Diagnostic System Operation

The ECM of the DEC system is equipped with self-diagnostic capabilities which detect faults in the system (EGR system or Vehicle Speed Sensor). The system does not include a "CHECK ENGINE" light in the dash. Instead, trouble codes are indicated by using the Diesel Diagnostic Check (DDC) tester (J-34750 ).

An Assembly Line Communication Link (ALCL) connecter is used at the factory to check the system prior to shipment. The DDC tester is connected to the ALCL connector and cigar lighter (or BAT terminal in fuse panel) to diagnosis the DEC system. See Fig 1 . Trouble codes are indicated by a "Check Engine" light (CEL) on the DDC tester.

The DDC tester also has selectable diagnostic modes to evaluate a driveability problem. It is the only special tool required to diagnose the DEC system.

Fig 1: Diesel Diagnostic Check Tester This tester must be used to determine system failures.

Fig 2: ALCL Connector Identification Located under left side of dash on pickup models and under driver's seat on van models.

Fig 3: ECM Location ECM is under right side of dash on pickup models and under driver's seat on van models.

Fig 4: DEC System Component Locations for Trucks and Vans