P0502
DTC P0502 Vehicle Speed Sensor Circuit Low Input
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 greater than 3 mph (5 km/h). 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 Setting the DTC
- Vehicle speed is less than 2 mph (3 km/h).
- Engine speed is between 1700 and 3600 RPM.
- Throttle Position (TP) is between 0-1%.
- Engine vacuum is between 80-70 kPa.
- Above conditions met for 5 seconds.
- DTCs P0107, P0108, P0122, and P0123 not set.
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 greater than 3 mph (5 km/h). 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.
- A poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation may cause an Intermittent problem.
- Thoroughly check the VSS high and VSS low circuits for backed out terminals, broken locks, and for proper routing.
- Refer to Symptoms.
Test Description
Number(s) below refer to the step number(s) 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 it 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 Isuzu Technical Communication System (ITCS)
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 where less than the 1300 ohms, the VSS high and low circuits are shorted together. If the
resistance value is 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 procedure must be performed. Refer to the latest Isuzu Technical Communication System (ITCS) information for programming procedures and also refer to the CKP System Variation Learn Procedure.