Engine Controls - Basic Testing Chevrolet Cutaway P30

INTRODUCTION

The following diagnostic steps help prevent overlooking simple problems and begin diagnosis for no-start conditions.

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. 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 this Section.

GENERAL MOTORS 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 article in this section 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.

Because fuel injectors on many models are triggered by ignition switch during cranking mode, DO NOT use ignition switch during compression tests on fuel injected vehicles. Use a remote starter to crank engine to prevent fire hazard or engine's oiling system contamination.

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.

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 (J26911). Nipple should be inserted into exhaust manifold AIR pipe.
2. Check At O2 Sensor Remove O2 sensor. Install backpressure tester in place of O2 sensor. After test is completed, coat O2 sensor threads with anti-seize compound.

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.

A-3, NO-START DIAGNOSIS

Note. For diesel information, see TESTS W/CODES - DIESEL article in this Section.

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 cracking, hardness and improper connections at both distributor cap and spark plugs.
3. Remove spark plugs. Check and replace as 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 proper viscosity and not contaminated with gasoline.

Scheme 175

1. Disconnect tachometer wire at distributor TACH terminal (if equipped). A shorted tachometer or tachometer circuit prevents vehicle from starting.
2. Check for battery voltage at BAT terminal at distributor with ignition on. (Scheme 176)- (Scheme 180). Repair as necessary.
3. Connect ST-125 spark tester to one end of plug wire, and crank engine. If spark is present, check fuel delivery.
4. If spark does not occur, disconnect 4-wire EST connector at distributor. If spark now occurs, replace pick-up coil in distributor
5. If spark does not occur, reconnect EST connector, and check voltage at TACH terminal at distributor with ignition on.
6. If voltage is less than one volt, repair faulty coil connection or replace faulty coil. If voltage is 1-10 volts, replace ignition module, and recheck for spark. If spark still does not occur, replace ignition coil.
7. If voltage reading at TACH terminal of distributor is greater than 10 volts, remove and invert distributor cap, keeping wires connected. Fabricate an HEI coil spark tester by trimming a spark plug boot and connecting trimmed boot to ST-125 spark tester. (Scheme 175) Crank engine.
8. If spark occurs, check cap for cracks, water or other defects. Check pick-up coil connector and ignition coil lead wire colors. Ignition coil with Yellow and Red wires should be used with a Yellow pick-up coil connector. Ignition coil with White and Red wires should be used with a Clear or Black pick-up coil connector.
9. If spark does not occur,turn ignition off, and disconnect pick-up coil leads from module. Turn ignition on. With voltmeter connected to distributor TACH terminal and fabricated coil spark tester connected, momentarily touch test light, connected to a remote voltage source (1.5-8.0 volts), to ignition module terminal "P".
10. If voltage at TACH terminal does not drop, check ignition module ground, and check for open in wires from ignition coil to module. If all is okay, replace ignition module.
11. If voltage at TACH terminal drops, check for spark at spark tester as test light is removed from terminal "P". If spark occurs, check pick-up coil connections and check for 500-1500 ohms resistance at pick-up coil leads. (Scheme 188) Repair as necessary.
12. If spark does not occur, 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 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). See A-7, BASIC FUEL SYSTEM CHECKS (GASOLINE).
2. Crank engine, and watch for injector spray. If injector spray occurs, go to step 5). If no spray occurs, disconnect injector harness, and check for battery voltage at harness.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 to one another.
3. If battery voltage is present on only one terminal, connect injector test light (also called a "node" light) to injector harness. Crank engine, and note light. If light flashes, check for stored ECM codes. See DIAGNOSTIC CIRCUIT CHECK. If no codes are present, refer to HARD START symptom in «TESTS W/O CODES»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article in this section. If light does not flash, momentarily touch test light from battery voltage to ECM RPM reference terminal (circuit No. 430).
4. Each time test light is removed from ECM 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 ECM. Prior to replacing ECM, check ECM power and ground circuits.
5. If injector spray occurred while cranking engine, disconnect injector harness,and crank engine.If injector spray or leakage occurs, a no-start condition could 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 article «TESTS W/O CODES»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-tests-wo-codes) in this section.

Fuel System (4.3L Turbo PFI)

1. Before checking fuel system for a no-start condition, check ignition for adequate spark. Check for proper fuel pump pressure and capacity. See A-7, BASIC FUEL SYSTEM CHECKS (GASOLINE).
2. Disconnect injector harness. Turn ignition on, and check for battery voltage at each injector harness. Battery voltage should be present on one side of each injector. If battery voltage is not present, check for blown injector power fuse. If battery voltage is present on both injector 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. Repeat on other injector connectors.If light flashes, check for stored ECM codes. If no codes are present, refer to HARD START symptom in the «TESTS W/O CODES»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-tests-wo-codes) article in this section.
4. If light does not flash, disconnect distributor 4-wire HEI-EST connector. Momentarily touch test light from battery voltage to ECM RPM reference wire (circuit No. 430) of 4-wire connector. Each time test light is removed from ECM 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 ECM.

