Engine Controls - Tests W/codes - Diesel: Diagnosis Chevrolet Sportvan G20

SELF-DIAGNOSTIC SYSTEM

The 6.2L light duty emissions system includes a self-diagnostic system which can determine input signal circuit malfunctions. Input signal circuits determine engine function control.

PRELIMINARY INSPECTION

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult to see vacuum hoses beneath the air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed CAREFULLY and THOROUGHLY as it can often lead to fixing a problem without further diagnosis.

DIAGNOSTIC PROCEDURE

1. Ensure all engine systems not related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all other non-DEC system problems are repaired.
2. The DEC system diagnosis should ALWAYS begin with the DIAGNOSTIC CIRCUIT CHECK chart. This determines if the DEC system and ECM are working properly. If trouble codes are displayed (other than Code 12), determine if codes are "hard" or "intermittent" trouble codes.
3. A "hard" code is one which is present when working on vehicle and the condition still exists. "Hard" codes will cause the SERVICE ENGINE SOON light to come on.
4. An "intermittent" code is one which does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see H - TESTING W/O CODES article in ENGINE PERFORMANCE.

ENTERING OR EXITING DIAGNOSTIC MODE

1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). (Scheme 28) The Diesel Electronic Control (DEC) system will enter the diagnostic mode.
2. In this mode, the ECM will display Code 12, by flashing the SERVICE ENGINE SOON light. Code 12 consists of one (1) flash, followed by a short pause, then 2 flashes in quick succession.
3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

Scheme 28

CLEARING TROUBLE CODES

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require that codes be cleared before using diagnostic chart. To clear codes, remove ECM memory battery voltage for 30 seconds.

If no hard fault codes are present (only intermittent codes exist), proceed to H - TESING W/O CODES article for intermittent diagnostic procedures.

SPECIAL TOOLS (DIAGNOSTIC)

Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line. These testers can save a great deal of time. For additional information, see owners manual included with tester.

TEST EQUIPMENT

A tachometer, test light, Digital Volt/Ohmmeter (DVOM) with a minimum 10-megohm input impedance, a vacuum gauge, and jumper wires are required to test and diagnose DEC system. A "Scan" tester may also be used to access data parameters. Tester will supply a visual reading of most inputs to the ECM and some outputs.

DIAGNOSTIC CHARTS

The following diagnostic flow-charts and circuit schematics are provided courtesy of General Motors Corp.

DIAGNOSTIC CIRCUIT CHECK

The ECM provides diagnostic logic to detect faults in the Diesel Electronic Control (DEC) system. When the ECM recognizes a fault, it can turn on SERVICE ENGINE SOON light and store codes. If the condition is corrected, the SERVICE ENGINE SOON light will turn off. The ECM recognizes errors in engine speed and EGR vacuum, as well as electrical faults involving the 5-volt reference circuit. The ECM controls the following

1. Exhaust Gas Recirculation (EGR)
2. Exhaust Pressure Regulation (EPR)
3. Torque Converter Clutch (TCC)
4. System Diagnosis
5. Cold Advance & Glow Plugs

To allow proper engine control, the ECM monitors the following inputs

1. Engine RPM
2. Manifold Absolute Pressure (MAP)
3. Throttle Position Sensor (TPS)
4. Vehicle Speed Sensor (VSS)
5. Coolant Temperature Sensor (CTS)

All diagnosis begins with DIAGNOSTIC CIRCUIT CHECK chart. After any DEC system repair, repeat the diagnostic circuit check.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This tests SERVICE ENGINE SOON light operation. With ignition on and engine off, the light should be on.
2. Grounding the diagnostic terminal allows ECM to flash Code 12 and any stored codes. The light must go on and off for a proper code. If light goes from bright to dim, this is not considered a code.

Diagnostic Circuit Check Diagram. Scheme 29

Diagnostic Circuit Check Flow Chart. Scheme 30

Diagnostic Aids

Check circuit No. 410 routing for a potential short to circuit No. 452 or ground. SCAN tester displays engine temperature in degrees Celsius. After tarting engine, temperature should rise to about 90°C and stabilize when thermostat opens. Use COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart to test coolant sensor at various temperature levels. An out-of-calibration sensor can result in poor driveability.

Code 14: Circuit Diagram CTS Sensor Signal Voltage Low. Scheme 31

Code 14: Flow Chart CTS Sensor Signal Voltage Low. Scheme 32

SCAN tester displays engine temperature in degrees Celsius. After starting engine, temperature should rise to about 90°C and stabilize when thermostat opens. If Code 12 or 21 are also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor connector.

Use COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart to test coolant sensor at various temperature levels. An out-of-calibration sensor can result in poor driveability.

Code 15: Flow Chart CTS Sensor Signal Voltage High. Scheme 33

Disregard Code 23 if SERVICE ENGINE SOON light goes out when throttle is returned to idle.

Code 23: Circuit Diagram & Flow Chart TPS Misadjusted. Scheme 34

With drive wheels turning, SCAN tester and speedometer reading should be close.

Code 24: Circuit Diagram VSS Circuit. Scheme 35

Code 24: Flow Chart VSS Circuit. Scheme 36

A vacuum leak may cause a Code 32. Check all vacuum hoses and components connected to vacuum hoses for leaks. Also check cruise control and A/C systems vacuum hoses and components.

Code 32: Flow Chart EGR Circuit Loop Error. Scheme 37