Circuit Description

CIRCUIT CHART
Circuit assignments for the Body Control Module (BCM) relating to heating and air conditioning:

ELECTRIC REAR DEFROST
When the rear defrost input circuit 292 to the BCM is momentarily switched to ground (as when the rear defrost switch is pressed), the BCM will respond by grounding circuit 193, the coil of the rear defrost relay.

This action energizes the rear defrost relay providing power to circuit 293 - rear defrost heater elements, and circuit 193- rear defrost LED telltale. Momentarily switching circuit 292 to ground again will result in the BCM turning the rear defrost relay Off.

The first time in an ignition cycle that 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.

A/C REQUEST
A/C request is made by the HVAC control head supplying voltage to circuit 603 to the BCM. The BCM will then request over the serial data link to the engine controller to operate the compressor. The BCM provides evaporator freeze protection by monitoring engine speed and ambient temperature.

HVAC FAN HI BLOW CONTROL
The BCM controls the fan operation when the HVAC fan switch is on HI. The BCM supplies voltage to the HVAC blower relay coil over circuit 52. This will close the relay contacts and provide battery voltage to the blower motor without going through the blower motor resistors.

The retailer can also enable the BCM to provide an A/C afterblow function to eliminate oder associated with A/C usage. This function operates by running the HVAC fan for a few minutes (approximately 30-40 minutes) after the vehicle has been turned Off, if the A/C has been used.