Circuit/System Description

WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

On vehicles equipped with 4 wheel drive, the output speed sensor is on the transfer case. The instrument panel mounted transfer case shift control switch consist of a 2HI position, 4HI and 4 Low. The transfer case shift control module continuously monitors the signal to determine the position selected by the vehicle operator. The transfer case shift control module communicates the status of the transfer case shift control switch to the transmission control module (TCM). When the switch is in the 4 Low position, the TCM multiples the transfer case output shaft speed by the transfer case ratio in 4 Low to determine the transmission output shaft speed. The TCM uses this information to provide earlier upshifts and also to prevent an over-speed condition in 4 Low.