Circuit/System Description
WARNING: This page is about a different car, the 2009 GMC Sierra and 2009 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
In Auto 4WD mode, the transfer case shift control module (TCCM) monitors the front and rear wheel speed sensors for slip. If the TCCM detects slip and is unable to correct for a difference between the front and rear speed sensors it will command the transfer case to apply the clutch.