Circuit/System Description

WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.

The internal mode switch (IMS), also known as the transmission manual shift shaft switch assembly, is a sliding contact switch attached to the manual shift shaft inside the transmission. The switch consists of 5 separate circuits, 4 of which indicate gear selector position to the transmission control module (TCM), and 1 that indicates Park/Neutral position to the engine control module (ECM). The ECM provides 12-volts to the Park/Neutral position (PNP) switch of the IMS. When the transmission in PARK or NEUTRAL, the switch closes and pulls the ECM voltage low, 0 volts. When the transmission is not in PARK or NEUTRAL, the ECM voltage is high, 12 volts. The ECM uses the PNP switch information in order to enable starter operation. The ECM park/neutral diagnostic monitors the park/neutral signal circuit to detect low voltage when the ECM expects to see high voltage and high voltage when the ECM expects to see low voltage.