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DTC U1500: ONSTAR(R) Serial Data Circuit Failure: Testing

WARNING: This page is about a different car, the 2001 Cadillac Seville. However, it is still accessible from the selected car via links, so may be relevant.
  1. If cellular communication diagnostic system check has been performed, go to next step. If cellular communication diagnostic system check has not been performed, perform CELLULAR COMMUNICATION DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. Turn ignition switch to OFF position. Disconnect Vehicle Communication Unit (VCU) 25-pin harness connector. VCU is located behind rear seat. sEE Figure . Turn ignition switch to RUN position. Measure voltage between ground and VCU harness connector terminal No. 18 (Tan/Black wire). See Fig 1 . Voltage should be 0.2 volts. If voltage is as specified, go to next step. If voltage is not as specified, go to step  5 .
  3. Measure voltage between ground and VCU harness connector terminal No. 12 (White wire). Voltage should be 5 volts. If voltage is as specified, go to next step. If voltage is not as specified, go to step  6 .
  4. Measure voltage between ground and VCU terminal No. 4 (Orange wire). Voltage should be 12 volts. If voltage is not as specified, go to step  7 . If voltage is as specified, go to step  8 .
  5. Inspect Tan/Black wire for a short to ground between ground and VCU harness connector terminal No. 18. See Fig 1 . If problem does not exist, go to step  9 . If problem exists, repair Tan/Black wire as necessary. After repair, go to step  12 .
  6. Inspect White wire for an open or short to voltage between VCU harness connector terminal No. 12 and Vehicle Interface Unit (VIU) harness connector C1 terminal B12. See Fig 1 and Fig 2 . If problem does not exist, go to step  9 . If problem exists, repair White wire as necessary. After repair, go to step  12 .
  7. Inspect Orange wire for an open or short to ground between rear fuse block and VCU harness connector terminal No. 4. If problem does not exist, go to step  9 . If problem exists, repair Orange wire as necessary. After repair, go to step  12 .
  8. Inspect VCU harness connector for poor connections. If problem does not exist, go to step  10 . If problem exists, repair VCU harness connector as necessary. After repair, go to step  12 .
  9. Inspect VIU harness connector for poor connections. If problem does not exist, go to step  11 . If problem exists, repair VIU harness connector as necessary. After repair, go to step  12 .
  10. Replace VCU. Program VCU. See VEHICLE COMMUNICATION UNIT/VEHICLE INTERFACE UNIT (ONSTAR(R))  under PROGRAMMING. After repair, go to step  12 .
  11. Replace VIU. Program VIU. See VEHICLE COMMUNICATION UNIT/VEHICLE INTERFACE UNIT (ONSTAR(R))  under PROGRAMMING. After repair, go to next step.
  12. Using scan tool, clear DTCs. Operate vehicle within code enable criteria. See CODE ENABLE CRITERIA  . If DTC resets, repeat test beginning at step  2 .
Fig 1: Identifying Vehicle Communication Unit Harness Connector Terminals
G00050901Courtesy of GENERAL MOTORS CORP.
Fig 2: Identifying Cellular Telephone & Vehicle Interface Unit Harness Connectors & Terminals
G00011708Courtesy of GENERAL MOTORS CORP.
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