Scheme 176

Scheme 177

Scheme 178

Scheme 179

Scheme 180

Circuit Description

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem.

Note. Numbers below refer to circled numbers on diagnostic chart.

Test Description

1. This test will determine if ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. By touching and removing a test light, connected to 12 volts, to circuit 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are OK.
2. This step tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.
3. This test checks for continuity to ECM.

Scheme 181

Scheme 182

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem. If both injector circuits fail to blink when tested, diagnose one injector circuit at a time.

Note. Numbers below refer to circled numbers on diagnostic chart.

1. This test will determine if ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. By touching and removing a test light, connected to 12 volts, to circuit 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are OK.
2. This step tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.
3. This test checks for continuity to ECM.

Scheme 183

Scheme 184

This chart should only be used if diagnosis in CHART A-3 indicated an injector circuit problem. If both injector circuits fail to blink when tested, diagnose on injector circuit at a time.

Note. Numbers below refer to circled numbers on diagnostic chart.

1. This test will determine if the ignition module is generating a reference pulse, if the wiring is at fault or if the ECM is at fault. By touching and removing a test light, connected to 12 volts, to circuit 430, a reference pulse sould be generated. If injector test light blinks, the ECM and wiring are OK.
2. This step tests for 12 volts to the injector. It will also determine if there is a short to voltage on the ECM side of the circuit.
3. This test checks for continuity to the ECM. For circuit schematic (Scheme 190)

Scheme 185

Model 700 TBI Unit (2.5L)

Place transmission gear selector in Park (Neutral on manual transmissions). Set parking brake, and block drive wheels. Loosen fuel filler cap to relieve tank pressure. Disconnect 3-terminal electrical connector at fuel tank. Start engine, and allow to run until engine stalls due to lack of fuel. Crank engine 3 seconds to remove any fuel pressure left in fuel lines. Fuel lines are now safe for servicing.

Model 220 TBI Unit (Except 2.5L)

Port Fuel Injection (4.3L Turbo)

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

1. Disconnect fuel pump at rear body connector. Start engine, and run it 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. Install gauge bleed hose in container. Open bleed valve to bleed fuel pressure.

FUEL SYSTEM PRESSURE TEST

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 equipped 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/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-basic-testing) . 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-29658A) and Adapter (J-2968A-85) between steel fuel line and flexible hose, ahead of in-line fuel filter.
3. Turn ignition on, and note reading on pressure gauge. If fuel pressure is 9-13 psi (.63-.91 kg/cm 2 ), no problems are present. If pressure is less than 9 psi (.63 kg/cm 2 ), go to step 5). If pressure is greater than 13 psi (.91 kg/cm 2 ), 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 9-13 psi (.63-.91kg/cm 2 ), correct restriction in fuel return line between disconnected point and fuel tank. If fuel pressure is greater than 13 psi (.91 kg/cm 2 ), check for restricted return line (including fuel filter) downstream of pressure gauge. If no restrictions are present, replace fuel pressure regulator (TBI 700) or fuel meter cover/pressure regulator (TBI 220).
5. If fuel pressure was less than 9 psi (.63 kg/cm 2 ), check for restricted 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 ), replace fuel pressure regulator (TBI 700) or fuel meter cover/pressure regulator (TBI 220). If pressure is less than 9 psi (.63 kg/cm 2 ), 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. If fuel pump did not run in step 1), 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»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article in this section. 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 FUEL PUMP RELAY in «SYSTEM/COMPONENT TESTS»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article in this section.
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.

PFI (4.3L Turbo)

1. Relieve fuel pressure. See FUEL PRESSURE RELIEF. Connect Fuel Pressure Gauge (J-34730-1) to fuel pressure connector on fuel rail. Turn ignition on. With ignition on and engine off, pressure should be 35-38 psi (2.46-2.67 kg/cm 2 ). If no fuel pressure is present, go to step 5).
2. Start engine. Pressure should drop 3-10 psi (.21-.70 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 «SYSTEM/COMPONENT TESTS»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article in this section.
4. Gradually pinch off fuel delivery line between gauge and fuel rail. If fuel pressure increases to within specifications, replace fuel pressure regulator. If fuel pressure does not increase with line pinched, check for faulty in-tank fuel pump or partial blocked fuel strainer.
5. 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»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article in this 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. Refer to the «SYSTEM/COMPONENT TESTS»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-systemcomponent-tests) article in this section.

