Circuit/System Description
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The engine control module (ECM) provides 12 volts to the Park/Neutral switch circuit of the transmission internal mode switch (IMS). The IMS is a sliding contact assembly that attaches to the manual shaft inside of the transmission. The switch consists of five separate circuits, four of which indicate the gear selector position to the transmission control module (TCM), and one that indicates the Park/Neutral position to the ECM. When the transmission is in Park or Neutral, the switch closes and pulls the ECM voltage low on the signal circuit. When the transmission is not in Park or Neutral, the ECM voltage is high, near 12 volts. The ECM uses the switch in order to enable starter motor operation.