For 1990-2009 cars

#### Vehicle Interface Unit (VIU)

The VIU voltage is supplied through three Fuses the GAUGES Fuse in the IP Fuse Block, the RADIO BATT fuse in the IP Fuse Block and through the RADIO Fuse in the IP Fuse Block. The VIU is grounded to G200. The VIU is capable of controlling the systems when command to do so by the OnStar® Call Center. The VIU controls the Locks through the Power Door Lock Relay Feed -- Unlock Coil circuit and the Power Door Lock Relay Feed -- Lock Coil Circuit . The VIU controls the horn by triggering the Horn Relay Output Coil circuit low. The VIU can turn on the Theft Alarm by controlling the Vehicle Antitheft System Signal -- Headlamp Alarm circuit. The VIU also interfaces and communicates with the Radio through the Diagnostic Signal -- Entertainment and Comfort Bus. The OnStar® audio is played through the radio on the Audio RF+ circuit, the Audio LF+ circuit and the Audio Common Circuit. The VIU receives an input from the Inflatable Restraint Sensing and Diagnostic Module through the Serial Data Signal -- UART circuit. The GPS antenna is connected to the VIU. The OnStar® system communicates wtih the rest of the vehicle through the Class 2 Data Link Communications Circuit.

#### Vehicle Communications Unit (VCU)

The VCU is supplied power from the VIU through the XCVR BATT and the XCVR IGN circuit. The VCU is grounded through the XCVR DIG GND circuit to the VIU. The Cellular antenna connects to the VCU. The VCU uses a three wire Bus to communicate with the VIU. This three wire Bus is a dedicated communication line between the VCU and the VIU. The audio commutations are made to the VCU through the VIU.

#### OnStar® Three Button Assembly

The Button Assembly is feed power from the VIU through the Keypad Power circuit. It is grounded with the Keypad GND circuit to the VIU. All Thee Button Assembly signal and return circuits go to the VIU.