Circuit/System Description
WARNING: This page is about a different car, the 2018 GMC Yukon XL, 2018 GMC Yukon, 2018 Chevrolet Tahoe, and 2018 Chevrolet Suburban. However, it is still accessible from the selected car via links, so may be relevant.
The all wheel drive (AWD/4WD Auto) indicator circuit consists of a light emitting diode (LED) feed circuit, a LED and a LED control circuit. The indicator dimming control circuit is the LED feed circuit and continuously supplies B+ to the 2WD HI, 4WD Auto, 4WD HI and 4WD LO LEDs. When the AWD mode has been selected by the driver, the transfer case shift control module (TCCM) provides a ground through the AWD indicator control circuit illuminating the LED.
The TCCM tests the AWD indicator circuit for unexpected voltage by monitoring the B+ supplied by the BCM on the indicator dimming control circuit by grounding the AWD indicator control circuit for 2 seconds during the Bulb Test at key ON.