Code 24: Vehicle Speed Sensor (VSS) (W/Digital Cluster)

Vehicle speed is sensed by a Vehicle Speed Sensor (VSS). This is comprised of a sensor unit mounted on the transmission, and a VSS Buffer mounted behind the instrument panel (I.P.). The Buffer is supplied 12 volts by the ignition and the ECM. The output from the Buffer to the ECM is a "toggling" of voltage. As the vehicle speed increases the 12 volt signal from the ECM is turned "ON" and "OFF", the faster the speed, the more rapidly the signal toggles. This tells the ECM how fast the vehicle is going. Code 24 says that the ECM has seen the following:

• Vehicle Speed Signal (voltage at term. "16") not rising and falling.
• RPM within an upper and lower limit.
• Engine load (TPS) above a given value.
• Not in Park or Neutral.
• For more than a given time.

This could keep the Torque Clutch Converter from applying and affect idle speed control motor.

1. This test checks if there is a VSS signal to the ECM while the drive wheels are turning. Normal voltage will vary from under 3 volts to more than 6 volts as the wheels turn. If the voltage varies while the wheels are turned, the problem is not in the VSS circuit. Fault could be ECM connector, the ECM or TPS setting.
2. This test checks for proper voltage from ECM to Buffer connector term. "F". If the voltage output from the ECM to the Buffer is in the normal 10-12 volt range the fault is in the buffer connections or the buffer. Low voltage indicates a ground or open to or in the ECM.

Fig 1: Code 24: VSS (Digital Cluster) Flow Chart