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Engine Controls - Basic Testing: Other Chevrolet Pickup K1500

Testing & Diagnostics 12 illustrations ~4365 words

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.

WARNINGBecause 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 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.
  3. Diagnosis Step 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 ). Step 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.

Scheme 15

Scheme 15: NO START - ENGINE CRANKS OKAY (4.3L WITH CMFI)
Scheme 15: NO START - ENGINE CRANKS OKAY (4.3L WITH CMFI)

Scheme 16

Scheme 16
Scheme 16

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 17)- (Scheme 24). 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 "+" terminal on module 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). See «A-7, BASIC FUEL SYSTEM CHECKS (GASOLINE)»(ref-19434-S09913332462001010200000) .
  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 (also called a "noid" light) to injector harness. Crank engine, and note light. If light flashes, check for stored ECM codes. See DIAGNOSTIC CIRCUIT CHECK (GASOLINE). If no codes are present, refer to HARD START symptom in TESTS W/O 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 - Purple/White wire).
  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 test light flashes, suspect a faulty ignition module or wiring. If wiring is okay, replace faulty ECM. Before replacing ECM, check ECM 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 article in this section.

Fuel System (4.3L CPI)

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 (GASOLINE).

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 TESTS W/O 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 17

Scheme 17
Scheme 17

Scheme 18

Scheme 18
Scheme 18

Scheme 19

Scheme 19
Scheme 19

Scheme 20

Scheme 20
Scheme 20

Scheme 21

Scheme 21
Scheme 21

Scheme 22

Scheme 22
Scheme 22

Scheme 23

Scheme 23
Scheme 23

Scheme 24

Scheme 24
Scheme 24

NO START - ENGINE CRANKS OKAY (3.8L WITH C(3)I)

Note. Before performing following tests, check battery condition, engine cranking speed and fuel level in tank.

Model 700 TBI Unit (2.5L)

Place transmission gear selector in Park (Neutral on M/T). 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)

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.

CPI & Port Fuel Injection

Fuel system is under pressure, which must be relieved before 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 allow it to run until 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.

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»(ref-19434-S33811069032001010200000) . 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-.91 kg/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. 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. 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 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 SYSTEM/COMPONENT 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.

CPI (4.3L)

  1. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(ref-19434-S33811069032001010200000) . 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 54-64 psi (3.8-4.5 kg/cm 2 ) and should hold steadily.
  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 no fuel pressure is 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 pressure does not approach 64 psi (4.5 kg/cm 2 ), replace CPI assembly. If pressure approaches 64 psi (4.5 kg/cm 2 ), no problem is present.
  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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT TESTS article.
  6. Gradually pinch off flexible fuel return line between CPI assembly and fuel tank. If fuel pressure increases to within specifications, 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 partial 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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT TESTS 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»(ref-19439) article.
  9. Relieve fuel pressure. Disconnect flexible fuel return line. Attach 5/16 ID 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.

PFI (4.3L Turbo)

  1. Relieve fuel pressure. See «FUEL SYSTEM PRESSURE RELIEF»(ref-19434-S33811069032001010200000) . 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 ).
  2. If pressure is as specified, go to next step. If pressure is greater than specification, replace faulty pressure regulator. If pressure is less than specification, go to step 4). If no fuel pressure is present, go to step 6).
  3. Start engine. Pressure should drop 3-10 psi (.21-.70 kg/cm 2 ). If pressure does not drop, replace faulty pressure regulator. If pressure drops, 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.
  4. 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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT TESTS article .
  5. 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.
  6. 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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT 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.
  7. If fuel pressure is present with voltage applied to test connector, test fuel pump relay and voltage supply to relay. See «SYSTEM/COMPONENT TESTS»(ref-19439) article.

