Circuit Description
The vehicle speed sensor (VSS) system is a pulse generator consisting of a speed sensor assembly, located in the case extension, and a toothed vehicle speed sensor reluctor wheel, which is pressed onto the final drive assembly. As the vehicle drives forward, the vehicle speed sensor reluctor wheel also rotates. This rotation produces a variable AC signal in the pickup coil that is proportional to vehicle speed.
The AC signal generated has a frequency and amplitude (voltage). The PCM uses the frequency portion of the AC signal to calculate the vehicle speed. The system is calibrated to produce 4000 pulses per mile and it is the amount of time it takes to produce those pulses that determines the vehicle speed. The voltage portion of the signal is used for diagnostic purposes only. The voltage will vary from 0.5 volts AC at 100 RPM to more than 200 volts AC at 6000 RPM.
This information is then sent to the PCM to indicate how fast the vehicle is traveling and to develop the shift pattern for the transmission. Other systems also use VSS information:
| • | TCC apply and release |
| • | Cruise control system |
| • | Fuel delivery systems |
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
DTC P0502 sets during the second consecutive trip in which the following conditions occur:
| • | No MAP sensor DTCs P0106, P0107, P0108, P1106 or P1107. |
| • | No TP sensor DTCs P0121, P0122, P0123, P1121 or P1122. |
| • | The transmission is not in PARK or NEUTRAL. |
| • | The input shaft speed is greater than 1500 RPM. |
| • | The throttle position angle is greater 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 shaft speed is less than 150 RPM for 2.5 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the Malfunction Indicator Lamp (MIL). |
| • | The PCM commands maximum line pressure. |
| • | The PCM freezes shift adapts. |
| • | The PCM calculates vehicle speed from the automatic transmission input shaft speed (A/T ISS) sensor and the commanded gear. |
| • | The PCM stores DTC P0502 in PCM history. |
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 the PCM history. |
| • | The PCM clears the DTC from the PCM history if the vehicle completes 40 consecutive 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
| • | Check for PRNDL DTC P0705. DTC P0705 affects transmission operation. This may set DTC P0502 falsely. |
| • | Check for electromagnetic interference (EMI) in circuits 400 (YEL) and 401 (PPL). EMI may be induced by a misrouted wiring harness along the spark plug wires. |
| • | Inspect the wiring at the PCM, the vehicle speed sensor (VSS) connector and all other circuit connecting points for the following conditions: |
| - | A bent terminal |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Poor terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | 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.
-
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? | -- | Go to Powertrain On Board Diagnostic (OBD) System Check in Engine Controls | |
Important: Before clearing the DTCs, use the Scan Tool in order to record the Freeze Frame and Failure Records for reference. Using the Clear Info function will erase the stored Freeze Frame and Failure Records from the PCM. 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 shaft speed increase when wheel speed increases? | -- | Go to Diagnostic Aids | ||
Is the measured voltage greater than the specified value? | 0.5 volts | |||
4 |
Is the resistance greater than the specified value? | 1864 ohms | ||
Refer to General Electrical Diagnosis in Wiring Systems. Was a condition found and corrected? | -- | |||
6 | Does the DMM display a value less than the resistance specified? | 981 ohms | ||
7 |
Refer to General Electrical Diagnosis in Wiring Systems. Was a condition found and corrected? | -- | ||
8 |
Is either resistance less than the specified value? | 50 K ohms | ||
9 |
Refer to General Electrical Diagnosis in Wiring Systems. Was a condition found and corrected? | -- | ||
10 |
Is the resistance within the specified range? | 981 ohms to 1864 ohms | ||
11 |
Was a condition found? | -- | Go to Diagnostic Aids | |
12 | Correct the vehicle speed sensor reluctor wheel alignment or replace the vehicle speed sensor reluctor wheel. Refer to Unit Repair. Is the alignment correct or the replacement complete? | -- | -- | |
13 | Replace the Vehicle Speed Sensor. Refer to Vehicle Speed Sensor Replacement . Is the replacement complete? | -- | -- | |
14 | Replace the PCM. Refer to Powertrain Control Module Replacement/Programming in Engine Controls. Is the replacement complete? | -- | -- | |
15 | Perform the following procedure in order to verify the repair:
Has the test run and passed? | -- | System OK |
