P0502

Diagnostic Chart:

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 will generate AC voltage that will vary from 0.5 volts at 100 RPM to more than 100 volts at 6000 RPM.

This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission. Other systems also use VSS information:

^ Torque Converter Clutch (TCC) apply and release

^ Cruise control system

^ Fuel delivery systems

If the PCM detects a low vehicle speed with a high engine speed while in a drive range, then DTC P0502 sets. DTC P0502 is a type B DTC.

CONDITIONS FOR SETTING THE DTC

^ No MAP DTCs P0106, P0107, P0108 P1106 or P1107.

^ No TP DTCs P0121, P0122, P0123, P1121 or P1122.

^ The transmission is not in Park or Neutral.

^ The input speed is more than 1500 RPM.

^ The throttle angle is more than 12%.

^ The MAP is 0-105 kPa (0-15 psi).

^ The engine torque is 55-200 N.m (40-150 lb. ft.).

^ 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 PCM commands maximum line pressure.

^ The PCM disables shift adapts.

^ The PCM calculates vehicle speed from the Automatic Transmission Input Speed Sensor (A/T ISS) and from the commanded gear.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The PCM turns OFF the MIL after three consecutive trips without a failure reported.

^ A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.

^ 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 PRNDL DTC P0705. DTC P0705 affects transmission operation. This may set DTC P0502 falsely.

^ Check for Electromagnetic Interference (EMI) in circuits 400 and 401. EMI may be induced if the harness is routed along the spark plug wires.

^ Inspect the wiring at the PCM, the Vehicle Speed Sensor (VSS) connector and all other circuit connecting points for the following conditions:

- A bent terminal

- A backed out terminal

- A damaged terminal

- Poor 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.

IMPORTANT: Place the gear select lever into the Neutral position before rotating the wheel.

his 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.