Rear Defogger Switch and HVAC Control Module

The vehicle is equipped with one of two different types of HVAC systems which affect rear window defogger operation. These are:

• Manual air temperature control (RPO C60)
• Automatic air temperature control (RPO C68)

While the body control module (BCM) controls the voltage applied to the rear defog relay on both systems, the way in which the BCM receives rear defog switch information differs depending upon which type of HVAC System the vehicle uses.

On vehicles equipped with C68, rear defog switch activations are detected and interpreted by the HVAC control module, which then sends a GMLAN serial data message requesting rear window defogger operation to the BCM. At the same time, the HVAC control module illuminates the rear defog indicator LED on the switch button.

On vehicles equipped with C60, the rear defog switch is hard wired to the BCM. The BCM supplies voltage, through the rear defog switch signal circuit, to one side of the switch contacts. The other side of the switch contacts are grounded to G201. When the switch button is pressed, the switch contacts are closed to ground and the BCM interprets the flow of voltage as a specific rear defogger request. The BCM then supplies voltage to the rear defog relay and simultaneously to the rear defog indicator LED on the switch button, via the rear defog indicator supply voltage circuit. The rear defog indicator is grounded to G201.

Specific circuit and component failures may produce different symptoms depending upon which type of HVAC System the vehicle uses. When diagnosing a failure of the rear window defogger feature, carefully follow the Test Descriptions, as well as the step by step instructions of the appropriate diagnostic table in order to avoid misdiagnosis.