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 Powertrain Control Module (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 SETTING THE DTC

^ No MAF P0101 P0102 or P0103 DTCs are set.
^ No Throttle Position (TP) sensor P0121, P0122 or P0123 DTCs are set.
^ No Automatic Transmission Input (Shaft) Speed sensor P0716 or P0717 DTCs are set.
^ No Automatic Transmission Fluid Pressure (TFP) Valve Position Switch P1810 DTC is set.
^ The transmission is not in Park, Neutral, or Reverse.
^ The delivered torque is greater than 109 Nm (80 lb. ft).
^ The TP angle is greater than 12.8°.
^ The vehicle speed is 8 km/h (5 mph) or less for 2 seconds.
^ The input shaft speed is 2000 RPM or greater.

ACTION TAKEN WHEN THE DTC SETS

^ The PCM illuminates the Malfunction Indicator Lamp (MIL) after two consecutive trips with a failure reported.
^ SERVICE ENGINE SOON, SPEED SENSOR FAULT displays on the Driver Information Center (DIC).
^ The PCM commands default line pressure.
^ The PCM disables steady state adapts.
^ The PCM disables upshift adapts.
^ The PCM inhibits the TCC.
^ The PCM inhibits torque management.
^ The PCM calculates vehicle speed from the Automatic Transmission Input (Shaft) Speed sensor and commanded gear.
^ The PCM disables the cruise control.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The PCM turns OFF the MIL after three consecutive ignition cycles without a failure reported.
^ A scan tool can clear the DTC from the PCM history. The PCM clears the DTC from the PCM history if the vehicle completes 40 warm-up cycles without a failure reported.
^ The PCM cancels the DTC default action when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM.

DIAGNOSTIC AIDS

^ Inspect the wiring for poor electrical connections at the PCM. Inspect the wiring for poor electrical connections at the VSS 2-way connector. Inspect for the following problems:

- A bent terminal
- A backed out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire

^ 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 or Failure Record information before proceeding further.
4. This step has the scan tool verify that the correct calibration is in the PCM.
6. This step tests for a resistance value greater than 1950 ohms which would indicate a possible open in circuit 400 or 401.
8. 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 401 is a possible short to ground.
10. This step performs a resistance check on the VSS, if no opens, shorts to ground or shorts together were found on the 400 or 401 circuits.