U1500

CIRCUIT DESCRIPTION
The Vehicle Communication Unit (VCU) and the Vehicle Interface Unit (VIU) communicate using the OnStar(R) Serial Data (-) circuit, OnStar(R) Serial Data (+) circuit and the OnStar(R) Serial Data Return circuit. The DTC U1500 is set when the VIU detects no communication between the VIU and the VCU.

CONDITIONS FOR SETTING THE DTC
^ The ignition must be in the RUN or ACC position.
^ The system voltage is at least 9.5 volts and no more than 15.5 volts.
^ All the above conditions are present for greater than 300 ms.

ACTION TAKEN WHEN THE DTC SETS
^ The VIU does not communicate with the VCU.
^ No calls can be placed.
^ The OnStar(R) status LED turns RED.

CONDITIONS FOR CLEARING THE DTC
^ The VIU does not detect a open on the battery positive voltage circuit.
^ The VIU does not detect a short to ground on the serial data return circuit.
^ The VIU does not detect a short to B+ on serial data (-) circuit.
^ A history DTC clears after 50 malfunction free ignition cycles.
^ The VIU receives the clear DTC command from the scan tool.

TEST DESCRIPTION

Steps 1-7:

Steps 8-12:

The numbers below refer to the step numbers on the diagnostic table.
2. By disconnecting the VCU you can check the three wire bus supply and carrier voltages supplied by the VIU.
3. By disconnecting the VCU you can check the three wire bus supply and carrier voltages supplied by the VIU.