P0502

Circuit Description

The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case, and a toothed speed sensor rotor, which is pressed onto the final drive assembly. As the vehicle drives forward, the speed sensor rotor also rotates. This rotation produces a variable voltage signal in the pickup coil that is proportional to vehicle speed.

The VSS generates AC voltage varying from 0.5 volts at 100 RPM to more than 100 volts at 6000 RPM. The PCM uses this signal to determine vehicle speed, and to develop the shift pattern for the transmission. Other systems also use VSS information:

^ TCC apply and release
^ Cruise control system
^ Fuel delivery systems

When the PCM detects an unrealistically low vehicle speed with a high engine speed while in a drive gear range, then DTC P0502 sets. DTC P0502 is a type A DTC.

Conditions for Running the DTC

^ No MAP DTCs P0107 or P0108.
^ No TP DTCs P0122 or P0123.
^ The transmission is not in Park or Neutral.
^ The engine speed is more than 3100 RPM.
^ The throttle angle is more than 15%.
^ The MAP is 0-105 kPa (0-15 psi).
^ The engine torque is 55-200 N.m (40-150 lb ft).

Conditions for Setting the DTC

DTC P0502 sets when the output speed is less than 150 RPM for 2.5 seconds.

Action Taken When the DTC Sets

^ The PCM illuminates the malfunction indicator lamp (MIL).
^ The transmission defaults to 3rd gear.
^ The PCM inhibits TCC operation.

Conditions for Clearing the MIL/DTC

^ The PCM turns OFF the MIL during the third consecutive trip in which the diagnostic test runs and passes.
^ A scan tool clears the DTC from PCM history.
^ The PCM clears the DTC from the PCM history if the vehicle completes 40 consecutive warm-up cycles without an emission related diagnostic fault occurring.
^ The PCM cancels the DTC default actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

Diagnostic Aids

^ Check for Electromagnetic Interference (EMI) in circuits 400 and 401. EMI may be induced if the harness is routed along the spark plug wires or alternator wires.
^ Inspect the wiring at the PCM, the vehicle speed sensor (VSS) connector and all other circuit connecting points for the following conditions:
- A backed out terminal
- A damaged terminal
- Reduced terminal tension.
- A chafed wire
- A broken wire inside the insulation
- Moisture intrusion
- Corrosion
^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ Ensure that the VSS is securely tightened to the case extension.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

2. This step verifies that the PCM is receiving a signal from the vehicle speed sensor.
3. This step verifies that there are no opens in the circuitry from the vehicle speed sensor to the PCM connector.
4. This step performs a resistance test on the VSS circuits from the PCM connector.