM and DLC terminal. If circuits are okay, replace faulty PCM or PROM (MEM-CAL).
2. Turn ignition off. Disconnect PCM connectors. Turn ignition on. If MIL is on, check for short to ground in MIL driver circuit between MIL and PCM driver terminal. (Scheme 291)- (Scheme 294). Also, see See appropriate WIRING DIAGRAM in the «WIRING DIAGRAMS»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) section at the end of this article for terminal identification.
3. If light is off with PCM connectors disconnected, turn ignition off. Reconnect PCM connectors. Turn ignition on with engine off and diagnostic terminal not grounded. Using a test light connected to ground, backprobe PCM terminal circuit to DLC terminal "B". (Scheme 291)- (Scheme 294). If MIL does not flash Code 12, go to next step. If MIL flashes Code 12, check for open in DLC test terminal "B" circuit to PCM. If circuit is okay, check for open in DLC terminal "A" circuit to PCM and PCM ground. (Scheme 291)- (Scheme 294).
4. Check PROM (MEM-CAL) for proper installation. If installation is okay, replace PCM using original PROM (MEM-CAL). Recheck for Code 12. If Code 12 is not present, replace PROM (MEM-CAL). If Code 12 is present, system is okay.

ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

Note. Use of Tech 1 scan tool is required to perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK.

The OBD System Check determines

1. If Malfunction Indicator Light (MIL) works.
2. If PCM/VCM is operating and can recognize a fault.
3. If any Diagnostic Trouble Codes (DTCs) 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. After performing necessary tests as described in the OBD system circuit check, if no codes are indicated and driveability problems still exist, see TESTS W/O CODES article and SCAN TESTER usage in TESTS W/CODES article.

1. Turn ignition on with engine off. Observe Malfunction Indicator Light (MIL). If MIL illuminates, go to next step. If MIL does not illuminate, go to «A-1, MALFUNCTION INDICATOR LIGHT (MIL)»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) .
2. Turn ignition off. Install Tech 1 scan tool and follow scan tool manufacturer's instructions to proceed with test. Turn ignition on. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, go to «A-2, NO SCAN TOOL DATA»(/chevrolet/caprice-impala-ss/1995-1996/remont/testing-diagnostics/#engine-controls-basic-diagnostic-procedures-43l) .
3. Attempt to start engine. If engine starts and runs, go to next step. If engine does not start or starts and dies, see A-3, NO-START DIAGNOSIS. Repair as necessary.
4. Using scan tool, observe DTC status LAST TST FAIL function. If any DTC is stored, save freeze frame and fail record information using CAPTURE INFO feature. If a DTC is indicated as last test failed, diagnose affected DTC. See TESTS W/CODES article. If DTC is not indicated, go to next step.
5. Using scan tool, display DTC fail records. If a DTC fail record is stored, save freeze frame and fail record information using scan tool CAPTURE INFO feature. If a fail record is stored, diagnose affected DTC. See TESTS W/CODES article.

Scheme 295

Scheme 296

Diagnostic Circuit Check & Work Flow Chart. Scheme 297

SUMMARY

If faults were not found while performing BASIC DIAGNOSTIC PROCEDURES, proceed to TESTS W/CODES - 4.3L article. If no hard codes are found in self-diagnostics, or vehicle does not have a self-diagnostic system, proceed to TESTS W/O CODES - GASOLINE article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

Wiring Diagram (Astro & Safari 4.3L - 1 Of 2). Scheme 298

Wiring Diagram (Astro & Safari 4.3L - 2 Of 2). Scheme 299

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 1 Of 3). Scheme 300

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 2 Of 3). Scheme 301

Wiring Diagram ("C" & "K" Series Pickup, Sierra, Suburban, Tahoe & Yukon - 4.3L With A/T - 3 Of 3). Scheme 302

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 1 Of 3). Scheme 303

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 2 Of 3). Scheme 304

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 4.3L With M/T - 3 Of 3). Scheme 305

Wiring Diagram ("P" Series - 4.3L With M/T - 1 Of 2). Scheme 306

Wiring Diagram ("P" Series - 4.3L With M/T - 2 Of 2). Scheme 307

Wiring Diagram ("P" Series - Commercial & Motorhome - 4.3L With 4L80-E - 1 Of 3). Scheme 308

Wiring Diagram ("P" Series - Commercial & Motorhome - 4.3L With 4L80-E - 2 Of 3). Scheme 309

Wiring Diagram ("P" Series - Commercial & Motorhome - 4.3L With 4L80-E - 3 Of 3). Scheme 310

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/PCM - 1 Of 2). Scheme 311

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/PCM - 2 Of 2). Scheme 312

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM-A - 1 Of 3). Scheme 313

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM-A - 2 Of 3). Scheme 314

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN W W/VCM-A - 3 Of 3). Scheme 315

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/PCM - 1 Of 2). Scheme 316

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/PCM - 2 Of 2). Scheme 317

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/VCM - 1 Of 2). Scheme 318

Wiring Diagram ("S" & "T" Blazer, Jimmy, Pickup & Sonoma - 4.3L VIN Z W/VCM - 2 Of 2). Scheme 319

Wiring Diagram (Van - 4.3L With 4L60-E - 1 Of 3). Scheme 320

Wiring Diagram (Van - 4.3L With 4L60-E - 2 Of 3). Scheme 321

Wiring Diagram (Van - 4.3L With 4L60-E - 3 Of 3). Scheme 322

Wiring Diagram (Van - 4.3L With 4L80-E - 1 Of 3). Scheme 323

Wiring Diagram (Van - 4.3L With 4L80-E - 2 Of 3). Scheme 324

Wiring Diagram (Van - 4.3L With 4L80-E - 3 Of 3). Scheme 325

See also:
• SYSTEM/COMPONENT TESTS - 4.3L
• ADJUSTMENTS - 4.3L
• INJECTOR CIRCUIT DIAGNOSIS
• INJECTOR COIL TEST
• DIAGNOSTIC CIRCUIT CHECK
• FUEL SYSTEM PRESSURE TEST
• FUEL SYSTEM PRESSURE RELIEF
• ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK