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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 83 (ENGINE CONTROLS - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0502: VSS CIRCUIT - LOW INPUT (M/T WITHOUT ABS)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Saturn LW200, 2002 Saturn L200, and 2002 Saturn L100. However, it is still accessible from the selected car via links, so may be relevant.
  1. Drive vehicle. Using scan tool, monitor vehicle speed. If vehicle speed is displayed, problem is intermittent. See DIAGNOSTIC AIDS  . If vehicle speed is not displayed, go to next step.
  2. Turn ignition off. Disconnect vehicle speed signal conditioner module harness connector located above DLC. Turn ignition on, with engine off. Using a DVOM, check voltage between ground and conditioner module harness connector terminal No. 5 (Brown wire). If battery voltage is present, go to next step. If battery voltage is not present, check for open or short in circuit. See WIRING DIAGRAMS article. Repair as necessary.
  3. Measure voltage between vehicle speed signal conditioner module harness connector terminals No. 1 (Black wire) and No. 5 (Brown wire). If battery voltage is present, go to next step. If battery voltage is not present, repair open in Black wire circuit.
  4. Using a test light connected to ground, quickly tap vehicle speed signal conditioner module harness connector terminal No. 2 (White wire). Observe vehicle speed reading scan tool. If vehicle speed changes, go to step  6 . If vehicle speed does not change, go to next step.
  5. Using DVOM, check voltage between ground and harness connector terminal No. 4 (White wire). If voltage reading is 0-4 volts, check for loose connection or open or short in White wire circuit. Repair as necessary. If circuit or terminals are okay, replace PCM. If voltage reading is 4-6 volts, replace PCM. If voltage reading is more than 6 volts, check for short to voltage in White wire circuit. Repair as necessary.
  6. Turn ignition off. Disconnect right front wheel speed sensor. Using DVOM, measure voltage on Dark Green wire and Tan wire at wheel speed connector. If voltage is present, repair short to voltage on Dark Green or Tan wire. If voltage is not present, go to next step.
  7. Check continuity between Dark Green wire and ground, and then between Tan wire and ground. If continuity exists in either test, repair short to ground in appropriate wire. If continuity does not exist in either test, go to next step.
  8. Reconnect right front wheel speed sensor. Using DVOM, measure resistance between vehicle speed signal conditioner module harness connector terminals No. 2 (Dark Green wire) and No. 3 (Tan wire). If resistance is 1000-2000 ohms, go to next step. If resistance is not 1000-2000 ohms, check for open in circuits or for loose connection. Repair as necessary. If circuits or terminals are okay, replace right front wheel speed sensor.
  9. Using DVOM, check AC voltage between vehicle speed signal conditioner module harness connector terminals No. 2 (Dark Green wire) and No. 3 (Tan wire), while manually spinning wheels. If AC voltage does not increase when wheels are spun, go to next step. If AC voltage increases when wheels are spun, check for loose connection at connectors. Repair as necessary. If terminals are okay, replace vehicle speed signal conditioner module.
  10. Check gap between sensor and reluctor wheel. Check for missing or damaged reluctor wheel or wheel speed sensor. Repair or replace as necessary.
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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.