Component Tests and General Diagnostics

ECM Main Relay Diagnosis

Diagnostic Instructions
- Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
- Review Strategy Based Diagnosis for an overview of the diagnostic approach.
- Diagnostic Procedure Instructions provide an overview of each diagnostic category.

Circuit/System Description
The engine relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The engine control module (ECM) supplies the ground path to the relay coil control circuit via an internal integrated circuit called an output driver module (ODM). The ODM output control is configured to operate as a low side driver for the engine relay. The ODM for the engine relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the engine relay ON, ignition 1 voltage is supplied to the ECM, and to several additional circuits.

Diagnostic Aids
This test procedure requires that the vehicle battery has passed a load test and is completely charged.

ECM Main Relay Diagnosis