PFI (3.8L)

  1. Relieve fuel pressure using J 34730-1 or equivalent. See «FUEL SYSTEM PRESSURE RELIEF»(ref-19434-S33811069032001010200000) . Turn ignition on. With ignition on and engine off, pressure should be 41-47 psi (2.84-3.25 kg/cm 2 ).
  2. If pressure is as specified, go to next step. If pressure is greater than specification, replace faulty pressure regulator. If pressure is less than specification, go to step 4). If no fuel pressure is present, go to step 6).
  3. Start engine. Pressure should drop 3-10 psi (.21-.70 kg/cm 2 ). If pressure does not drop, replace faulty pressure regulator. If pressure drops, 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.
  4. 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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT TESTS article.
  5. 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.
  6. 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»(ref-19439-S20468294722001010200000) in SYSTEM/COMPONENT TESTS 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.
  7. If fuel pressure is present with voltage applied to test connector, test fuel pump relay and voltage supply to relay. See «SYSTEM/COMPONENT TESTS»(ref-19439) article.

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. see scheme 14- see scheme 21. 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.

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

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 25) 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 25

Scheme 25
Scheme 25

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 26) Flex pick-up coil leads by hand to check for 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 26) Check for intermittent open by flexing wires and connectors. If resistance is not 500-1500 ohms, replace pick-up coil.

Scheme 26

Scheme 26
Scheme 26

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 ECM terminal. (Scheme 17)- (Scheme 24). If tach pulse signal is not present, vehicle will not run.

Disconnect tachometer wire if equipped. A shorted tachometer will prevent vehicle from starting. Disconnect cam sensor, and attempt to start engine. If engine starts, see appropriate cam sensor trouble code in appropriate TESTS W/CODES article in this section. Check for adequate spark using Spark Tester (ST-125). Check for spark on plug wires No. 1, 3 and 5 (one at a time). Leave matching plug wire connected while checking for spark. 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 module. If battery voltage is not present, check for blown ignition fuse. If fuse is not blown, check for open between fuse and ignition module.

Disconnect ignition coil leads. Use an ohmmeter to check ignition coil resistance. Primary resistance should be .5-.9 ohm. Secondary resistance should be 5000-8000 ohms. Replace ignition coil if not within specification.

Tach Pulse (RPM Reference) 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 ECM terminal. (Scheme 17)- (Scheme 24).

IDLE SPEED & IGNITION TIMING (GASOLINE)

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

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 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 WIRING DIAGRAMS article in this section for power terminal identification. If power to ECM power terminals is not available, check for open in power circuits. If power to ECM power terminals is available, check for poor ECM ground circuits. If circuits are okay, replace faulty ECM.
  3. Turn ignition off. Disconnect ECM connectors. Turn ignition on, and connect ECM MIL driver terminal to ground using a test light. See WIRING DIAGRAMS article in this section in ENGINE PERFORMANCE for power terminal identification.
  4. If MIL is now on, repair MIL driver terminal connections at ECM, or replace faulty ECM. 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 ECM and bulb, short in driver circuit to voltage or open in ignition feed to MIL.

A-2, MALFUNCTION INDICATOR LIGHT (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 step 2). If Code 12 now flashes, problem is a no serial data. Check serial data circuit for open or short to ground, or short to voltage between ECM and DLC terminal. If circuits are okay, replace faulty ECM or PROM (MEM-CAL).
  2. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If MIL is on, check for short to ground in MIL driver circuit between MIL and ECM driver terminal. see scheme 14- see scheme 21. Also, see WIRING DIAGRAMS article in this section in ENGINE PERFORMANCE for terminal identification.
  3. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off and diagnostic terminal not grounded. Using a test light connected to ground, backprobe ECM terminal circuit to DLC terminal "B". see scheme 14- see scheme 21. If MIL does not flash Code 12, go to step 4). If MIL flashes Code 12, check for open in DLC test terminal "B" circuit to ECM. If circuit is okay, check for open in DLC terminal "A" circuit to ECM and ECM ground. see scheme 14- see scheme 21.
  4. Check PROM (MEM-CAL) for proper installation. If installation is okay, replace ECM 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.

SUMMARY

If faults were not found while performing BASIC TESTING, proceed to appropriate TESTS W/CODES article in this section. 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.

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