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HOMESERVICE MANUALSBUICK1996ROADMASTER BASEREPAIR AND DIAGNOSIS (SINGLE PAGE)TRANSMISSIONAUTOMATIC TRANSAUTOMATIC TRANSMISSION DIAGNOSIS - 4L60-EDIAGNOSTIC TESTS - PASSENGER CARSP0502, VEHICLE SPEED SENSOR CIRCUIT (LOW INPUT)DIAGNOSTIC PROCEDURES
1996 Buick Roadmaster Base

Diagnostic Procedures

1996 Buick Roadmaster BaseSECTION Diagnostic Procedures
  1. Connect scan tool to DLC. Turn ignition switch to ON position. DO NOT  start engine. Using scan tool, record freeze frame and failure records for reference. Data will be lost when DTCs are cleared later in this test. Raise and support vehicle.
  2. Start engine and let idle. Disable traction control system (if equipped). Shift gear selector to any forward position. Select "Output Speed" on scan tool. With drive wheels rotating, if transmission output speed does not increase when wheel speed increases, go to next step. If transmission output speed increases when wheel speed increases, see DIAGNOSTIC AIDS below.
  3. Turn ignition off. Disconnect appropriate PCM connector connected to VSS. See WIRING DIAGRAMS  . Connect voltmeter between VSS terminals at appropriate PCM connector terminals. Rotate drive wheels and observe voltmeter display. If voltage is less than .5 volt, go to next step. If voltage is more than .5 volt, check PCM connector for damaged or backed out connector terminals, weak terminal tension and poor connections. Repair as necessary, then go to step 7. If terminals and connections are okay, replace PCM, then go to step 7.
  4. Using an ohmmeter, measure resistance between VSS terminals at appropriate PCM connector. See WIRING DIAGRAMS  . If resistance is more than 2700 ohms, check for poor connection or open circuit in VSS wiring. Repair as necessary, then go to step 7. If wiring is okay, go to next step. If resistance is more than 1480 ohms, go to next step. If resistance is less than 1480 ohms, check for short to ground, or for VSS wires shorted together. Repair as necessary, then go to step 7. If wiring is okay, go to next step.
  5. Reconnect appropriate PCM connector. Disconnect VSS connector. Measure resistance between VSS terminals. If resistance is not 1480-2700 ohms, go to next step. If resistance is 1480-2700 ohms, check PCM connector for damaged or backed out connector terminals, weak terminal tension and poor connections. Repair as necessary, then go to step 7. If terminals and connections are okay, replace PCM, then go to step 7.
  6. Remove VSS from transmission. Inspect output speed sensor rotor for damage or misalignment. Repair as necessary, then go to next step. If rotor is okay, replace VSS, then go to next step.
  7. After repair is complete, select DTC on scan tool. Select "Clear Info" function. Select "Specific DTC" and enter DTC "P0502". Test drive vehicle. Ensure transmission output speed is more than 251 RPM for 2 seconds. If DTC P0502 is not present, repair is complete. If DTC P0502 is still present, repeat test.
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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.