Diagnostic System Check - DEC Check Engine Light: (VIN T, V)

Fig. 003 - Chart for DEC Check Engine Light.:

DIESEL SYSTEM DIAGNOSTIC CHECK

A "CHECK ENGINE" Light (CEL) will come "ON" to indicate a fault has occurred in the system, but will not flash or store a fault code. The type of fault can be determined by CEL operation. The CEL will be "ON" steady for a vacuum error, or engine speed below 400 RPM. If a vacuum error is recognized, the ECM will turn the EGR "OFF". Vacuum errors include EGR, EPR, MAP and Vacuum Systems. The CEL may also be "ON" under certain load and RPM conditions. When the CEL is "throttle position" related, the fault will be in Engine Speed, VSS, or MVS systems.
It takes the ECM approximately 10 seconds to recognize a fault except for Engine Speed below 400 rpm which is recognized immediately. If a vacuum error is detected by the MAP sensor the CEL will come "on" and stay "on" until the key is cycled "off" and "on", even if the vacuum error is corrected. All other faults detected by the ECM that cause a CEL to appear will also allow the CEL to go "off" in 10 seconds if the fault is repaired or intermittent without first shutting the engine "off".
The Diesel Electronic Control Module (ECM) is a multifunction engine controller. The ECM controls the following:
1) Exhaust Gas Recirculation (EGR)
2) Exhaust Pressure Regulation Control (EPR)
3) Transmission Converter Clutch Control (TCC)
4) System Diagnosis
The ECM monitors the following inputs to allow proper engine control of the above:
1) Engine RPM
2) Vehicle Speed (VSS)
3) Absolute Pressure (MAP) used to monitor EGR vacuumcircuit
4) Metering Valve Sensor (MVS)
5) Engine temperature
6) 4th Gear input (if 4 speed transmission equipped)
The ECM is serviced as an assembly. See Section 8C, "Instrument Panel", for ECM replacement.
All diagnosis should start with the Diesel System Diagnostic Check found on the following page.
A brief description of operation is included with each System Check Chart. VIN T, VIN V

Fig. 006 - WIRING DIAGRAM for DEC Engine Light Check. VIN T, VIN V:

ENGINE LIGHT CHECK

The "CHECK ENGINE" light is used to indicate that the ECM has recognized a fault, and to aid in diagnosis of that fault. It is supplied 12V by the ECM fuse, and the ground is completed by the ECM to turn the light "ON". The light should normally be "ON" with the key "ON", and should go "OFF" when engine speed exceeds 400 RPM.

Fig. 005 - Chart for DEC Engine Light Check.:

1. Checks for power to ECM. Should be battery voltage.
2. Checks for an open circuit to ECM. Normal voltage to ECM at Terminal "E" should be battery voltage.
3. Checks for a good ECM ground. If there is a poor connection to ground, voltage at Terminal "K" will be above 1 volt. The poorer the ground, the higher the voltage.