P0502

Circuit Description
The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto the final drive carrier assembly. As the vehicle drives forward, the vehicle speed sensor reluctor wheel also rotates. This rotation produces a variable AC signal in the pickup coil that is proportional to vehicle speed. The AC signal generated has a frequency and amplitude (voltage). The PCM uses the frequency portion of the AC signal to calculate the vehicle speed. The system is calibrated to produce 4000 pulses per mile and it is the amount of time it takes to produce those pulses that determines the vehicle speed. The voltage portion of the signal is used for diagnostic purposes only. The voltage will vary from 0.5 volts AC at 100 RPM to more than 200 volts AC 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:
^ 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 Running the DTC
^ No MAP sensor DTCs P0106, P0107, P0108, P1106 or P1107.
^ No TP sensor DTCs P0121, P0122, P0123, P1121 or P1122.
^ The transmission is not in PARK or NEUTRAL.
^ The input shaft speed is greater than 1500 RPM.
^ The TP angle is greater than 12%.
^ 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
The output shaft speed is less than 150 RPM for 2.5 seconds.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the conditions for setting the DTC are met.
^ The PCM commands maximum line pressure.
^ The PCM freezes shift adapts.
^ The PCM calculates vehicle speed from the A/T ISS sensor and the commanded gear.
^ The PCM stores DTC P0502 in PCM history.

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 the 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 switch is OFF long enough in order to power down the PCM.

Diagnostic Aids
^ Check for electromagnetic interference (EMI) in circuits 400 (YEL) and 401 (PPL). EMI may be induced by a misrouted wiring harness along the spark plug wires.
^ Inspect the wiring at the PCM, the 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 VSS.
3. This step tests the ability of the VSS to produce an AC voltage as well as the integrity of the wiring from the VSS to the PCM connector.
4. This step performs a resistance test on the VSS circuits from the PCM connector.
10. A YES answer indicates an intermittent condition.
11. A NO answer indicates an intermittent condition.