U1500

DTC 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. DTC U1500 is set when the VIU detects no communication between the VIU and the VCU.

CONDITIONS FOR RUNNING 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.

CONDITIONS FOR SETTING THE DTC
- The VIU detects an open, short to ground or short to B+ on the serial data return circuit.
- The VIU detects an open, short to ground or short to B+ on serial data (-) circuit.
- The VIU detects an open, short to ground or short to B+ on serial data (+) circuit.
- The VIU detects an open on the battery positive voltage circuit.
- 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� status LED turns RED.

CONDITIONS FOR CLEARING THE DTC
- The VIU does not detect an open, short to ground or short to B+ on the serial data return circuit.
- The VIU does not detect an open, short to ground or short ro B+ on serial data (-) circuit.
- The VIU does not detect an open, short to ground or short ro B+ on serial data (+) circuit.
- The VIU does not detect an open on the battery positive voltage 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 - 4:

Steps 5 - 9:

Steps 10 - 14:

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