Vehicle Speed Sensor: Testing and Inspection
System Schematic:
To test the Vehicle Speed Sensor (VSS), located in the speedometer, proceed as follows.
1. Connect an analog voltmeter between ECU pin 19 and ground.
NOTE: To test pins do not remove the connector from the ECU, instead insert a straight pin or needle from the back side of the connector to access the pin.
2. Turn the ignition switch to the "on" position, without starting the engine.
3. Raise one drive wheel off the ground and manually rotate the wheel slowly.
4. Observe the voltmeter, the voltage should oscillate between a positive voltage and 0 volts, as the wheel is rotated. If this occurs the Vehicle Speed Sensor system is operating properly.
5. If the voltmeter remains steady at reference voltage, there is either an open circuit between the VSS and the ECU, or the VSS is faulty. Before replacing the VSS check for an open circuit in the VSS or instrument cluster grounds.
6. If the voltmeter remains steady at 0 volts, there is either no reference voltage from the ECU, a ground between the ECU and the VSS, or the VSS is faulty.
7. To determine if the VSS is grounded or faulty, turn the ignition switch "off" and test for continuity between ECU pin 19 and ground (using the same test method as in step #3). If continuity alternates, the VSS is operating properly (the ECU is bad). If continuity remains constant, either there is a ground between the VSS and the ECU, or the VSS is faulty.
Note:
There should be continuity and non-continuity 4 times for every rotation of the speedometer shaft.
HARNESS TEST
1. Disconnect the ECU connector. Raise the drive wheels off the ground.
2. Using a Ohmmeter, check the speed sensor output circuit for continuity between the ECU connector terminal 19 and ground with the drive wheels turning.
Continuity: Should Alternate On And Off.
3. Reconnect the ECU connector, remove the 12 pin instrument cluster connector and turn on the ignition. Measure the power supply voltage between instrument connector terminal "A" (Yellow/White Wire) and ground.
Voltage: 4.5 - 4.9 Volts.
4. Check for continuity of the ground circuit, between the 16 pin instrument cluster connector terminal "B" (Black Wire) and ground.
Continuity: Should Exist.
If any of the previous tests produce unsatisfactory results, the harness will need to be repaired or replaced. Once repairs have been completed, road test the vehicle to confirm that the repair has corrected the problem.
If the same problem reoccurs, it is possible that there is an intermittent failure of the component or the ECU. Check for looseness at all harness junctions and test for an intermittent failure.