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Home >> Pontiac >> 2002 >> Firebird Formula, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics - 5.7L >> Diagnostic Tests >> DTC P0502: Vehicle Speed Sensor Low Output (A/T) >> Diagnostic Procedures

Diagnostic Procedures

  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: Before clearing the DTC, use the scan tool in order to record the Freeze Frame and Failure Records. Using the Clear Info function erases the Freeze Frame and Failure Records from the PCM.
  2. Install a scan tool. Turn ON the ignition, with the engine OFF. Record the DTC Freeze Frame and Failure Records. Clear the DTC. Raise and support the rear axle assembly. Start the engine. Place the transmission in any drive range. With the drive wheels rotating, does the scan tool Transmission OSS increase with the drive wheel speed? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
  3. Turn OFF the ignition. Disconnect the PCM connector C2. Using the Digital Multimeter (DMM) and the J 35616-A GM Terminal Test Kit, measure the resistance between harness connector terminals C2-20 and C2-21. Does the resistance measure 1377-3355 ohms? If yes, go to next step. If no, go to step  7.
  4. Place the transmission in Neutral. Select AC volts. Prevent one rear wheel from turning. Rotate the other rear wheel by hand, ensuring that the driveshaft is turning. Does the voltage measure greater than 0.5 V? If yes, go to next step. If no, go to step  12.
  5. Measure the resistance from terminal C2-21 to ground. Does the resistance measure greater than 50 k/ohms? If yes, go to next step. If no, go to step  9.
  6. Reconnect the PCM connector C2. Disconnect the engine wiring harness from the VSS assembly. Turn ON the ignition, with the engine OFF. Test the high circuit (CKT 821) of the VSS assembly for a short to power. Test the low circuit (CKT 822) of the VSS assembly for a short to power. Did you find and correct the condition? If yes, go to step  15. If no, go to step  14.
  7. Disconnect the engine wiring harness from the VSS assembly. Measure the resistance of the VSS assembly. Does the resistance measure 1377-3355 ohms? If yes, go to next step. If no, go to step  13.
  8. Was the resistance measured in step  3 greater than 3355 ohms? If yes, go to step  10. If no, go to step  11.
  9. Test the high circuit (CKT 821) of the VSS assembly for a short to ground. Test the low circuit (CKT 822) of the VSS assembly for a short to ground. Did you find and correct the condition? If yes, go to step  15.
  10. Test the high circuit (CKT 821) of the VSS assembly for an open. Test the low circuit (CKT 822) of the VSS assembly for an open. Repair as necessary. After repairs, go to step  15.
  11. Test the high circuit (CKT 821) and the low circuit (CKT 822) of the VSS assembly for a short together. Repair as necessary. After repairs, go to step  15.
  12. Remove the VSS assembly. See appropriate AUTOMATIC TRANSMISSION article in TRANSMISSION SERVICING. Inspect the output shaft speed sensor rotor for damage or misalignment. Inspect the case extension bushing for wear. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  13. Replace the VSS assembly. After repairs, go to step  15.
  14. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  15. Perform the following procedure in order to verify the repair:
    • Select DTC.
    • Select Clear Info.
    • Operate the vehicle, so that the transmission output speed is greater than 250 RPM for 2 seconds.
    • Select Specific DTC.
    • Enter DTC P0502.

    Has the test run and passed? If yes, system is okay. If no, go to step  1.

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