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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 170 (BODY CONTROL MODULES)DIAGNOSTIC TESTSDTC U1500: ONSTAR® (GENERATION 2.6) SERIAL DATA CIRCUIT MALFUNCTIONTEST PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Test Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Test Procedure
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. 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. Turn ignition switch to RUN position. Measure voltage between ground and VCU connector terminal No. 11 (Light Blue/White wire). See Figure. If voltage is greater than 4 volts, go to next step. If voltage is not greater than 4 volts, go to step  7.
  3. Measure voltage between ground and VCU harness connector terminal No. 12 (White wire). If voltage is greater than 4 volts, go to next step. If voltage is not greater than 4 volts, go to step  8.
  4. Measure voltage between ground and VCU terminal No. 4 (Orange wire). If battery voltage is present, go to next step. If battery voltage is not present, go to step  9.
  5. Turn ignition switch to OFF position. Reconnect VCU connector. Disconnect VIU connector C1. Remove OnStar® serial data return circuit from VIU connector C1 terminal B11 and reconnect VIU connector C1. See Figure. Connect a DVOM between OnStar® serial data return circuit and a good ground. Turn ignition switch to ON position. Set DVOM to DC voltage scale and enable MIN/MAX feature. Allow meter to record for 45 seconds. If maximum voltage is not less than 4 volts, go to next step. If maximum voltage is less than 4 volts, go to step  10.
  6. If minimum voltage is greater than 0.1 volt, go to step  11. If minimum voltage is not greater than 0.1 volt, go to step  13.
  7. Inspect Light Blue/Black wire for a short to ground between VCU connector terminal No. 11 and VIU connector C1 terminal B11. See Figure and Figure . If problem does not exist, go to step  13. If problem exists, repair Light Blue/Black wire as necessary. After repair, go to step  14.
  8. Inspect White wire for an open or short to voltage between VCU connector terminal No. 12 and Vehicle Interface Unit (VIU) connector C1 terminal B12. See Figure and Figure . If problem does not exist, go to step  13. If problem exists, repair White wire as necessary. After repair, go to step  14.
  9. Inspect battery supply voltage (Orange wire) for an open or short to ground between VCU connector terminal No. 4 and VIU connector C1 terminal A16. If problem does not exist, go to step  13. If problem exists, repair Orange wire as necessary. After repair, go to step  14.
  10. Inspect OnStar® serial data return circuit (Tan/Black wire) between Vehicle Interface Unit (VIU) connector C1 terminal B11 and VCU terminal No. 18 for an open or short to ground. If problem does not exist, go to step  12. If problem exists, repair OnStar® serial data return circuit (Tan/Black wire) as necessary. After repair, go to step  14.
  11. Inspect OnStar® serial data return, OnStar® serial data (-) and OnStar® serial data (+) at VIU harness connector terminals B11, B12 and B13, respectively for short to voltage. If problem does not exist, go to next step. If problem exists, repair VIU harness connector as necessary. After repair, go to step  14.
  12. Inspect VIU harness connector for poor connections. If problem does not exist, replace VIU. See ONSTAR® MODULES  under REMOVAL & INSTALLATION. Program VIU. See VEHICLE COMMUNICATION UNIT/VEHICLE INTERFACE UNIT (ONSTAR®)  under PROGRAMMING. After repair, go to next step.
  13. Inspect VCU harness connector for poor connections. If problem does not exist, replace VCU. See ONSTAR® MODULES  under REMOVAL & INSTALLATION. Program VCU. See VEHICLE COMMUNICATION UNIT/VEHICLE INTERFACE UNIT (ONSTAR®)  under PROGRAMMING. After repair, go to step  14.
  14. Using scan tool, clear DTCs. Operate vehicle within code enable criteria. See CODE ENABLE CRITERIA . If DTC resets, repeat test beginning at step  2.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.