P0502

Circuit Description
Vehicle speed information is provided to the powertrain control module (PCM)/engine control module (ECM) by the Vehicle Speed Sensor (VSS). The VSS is a permanent magnet generator that is mounted in the transaxle and produces a pulsing voltage whenever vehicle speed is over 3 mph (5 kph). The A/C voltage level and the number of pulses increases with vehicle speed. The PCM/ECM converts the pulsing voltage into mph (kph) and than supplies the necessary signal to the instrument panel for speedometer/odometer operation and to the cruise control module and multi-function alarm module operation. This Diagnostic Trouble Code (DTC) will detect if vehicle speed is reasonable according to engine rpm and load.

Conditions for Setting the DTC
- DTC(s) P0106, P0107, P0108, P0117, P0118, P0121, P0122, P0123, P0201, P0202, P0203, P0204, P0300, P0351, P0352, P0402, P0404, P0405, P0406, P1404 and P1627 are not set.
- Engine running.
- Ignition voltage is between 11 - 16 volts.
- Coolant temperature is greater than 60 °C (140 °F).

Power Test
- Vehicle speed is less than 4 kph (6.4 mph).
- Engine load is greater than 50 kPa.
- Throttle Position (TP) sensor is between 25 and 70%.
- The rpm is between 2700 and 6000.

Deceleration Test
- Vehicle speed is less than 4 kph (6.4 mph).
- MAP is less than 28 kPa.
- TP sensor is less than equal to 0.8%.
- The rpm is between 1500 and 3500.
- The change in RPM is less than 75 RPM/cycle.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The PCM/ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn off after three 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.
- DTC(s) can be cleared by using the scan tool.

Diagnostic Aids
An intermittent problem may be caused by a poor connection, rubbed-through wire insulation, or a wire that is broken inside the insulation.

VSS signal circuit should be thoroughly checked for the following conditions:
- Backed-out terminals
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection
- Physical damage to the wiring harness

Ensure the VSS is correctly torqued to the transmission housing.
Refer to "Intermittents".

Steps 1 - 4:

Steps 5 - 13:

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) 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 kph). This step determines if DTC P0502 is the result of a hard failure 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/ECM is receiving a signal from the vehicle speed sensor.
6. A resistance reading that is higher than the specified value indicates that the VSS circuitry is open.
11. The replacement PCM/ECM must be reprogrammed. Refer to the latest Techline procedure for PCM/ECM reprogramming.