Engine Controls - Tests W/codes - Diesel: Diagnosis GMC Pickup C2500

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 air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chafed 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 fix a problem without requiring 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 non-DEC system problems are repaired.
2. ALWAYS begin diagnosis with checking diagnostic circuit to determine if DEC system and ECM are working properly. See «DIAGNOSTIC CIRCUIT CHECK»(ref-10393-S05722031492000110900000) chart in this article. If trouble codes (other than Code 12) are displayed, determine if they are hard or intermittent trouble codes.
3. A hard code is present while working on vehicle, and problem condition persists. Hard codes will cause SERVICE ENGINE SOON light to come on.
4. An intermittent code does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnostic procedures, see TESTS W/O CODES article.

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 144) The Diesel Electronic Control (DEC) system will enter diagnostic mode.
2. In this mode, ECM will display Code 12 by flashing the SERVICE ENGINE SOON light once, 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 diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper from ALDL connector.

Scheme 144

CLEARING TROUBLE CODES

Note. To prevent ECM damage, ensure ignition is in OFF position when disconnecting or reconnecting power to ECM.

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require codes to be cleared before using diagnostic chart. To clear codes, remove power to ECM for 30 seconds by either disconnecting battery cable and ECM pigtail to battery, or removing ECM "B" fuse.

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

SPECIAL TOOLS (DIAGNOSTIC)

Special scan testers, plugged into the ALDL connector, may be used to read trouble codes, and check voltages in system on serial data line. These testers can save a great deal of time. For additional information, see owner's 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 Diesel Electronic Control (DEC) system. A scan tester may also be used to access data parameters. Tester will supply a visual reading of most inputs, and some outputs, to ECM.

DIAGNOSTIC CHARTS

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

DIAGNOSTIC CIRCUIT CHECK

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

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, 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)

Begin all diagnosis with diagnostic circuit check. After any DEC system repair, repeat diagnostic circuit check.

Note. Test numbers refer to numbers on diagnostic chart.

1. This tests SERVICE ENGINE SOON light operation. With ignition on and engine off, light should be on.
2. Grounding ALDL diagnostic terminal "B" allows ECM to flash Code 12 and any stored codes. Light must flash on and off to be considered a proper code. If light just goes from bright to dim, it is not considered a code.

Scheme 145

Scheme 146

Scheme 147

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 starting engine, temperature should rise to about 90°C and stabilize when thermostat opens. Test coolant sensor at various temperature levels. See COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart. An out-of-calibration sensor can cause poor driveability.

Scheme 148

Scheme 149

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 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor connector. Test coolant sensor at various temperature levels. See COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart. An out-of-calibration sensor can cause poor driveability.

Scheme 150

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

Scheme 151

With drive wheels turning, scan tester and speedometer reading should closely match.

Scheme 152

Scheme 153

Scheme 154

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.

Scheme 155