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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Automatic Transmission Diagnosis - 4L60-E >> Diagnostic Tests >> DTC P0502: VSS Circuit - Low Input >> Diagnostic Procedure

Diagnostic Procedure

  1. Did you perform the diagnostic system check? If so, go to next step. If not, see DIAGNOSTIC SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. Connect scan tool. Turn ignition on, engine off. Using scan tool, record freeze frame and failure records for reference, and then clear DTCs. Raise and support rear axle assembly. Start engine. Disable traction control system (if equipped). Place transmission in any drive range. Monitor output speed sensor parameter on scan tool with wheels rotating. If output speed increases with wheel speed, see DIAGNOSTIC AIDS . If output speed does not increase with wheel speed, go to next step.
  3. Turn ignition off. Disconnect appropriate PCM harness connector connected to VSS. See WIRING DIAGRAMS . Measure resistance between VSS terminals at appropriate PCM harness connector(s). If resistance is 976-2354 ohms (Corvette and 4WD) or 1377-3355 ohms (except Corvette and 4WD), go to next step. If resistance is not as specified, go to step  7.
  4. Place transmission in Neutral. Using DVOM, select AC volt scale. Hold one wheel from rotating. Rotate other wheel by hand, ensuring drive shaft rotates. If voltage is greater than .5 volt AC, go to next step. If voltage is not greater than .5 volt AC, go to step  12.
  5. Measure resistance between ground and VSS input circuit at PCM harness connector. If resistance is greater than 50 k/ohms, go to next step. If resistance is not greater than 50 k/ohms, go to step  9.
  6. Reconnect PCM harness connector. Disconnect VSS harness connector. Turn ignition on, engine off. Check VSS input and ground circuits for short to voltage. Repair as necessary. After repairs, go to step  15. If circuits are okay, go to step  14.
  7. Disconnect VSS harness connector. Measure resistance of VSS. If resistance is 976-2354 ohms (Corvette and 4WD) or 1377-3355 ohms (except Corvette and 4WD), go to next step. If resistance is not as specified, go to step  13.
  8. If resistance measured in step  3 was greater than 2354 ohms (Corvette and 4WD) or 3355 ohms (except Corvette and 4WD), go to step  10. If resistance is not greater than specified value, go to step  11.
  9. Check VSS input and ground circuits for short to ground. Repair as necessary. After repairs, go to step  15.
  10. Check for open in VSS input and ground circuits. Repair as necessary. After repairs, go to step  15.
  11. Check VSS input and ground circuits for short together. Repair as necessary. After repairs, go to step  15.
  12. Remove VSS assembly. See appropriate DIFFERENTIALS & DRIVE AXLES article in DRIVELINE/AXLES. Check output shaft speed sensor rotor for damage or misalignment. Check case extension bushing for wear. Repair as necessary. After repairs, go to step  15. If no problem is found, go to next step.
  13. Replace VSS assembly. See appropriate DIFFERENTIALS & DRIVE AXLES article in DRIVELINE/AXLES. After repairs, go to step  15.
  14. Replace PCM. After replacement, program PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article in ENGINE PERFORMANCE. After repairs, go to next step.
  15. Using scan tool, clear DTCs. Test drive vehicle. Ensure transmission output speed is greater than 500 RPM for about 3 seconds. Check for DTCs. If DTC P0502 does not return, system is okay. If DTC P0502 returns, go to step  1.
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