Electric Rear Defrost Feedback Circuit Malfunction - HVAC System - Automatic

Electric Rear Defrost Feedback Circuit Malfunction:

CIRCUIT DESCRIPTION
When the rear defog switch is pressed, the Automatic Control Head sends a class II message via data link on circuit 1807 to the body control module (BCM). The BCM will respond by grounding circuit 193, coil side of the rear defog relay.

This action energizes the rear defog relay providing power to circuit 293 (rear defog beater elements), circuit 683 (heated mirror relay coil) and circuit 683 (fear defrost LED telltale). Momentarily switching circuit 193 to ground again will result in the BCM turning the rear defrost relay "OFF."

The first time in an ignition cycle the rear defrost is activated, rear defrost will remain "ON" for 15 minutes or until the ignition switch is turned "OFF." If rear defrost is activated again during the same ignition cycle, rear defrost will remain "ON" for 7(1/2) minutes or until the ignition switch is turned "OFF."

TEST

Diagnostic Chart: