Engine Controls - Basic Testing: Diagnosis GMC Vandura P3500

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.

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.

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 180

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. see scheme 2- see scheme 6. 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 180) 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. see scheme 14 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.

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.

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.

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.

FUEL SYSTEM PRESSURE TEST

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.

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 184)

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 181)- (Scheme 184).

Scheme 181

Scheme 182

Scheme 183

Diagnostic Circuit Check & Work Flow. Scheme 184

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»(ref-10254) 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 185