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 automatic transmission manual shift shaft position assembly, also known as an internal mode switch (IMS), is a sliding contact switch that mounts to the lower control valve body. The switch has a sliding rail that connects to the transmissions' manual valve. The slide rail follows the position of the manual valve to provide manual valve location to the transmission control module (TCM) and engine control module (ECM). Internal to the switch are metal contact finger that attach to a slide rail. The fingers make contact with segmented stainless steel strips that run the length of the switch. Contact between the fingers and the stainless steel strips will cause the TCM signal voltage to be Low, 0 volts, when there is no contact between a finger and a strip, the TCM signal voltage will remain High, 12 volts. As a result of the different High and Low signal combinations, the TCM and ECM can decode the transmission manual valve position. For diagnostic purposes, the High and Low signals can be displayed on the scan tool. There are four signal circuits used to identify the manual valve position and are identified as parameter A, B, C and P, parity. The switch also has two additional circuits which are D, a common ground circuit and S, a Park/Neutral start circuit for the ECM. The P/N signal circuit goes from the switch directly to the ECM, using the transmission 16-way connector as a passing through only. The P/N circuit does not go into the TCM, only the ECM, where it can be viewed on the scan tool through the ECM. The TCM and ECM use the decoded information for engine controls, starter enable, as well as determining the transmission shift patterns and driver shift requests (DSR) usage.