FIELD SERVICE MODE CHECK

Note. O2 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 ALDL. (Scheme 189)- (Scheme 192). In closed loop mode, SERVICE ENGINE SOON light 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.

FUEL PUMP FLOW TEST

1. Disconnect fuel line at fuel filter inlet. Disconnect fuel injection pump electric shut-off solenoid wire (Pink).
2. Crank engine 15 seconds. If fuel pump does not supply approximately 1/2 pint of fuel in 15 seconds, go to FUEL SYSTEM PRESSURE TEST.

1. Turn engine off. Disconnect fuel line at inlet to fuel filter assembly. Install low pressure fuel gauge to fuel line.
2. Crank or run engine 10-15 seconds. If fuel pressure is not 5.8-8.7 psi (.41-.61 kg/cm 2 ), check fuel lines and fuel tank sending unit for restriction. If fuel lines and sending unit are okay, replace fuel pump.

HEI-EST DISTRIBUTOR

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

Spark

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.

Ignition Coil Power Source

1. Turn ignition on. On models with remotely mounted ignition coil, 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.
2. On models equipped with integrally mounted ignition coil, check voltage between BAT terminal and ground at distributor. If battery voltage does not exist, check for open circuit, blown ignition fuse or defective ignition switch.

Ignition Coil Resistance (Externally Mounted)

1. Remove coil connectors and secondary coil wire. In test "A", use high ohmmeter scale. Resistance value should be very high (infinite). (Scheme 186) 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. If no continuity exists, replace coil.

Scheme 186

Ignition Coil Resistance (Internally Mounted)

1. Turn ignition off. Remove distributor cap and coil assembly. Invert cap. Set ohmmeter to low scale. Connect leads to coil BAT and TACH terminals. (Scheme 187) If resistance exceeds approximately zero, replace ignition coil.
2. Set ohmmeter on high scale. Connect one lead to coil secondary terminal and other lead first to TACH terminal and then to ground terminal. If resistance reading in BOTH instances is infinite, replace ignition coil.

Scheme 187

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, and connect one lead to either pick-up coil lead and other lead to distributor housing. (Scheme 188) Flex pick-up coil leads by hand to check for intermittent shorts to ground. If reading is not always infinite, replace pick-up coil.
2. Connect ohmmeter between both pick-up coil leads. Check for intermittent opens by flexing wires and connectors. If resistance is not 500-1500 ohms, replace pick-up coil.

Scheme 188

TACH Pulse (RPM) Signal

Use a Scan tester connected to ALDL connector of vehicle to check for a tach pulse signal. RPM should be indicated on tester when engine is being cranked or is running. Tach pulse (RPM reference) is indicated as a voltage signal when a DVOM (minimum 10-megohm input impedance) is touched to circuit No.430 ECM terminal. (Scheme 176)- (Scheme 180).

IDLE SPEED & IGNITION TIMING (GASOLINE)

Ensure idle speed and ignition timing are set to specifications. For adjustment procedures, see ADJUSTMENTS article in this section.

DIAGNOSTIC CIRCUIT CHECK (GASOLINE)

Diagnostic Circuit Check determines

1. If SERVICE ENGINE SOON light works.
2. If ECM is operating and can recognize a fault.
3. If 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 192)

After performing necessary tests in diagnostic circuit check and if no codes are indicated and driveability problems still exist, refer to TESTS W/O CODES and SCAN TESTER USAGE in the TESTS W/CODES - GASOLINE article in this section.

1. Check SERVICE ENGINE SOON light operation. Turn ignition on with Scan tester not connected, ALDL test terminal not grounded and engine not running. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3).
2. Grounding ALDL diagnostic test terminal "B" now should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light goes from bright to dim or if light remains on and does not flash Code 12, see A-2, SERVICE ENGINE SOON LIGHT ALWAYS ON OR WON'T FLASH CODE 12.
3. If light begins to flash as soon as ignition is turned on, check for a short to ground on diagnostic test terminal wire between ALDL diagnostic terminal "B" and ECM terminal. If circuit is okay, replace ECM. (Scheme 189)- (Scheme 192).

Scheme 189

Scheme 190

Scheme 191

Diagnostic Circuit Check & Work Flow. Scheme 192

A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE

1. If SERVICE ENGINE SOON light 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 ECM fuse. If fusible links or ECM fuse are blown, repair short to ground.
2. If fusible links and ECM fuse are okay, turn ignition on, and check power circuits to ECM, including keep-alive memory and ignition feed. See the «WIRING DIAGRAMS»(/chevrolet/cutaway-p30/1985-1997/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in this section for power terminal identification. If power is not available to power terminals of ECM, check for opens in power circuits. If power is available to ECM power terminals, check for poor ECM ground circuits. If circuits are okay, replace faulty ECM.
3. If engine starts and SERVICE ENGINE SOON light does not illuminate, turn ignition off. Disconnect ECM connectors. Turn ignition on, and jumper ECM SERVICE ENGINE SOON light driver terminal to ground using a test light. See the «WIRING DIAGRAMS»(/chevrolet/cutaway-p30/1985-1997/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in this section for power terminal identification.
4. If light is now on, repair light driver terminal connections at ECM or replace faulty ECM. If light 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 ECM and bulb, short in driver circuit to voltage or open in ignition feed to SERVICE ENGINE SOON light.

