P0502

Circuit Description

The vehicle speed sensor (VSS) is a permanent magnet (PM) generator attached to the transmission case. The final drive assembly has a toothed wheel (VSS rotor). As the final drive assembly rotates, an AC voltage is induced and transmitted to the PCM. The PCM converts the AC voltage into a digital signal. The PCM uses the vehicle speed in order to determine shift timing, torque converter clutch (TCC) apply and release, and gear ratio. The VSS voltage will vary from 0.5 volts AC at 100 RPM to more than 100 volts AC at 8000 RPM. The VSS resistance value is 1300-1950 ohms.
If the PCM detects a low vehicle speed and a high input shaft speed sensor signal while in a drive range, then DTC P0502 sets. DTC P0502 is a Type B DTC.

Conditions for Running the DTC

^ No MAF DTCs P0101, P0102 or P0103.
^ No TP DTCs P0121, P0122 or P0123.
^ No A/T Input (shaft) Speed Sensor (ISS) DTCs P0716 or P0717.
^ No Transmission Fluid Pressure (TFP) manual valve position switch DTC P1810.
^ The transmission is not in PARK, NEUTRAL, or REVERSE.
^ The delivered torque is greater than 109 N.m (80 lb ft).
^ The TP angle is greater than 12.8 degrees.
^ The input shaft speed is 2000 RPM or greater.

Conditions for Setting the DTC

The vehicle speed is 8 km/h (5 mph) or less for 2 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.
^ SERVICE ENGINE SOON, SPEED SENSOR FAULT displays on the driver information center (DIC).
^ The PCM commands default line pressure.
^ The PCM inhibits TCC engagement.
^ The PCM disables the cruise control.
^ The PCM freezes steady state adapts.
^ The PCM freezes upshift adapts.
^ The PCM calculates vehicle speed from the automatic transmission input (shaft) speed sensor and commanded gear.
^ The PCM stores DTC P0502 in PCM history during the second consecutive trip in which the Conditions for Setting the DTC are met.

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 PCM history.
^ The PCM clears the DTC from PCM history if the vehicle completes 40 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

^ Inspect the wiring at the PCM and the VSS 2-way connector 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
^ When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.
^ Check for electromagnetic interferences (EMI) induced on circuits 400 and 401 by a misrouted harness along spark plug wires.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

2. This step checks for DTCs and for a vehicle speed signal. It is important to record Freeze Frame and Failure Record information before proceeding further.
4. This step tests for a resistance value greater than 1950 ohms which would indicate a possible open in circuit 400 or circuit 401.
6. This step verifies that the ohmmeter displays a value less than or equal to the specified VSS value. Resistance less than 1300 implies that the circuits are shorted together. Resistance within specification implies that circuit 400 or circuit 401 is a possible short to ground.
8. This step performs a resistance check on the VSS, if no opens, shorts to ground or shorts together were found on circuit 400 or circuit 401.