Circuit Description
The Vehicle Speed Sensor (VSS) is a Permanent Magnet (PM) generator attached to the transmission case extension. 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 this AC voltage into a digital signal. The PCM uses the vehicle speed to determine the shift timing, the TCC apply and release, and the gear ratio calculations. The VSS voltage will vary from 0.5 volts AC at 100 RPM to more than 100 volts AC at 6000 RPM. The VSS resistance value is 981-1864 ohms.
If the PCM detects a low vehicle speed with a high engine speed while in a drive range, then DTC P0502 sets. DTC P0502 is a type B DTC.
Conditions for Setting the DTC
| • | No MAP DTCs P0106, P0107, P0108, P1106 or P1107. |
| • | No TP DTCs P0121, P0122, P0123, P1121, or P1122. |
| • | The transmission is not in Park or Neutral. |
| • | The input speed is more than 2500 RPM. |
| • | The throttle angle is more than 12%. |
| • | The MAP is 0-105 kPa (0-15 psi). |
| • | The engine torque is 55-200 N·m (40-150 lb ft.). |
| • | The output speed is less than 150 RPM for 2.5 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the Malfunction Indicator Lamp (MIL) after 2 consecutive ignition cycles with a failure reported. |
| • | The PCM commands maximum line pressure. |
| • | The PCM disables shift adapts. |
| • | The PCM calculates the vehicle speed from the Automatic Transmission Input Speed Sensor (A/T ISS) and from the commanded gear. |
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 actions when the fault no longer exists and the ignition is OFF long enough in order to power down the PCM. |
Diagnostic Aids
| • | Check for PRNDL DTC P0705. DTC P0705 affects transmission operation. This may set DTC P0502 falsely. |
| • | Check for Electromagnetic Interference (EMI) in circuits 400 and 401. EMI may be induced if the harness is routed along the spark plug wires. |
| • | Inspect the wiring for poor electrical connections at the PCM. Inspect the wiring at the TCC brake switch. Look for the following problems: |
| - | A bent terminal |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Poor terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| • | When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change. |
| • | Ensure that the VSS is securely tightened to the case extension. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step verifies that the PCM is receiving a signal from the vehicle speed sensor.
-
Important: Place the gear select lever into the Neutral position before rotating the wheel.
This step verifies that there are no opens in the circuitry from the vehicle speed sensor to the PCM connector.
-
This step performs a resistance test on the VSS circuits from the PCM connector.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and Failure Records for reference. The Clear Info function will erase the data. Notice: Support the lower control arms in the normal horizontal position in order to avoid damage to the drive axles. Do not operate the vehicle in gear with the wheels hanging down at full travel. With drive wheels rotating, does transmission output speed increase when wheel speed increases? | -- | Go to Diagnostic Aids | ||
Is the measured voltage greater than the specified value? | 0.5 volts | |||
4 | Remove the VSS and connect an ohmmeter from terminal A to terminal B of the VSS. Is the measured resistance within the specified range? | 981-1864ohms | ||
Does the ohmmeter display a value greater than the specified value? | 1864ohms | |||
6 |
Refer to Troubleshooting Procedures, Section 8. Did you find and correct a problem? | -- | -- | |
7 | Does the ohmmeter display a value less than the resistance specified? | 981ohms | ||
8 |
Refer to Troubleshooting Procedures, Section 8. Did you find and correct a problem? | -- | -- | |
9 |
Refer to Troubleshooting Procedures, Section 8. Did you find and correct a problem? | -- | ||
10 | Replace the VSS. Refer to Vehicle Speed Sensor Replacement, in On-Vehicle Service. Is the replacement complete? | -- | -- | |
11 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming , Section 6. Is the replacement complete? | -- | -- | |
12 | In order to verify your repair, perform the following procedure:
Has the test run and passed? | -- | System OK |
