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