P0502

Schematic:

CIRCUIT DESCRIPTION
The Vehicle Speed Sensor (VSS) provides the Powertrain Control Module (PCM) with the vehicle speed information. The VSS is a permanent magnet generator that is mounted in the transmission. The VSS produces a pulsing voltage whenever the vehicle speed is more than 5 km/h (3 mph). The AC voltage level and the number of pulses increases with the vehicle speed. The PCM then supplies the signal to the instrument panel for the speedometer, the odometer, the electronic transmission, if equipped, and the cruise control operation. The PCM converts the pulsing voltage into mph the technician can use for calculations.

CONDITIONS FOR RUNNING THE DTC
^ Engine speed is between 1700 and 3600 RPM.
^ The throttle position (TP) is between 0-1 percent.
^ Engine vacuum is between 80-70 kPa.
^ Above conditions are met for 5 seconds.

CONDITIONS FOR SETTING THE DTC
Vehicle speed is less than 3 km/h (2 mph).

ACTION TAKEN WHEN THE DTC SETS
^ The Malfunction Indicator Lamp (MIL) will illuminate.
^ The PCM will record the operating conditions at the time that the diagnostic fails. The Freeze Frame and Failure Records buffers will store this information.
^ A history DTC stores.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The MIL will turn OFF after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
^ A history DTC will clear after 40 consecutive warm up cycles without a fault.
^ A scan tool can clear the DTCs.

DIAGNOSTIC AIDS
^ A scan tool should indicate a vehicle speed whenever the drive wheels are turning more than 5 km/h (3 mph). A DTC P0502 can be falsely set if the engine is braked-torqued in gear. Disregard if the DTC set while the drive wheels are not turning.
^ An intermittent problem may be caused by the following conditions:
- A poor connection
- Rubbed through wire insulation
- A wire that is broken inside the insulation
^ Thoroughly check the VSS high and VSS low circuits for the following conditions:
^ Backed out terminals
^ Broken locks
^ Proper routing
^ Refer to Symptoms. Symptom Related Diagnostic Procedures

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

TEST DESCRIPTION
Numbers below refer to the step numbers on the Diagnostic Table.
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
2. The permanent magnet generator only produces a signal if the drive wheels are turning greater than 3 mph (5 km/h). This step determines if DTC P0502 is the result of a hard malfunction or an intermittent condition.
3. Proper engine loads cannot be achieved in a shop environment to properly run the vehicle within the Freeze Frame Data conditions. It will be necessary to drive the vehicle on the road to obtain the proper engine loads.
4. This step verifies that the PCM is receiving a signal from the vehicle speed sensor.
5. Refer to service bulletin information for the latest calibration update.
6. Refer to the latest Techline information for programming procedures.
8. A resistance reading that is higher than the specified value indicates that the VSS circuitry is open.
10. If the displayed resistance were less than the 1300 ohms, the VSS high and low circuits are shorted together. If the resistance value within the specified value, check the VSS high and low circuits for a short to ground.
11. This checks the resistance of the VSS if no opens or shorts were found on the VSS high and low circuits.
13. Replacement PCMs must be reprogrammed and the Crankshaft Position System Variation Learn Procedure must be performed. Refer to the latest Techline information for PCM reprogramming and also refer to the PCM Programming.