Diagnostic Circuit Check

Circuit Description:
The ECM provides the diagnostic logic to detect faults in the systems that the ECM monitors or controls. The ECM, when it recognizes a fault, has the capability of turning a "Service Engine Soon" (SES) light "ON" and storing a code. Furthermore, if the condition corrects itself, the "Service Engine Soon" signal will be turned "OFF" immediately following the correction.
The ECM recognizes errors in engine speed, vehicle speed vacuum errors in the EGR vacuum loop, via the MAP sensor, and electrical faults involving the 5 volt reference circuit.
The ECM is a multifunction controller that controls the following:
1. Exhaust Gas Recirculation (EGR).
2. Exhaust Pressure Regulation (EPR).
3. Torque Converter Clutch Control (TCC).
4. System Diagnosis.
5. Cold Advance and Glow Plug.
The ECM monitors the following inputs, to allow proper control of the above.
1. Engine rpm.
2. Manifold Absolute Pressure (MAP) used to monitor EGR vacuum.
3. Throttle Position Sensor (TPS).
4. Vehicle Speed Sensor (VSS).
5. Coolant Temperature.
All diagnosis should start with the Diesel Diagnostic Circuit Check.
After any repair to the diesel electronic control system, the diesel diagnostic circuit check must be repeated.
A brief description of operation is included with each system check or code chart.

Test Description:
Numbers below refer to the circled numbers on the diagnostic chart.
1. Checks for proper operation of the "Service Engine Soon" light. With the key "ON," and the engine not running, the light should he "ON" steady.
2. Grounding the diagnostic terminal will flash a Code 12 and any stored codes. The light must go "ON" and "OFF," for a proper code. If the light goes from "bright" to "dim," this is not considered a code.