GM Full-Size Trucks 1980-1987 Repair Guide

Diesel Electronic Control System California Only


The Diesel Electronic Control (DEC) system is used on 6.2L (379) engines built for use in California to control the exhaust gas recirculation system, exhaust pressure regulation system and the transmission converter clutch.

The primary component in the system is the diesel Electronic Control Module (ECM). To effectively control all of the systems, the diesel ECM monitors several engine parameters. These parameters include the engine rpm signal which is derived from the engine speed sensor, the vacuum level in the EGR system which is sensed using a manifold absolute pressure sensor, the throttle position which is sensed through the throttle position sensor, and vehicle speed signal which is derived from the vehicle speed sensor.

The ECM is a reliable solid state computer, protected in a metal box and located in the cab. It is used to monitor and control all the functions of the Diesel Electronic Control System. As explained previously, the ECM can perform several functions at the same time but it also has the ability to detect certain faults within the system. When it detects a fault in the system, the ECM will record what system or circuit that is faulty in memory. The ECM records the fault in the form of a diagnostic trouble code. The diagnostic trouble codes recorded in the ECM's memory can later be accessed to aid in diagnosis of the problem. It should be understood that the ECM can not detect all the possible failures that may be encountered.


Diagnosis of a driveability and/or emissions problems requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to perform any repairs before performing the preliminary diagnostic steps. In many cases this will shorten diagnostic time and often cure the problem without further testing. Perform diagnosis in the order listed below.

Visual/Physical Underhood Inspection

This is possibly the most critical step of diagnosis, and should be performed first. A detailed examination of connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other component. Wiring should be checked carefully for any sign of strain, burning, crimping, or terminal pull-out from a connector. Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit.

Reading Diagnostic Trouble Codes

Unlike the Computer Control Command System found on gasoline engines, the diagnostic codes stored in the Diesel Electronic System can not be accessed using a simple jumper wire. The diesel diagnostic check tool or equivalent must be used to access any stored codes in the ECM's memory. To use the check tool, follow the operating instructions supplied with the unit.

Clearing Trouble Codes

Stored fault codes may be erased from memory at any time by removing power from the ECM for at least 30 seconds. It may be necessary to clear stored codes during diagnosis to check for any recurrence during a test drive, but the stored codes must be written down when retrieved. The codes may still be required for subsequent troubleshooting. Whenever a repair is complete, the stored codes must be erased and the vehicle test driven to confirm correct operation and repair.

The ignition switch must be OFF any time power is disconnected or restored to the ECM. Severe damage may result if this precaution is not observed.

Depending on the electric distribution of the particular vehicle, power to the ECM may be disconnected by removing the ECM fuse in the fusebox, or disconnecting the positive battery terminal. Disconnecting the battery cables to clear codes is not recommended, as this will also clear other memory data in the vehicle such as radio presets or clock.