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Home >> Cadillac >> 2002 >> Eldorado ETC >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - Self-Diagnostics - 4.6L >> Diagnostic Tests >> DTC P0502: Vehicle Speed Sensor Low Output >> 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.
    CAUTION: Support the lower control arms in the normal horizontal position in order to avoid damage to the drive axles. Do not operate the vehicle in gear with the wheels hanging down at full travel.
  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 drive wheels. Start and idle the engine. Place the transmission in DRIVE. Monitor Transmission OSS on the scan tool. With the drive wheels rotating, does the Transmission OSS RPM increase when wheel speed increases? If yes, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
  3. Turn OFF the ignition. Disconnect the PCM connector C1. Using the J 35616-A connector test adapter kit, connect the Digital Multimeter (DMM) between PCM harness connector terminals C1-56 and C1-57. Select AC volts on the DMM. Place the range selector in NEUTRAL. Rotate the right front drive wheel by hand, while observing the DMM display. Does the voltage measure greater than 0.5 V AC? If yes, go to step  14. If no, go to next step.
  4. Leave the DMM test leads connected. Measure circuit resistance. Is the circuit resistance 1300-1950 ohms? If yes, go to step  8. If no, go to next step.
  5. Is the circuit resistance greater than 1950 ohms? If yes, go to step  11. If no, go to next step.
  6. Leave the DMM test leads connected. Disconnect the VSS connector at the transmission. Is the circuit resistance less than 1300 ohms? If yes, go to next step. If no, go to step  13.
  7. Repair the short between the high signal circuit (CKT 400) and the low signal circuit (CKT 401). After repairs, go to step  15.
  8. Test the high signal circuit (CKT 400) of the VSS for a short to ground between the PCM and the VSS. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  9. Reconnect the PCM connector C1. Select DC volts on the DMM. Using the J 35616-A connector test adapter kit, connect the DMM between terminals "A" and "B" of the VSS vehicle harness. Turn ON the ignition with the engine OFF. Does the DMM display system voltage? If yes, go to next step. If no, go to step  12.
  10. Test the high signal circuit (CKT 400) of the VSS for a short to voltage. After repairs, go to step  15.
  11. Test the high signal circuit (CKT 400) of the VSS for an open. Test the low signal circuit (CKT 401) of the VSS for an open. Did you find and correct the condition? If yes, go to step  15. If no, go to step  13.
  12. Remove the VSS assembly. See appropriate AUTOMATIC TRANSMISSION article in TRANSMISSION SERVICING. Inspect the VSS and the transmission for the following:
    • Incorrect VSS.
    • VSS damage.
    • Excessive runout in the final drive carrier assembly.
    • Excessive VSS to speed sensor rotor gap.
    • Incorrect speed sensor rotor alignment.
    • Speed sensor rotor damage.

    Repair any of the above items as necessary. 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.
    • Drive the vehicle.
    • 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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