P0502

Circuit Description
Vehicle speed information is provided to the 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 Alternating Current (AC) voltage level and the number of pulses increases with vehicle speed. The ECM converts the pulsing voltage into mph (kph) and then 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 and P1404 are not set.
- Ignition voltage is between 11 - 16 volts.
- Coolant temperature is greater than -34.6 °F (-37 °C).

Power Test
- Vehicle speed is less than 5 mph (8 kph).
- Manifold Absolute Pressure (MAP) is greater than 55 kPa.
- Throttle Position (TP) sensor is between 25 and 60%.
- The rpm is between 2700 and 6000.

Deceleration Test
- Vehicle speed is less than 5 mph (8 kph ).
- MAP is less than 25 kPa.
- TP sensor is less than 1.0%.
- The rpm is between 1200 and 6000.
- The change in RPM is less than 90 RPM/cycle.
- P/N switch is OFF.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The 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 ECM is supplying a signal voltage to the vehicle speed sensor.
6. A resistance reading that is higher than the specified value indicates that the VSS ground circuitry is open.
11. The replacement ECM must be reprogrammed. Refer to the latest Techline procedure for ECM reprogramming.