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.
The internal mode switch (IMS) consists of 5 internal switches, A, B, C, P and Neutral Start (NS). The IMS is attached to the selector shaft, and the switches change state depending on the position of the selector shaft. The TCM uses four of the internal switches, A, B, C, and P, to determine the gear range that is selected. The ECM uses the NS switch as confirmation that the transmission is in neutral or park before the engine starter is engaged.