Circuit Description
WARNING: This page is about a different car, the 2008 Cadillac XLR. However, it is still accessible from the selected car via links, so may be relevant.
The OnStar® button assembly located on the inside rearview mirror consists of 3 buttons, Call/Answer, OnStar® Call Center, and OnStar® Emergency. The vehicle communication interface module (VCIM) supplies the OnStar® button assembly with 10 volts via the keypad supply voltage circuit. Each of the buttons, when pressed, completes the circuit across a resister allowing a specific voltage to be returned to the VCIM over the keypad signal circuit. Depending upon the voltage range returned, the VCIM is able to identify which button has been activated.