Circuit/System Description

WARNING: This page is about a different car, the 2018 GMC Yukon XL, 2018 GMC Yukon, 2018 Chevrolet Tahoe, 2018 Chevrolet Suburban 3500 HD, and 2018 Chevrolet Suburban. 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 sends the status of the transfer case range out on the serial data line to other control modules. The transmission control module (TCM) monitors the status of the transfer range. When the transfer case is in the 4 Low range, 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.