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:
- Exhaust Gas Recirculation (EGR)
- Exhaust Pressure Regulation (EPR)
- Torque Converter Clutch (TCC)
- System Diagnosis
- Cold Advance & Glow Plugs
To allow proper engine control, ECM monitors the following inputs:
- Engine RPM
- Manifold Absolute Pressure (MAP)
- Throttle Position Sensor (TPS)
- Vehicle Speed Sensor (VSS)
- 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.
- This tests SERVICE ENGINE SOON light operation. With ignition on and engine off, light should be on.
- 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.