Circuit Description
Vehicle speed information is provided to the powertrain control module (PCM) by the vehicle speed sensor (VSS). The VSS is a permanent magnet generator that is mounted in the transaxle and produces a pulsing voltage whenever vehicle speed is over about 5 km/h (3 mph). The AC voltage level and the number of pulses increases with vehicle speed. The PCM then supplies the necessary signal for the instrument panel for speedometer, odometer, electronic transmission (if equipped), and cruise control operation. The PCM converts the pulsing voltage into mph which can be used for calculations and can be displayed on the scan tool. Output of the generator can also be seen by using a digital voltmeter on the AC scale while rotating the generator.
Conditions for Setting
| • | DTCs P0107, P0108, P0122, P0123 are not set. |
| • | The engine speed is between 3,000-4,775 RPM. |
| • | Throttle position (TP) is between 15-99 percent. |
| • | Manifold absolute pressure is above 80 kPa or below 40 kPa. |
| • | Transmission input speed is above 3,000 RPM. |
| • | Above conditions are met for 5 seconds. |
Action Taken
Malfunction indicator lamp (MIL) illuminates.
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after passing three consecutive diagnostic tests. |
| • | A history DTC will clear after 40 consecutive ignition cycles have occurred without a fault. |
| • | History and current DTCs can be cleared by using a scan tool. |
Diagnostic Aids
| • | A scan tool should indicate a vehicle speed whenever the drive wheel are turning greater than 5 km/h (3 mph). DTC P0502 can be falsely set if the engine is braked-torqued in gear. Disregard if set while the drive wheels are not turning. |
| • | An intermittent problem may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation. |
| • | CKTs 400 and 401 should be thoroughly checked for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wiring connections, physical damage to the wiring harness or for proper routing. Refer to Intermittent Conditions . |
Test Description
The number below refers to the step number on the diagnostic table.
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The permanent magnet generator only produces a signal if the drive wheels are turning greater than 5 km/h (3 mph). This step determines if DTC P0502 is the result of a hard failure or an intermittent condition.
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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.
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At this point, the PCM is not sending vehicle speed data to the scan tool. If the scan tool is connected properly, check to see if the EEPROM and PCM are correct for the application before replacing the PCM. To re-program the PCM, refer to Section C1.
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Use signal generator testers J 33431 or J 38522. The VSS output is a 4,000 PPM signal. The scan tool will read 14.5 km/h (9 mph) with the testers set to 60 Hz. Signal generator adapter J 41059 is needed to perform this test.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Was the On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | |
Important: Running the vehicle in gear with the wheels hanging down at full travel will damage the drive axles. Does the scan tool display vehicle speed above 0 km/h (0 mph)? | -- | Go to Step 3 | Go to Step 4 | |
Does the scan tool display vehicle speed above the specified value? | 0 km/h (0 mph) | Go to Diagnostic Aids | Go to Step 4 | |
4 | Does the speedometer operate? | -- | Go to Step 5 | Go to Step 7 |
Verify that the EEPROM and the PCM are the correct applications and re-program if necessary. Was re-programming necessary? | -- | Go to Step 6 | ||
6 | Replace the PCM. Is the action complete? | -- | -- | |
Does the scan tool display vehicle speed above the specified value? | 60 Hz 0 km/h (0 mph) | Go to Step 9 | Go to Step 8 | |
8 | Inspect the VSS high and VSS low circuits for being open or shorted to ground or voltage and repair as necessary. Was a repair necessary? | -- | Go to Step 6 | |
9 | Replace the vehicle speed sensor in transaxle. Is the action complete? | -- | -- | |