A-2, SERVICE ENGINE SOON LIGHT ALWAYS ON OR WON'T FLASH CODE 12

1. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If SERVICE ENGINE SOON light is on, check for short to ground in light driver circuit between light and ECM driver terminal. See the «WIRING DIAGRAMS»(/chevrolet/cutaway-p30/1985-1997/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in this section for terminal identification.
2. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off. Using a DVOM, check voltage at ALDL diagnostic "test" terminal "B". (Scheme 189)- (Scheme 192). If voltage is greater than 9 volts, check for a short to voltage on ALDL terminal "B" wire between ECM and ALDL connector.
3. If voltage is less than 5 volts, use DVOM to backprobe appropriate ECM terminal. See the «WIRING DIAGRAMS»(/chevrolet/cutaway-p30/1985-1997/remont/wiring-diagrams-engine-performance/#wiring-diagrams) article in this section for terminal identification. If 5-6 volts is now present, repair open or short in wire between ECM and ALDL terminal "B". If voltage is 5-6 volts, go to step 4).
4. If voltage at terminal "B" of ALDL connector is 5-6 volts, jumper wire terminal at ECM to ground. If SERVICE ENGINE SOON light now flashes a Code 12 and ALDL terminal "A" was used when previously grounding terminal "B", check terminal "A" for open circuit.
5. If SERVICE ENGINE SOON light does not flash when ECM end of terminal "B" wire is jumpered to ground, check PROM/MEM-CAL for proper installation. If PROM/MEM-CAL is installed correctly, replace ECM. Install original PROM/MEM-CAL into replacement ECM. Repeat diagnostic circuit check. If Code 12 still does not flash, replace PROM/MEM-CAL. Replace PROM/MEM-CAL only after replacing ECM, as PROM/MEM-CAL is probably not at fault.

DIAGNOSTIC CIRCUIT CHECK (DIESEL)

Note. The 6.2L model with light duty emissions is equipped with a self-diagnostic system. For information on diagnostic circuit check and retrieving codes, see TESTS W/CODES - DIESEL article in this section.

Preliminary Inspection

1. Check all vacuum hoses for incorrect routing, restrictions, cuts or other damage. Inspect hoses beneath air cleaner assembly and other engine components.
2. Inspect all wires in engine compartment for correct and good connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. Perform preliminary inspection CAREFULLY and THOROUGHLY. A thorough inspection often leads to fixing a problem without further diagnosis.

Diagnostic Procedure

1. Ensure all engine systems not related to Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all problems not related to DEC system have been repaired. DEC system diagnosis should always begin with DIAGNOSTIC CIRCUIT CHECK chart in «TESTS W/CODES - DIESEL»(/chevrolet/cutaway-p30/1985-1997/remont/testing-diagnostics/#engine-controls-tests-wcodes-diesel) article in this section. Performing this check determines whether DEC system and ECM are working properly.
2. If trouble codes (other than Code 12) are displayed, determine whether codes are "hard" or "intermittent" trouble codes. A hard code remains until code-setting condition is repaired. Hard codes cause SERVICE ENGINE SOON light to come on.
3. An intermittent code does not reset itself and is not present when vehicle is repaired. An intermittent code is often caused by loose connections. SERVICE ENGINE SOON light goes out 10 seconds after fault goes away.

Scheme 193

SUMMARY

If no faults were found while performing BASIC TESTING , proceed to article TESTS W/CODES - DIESEL (for Diesel engines) or article TESTS W/CODES - GASOLINE (for Gas engines) or refer to article in TESTS W/CODES - TRANSMISSION in this section (if transmission codes are displayed). If no hard codes are found in self-diagnostics, or vehicle does not have a self-diagnostic system, proceed to TESTS W/O CODES article in this section for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.

See also:
• SYSTEM/COMPONENT TESTS
• ENGINE COOLING FAN
• TESTS W/CODES - DIESEL
• TESTS W/O CODES
• ADJUSTMENTS
• TESTS W/CODES - GASOLINE
• WIRING DIAGRAMS
• TESTS W/CODES - TRANSMISSION
• BASIC FUEL SYSTEM CHECKS