Code 24 - Vehicle Speed Sensor (VSS)
The VSS gives the ECM the car speed information. The ECM applies and monitors 12 volts on circuit 437. circuit 437 connects to the VSS which alternately grounds circuit 437 when the drive wheels are turning. This "pulsing" action takes place about 2000 times per mile and the ECM calculates vehicle speed based on the time between "pulses". Code 24 will set if: circuit 437 voltage is constant, engine speed is between 1400-3600 RPM, less than 2 percent throttle opening, low load condition (low air flow) and not in park or neutral. All conditions must be met for 4 seconds.
NOTE:
The following step numbers refer to the numbers on the Diagnostic Flow Charts.
- This test monitors the ECM voltage on circuit 437. With the drive wheels turning, the "pulsing" action will result in a varying voltage. This variation will be greater at low speeds to an average of 4-6 volts at about 20 MPH.
- Voltage less than 1 volt at the ECM connector indicates that the circuit 437 wire is shorted to ground. Disconnect circuit 437 at the VSS, if voltage now reads about 10 volts, the VSS is faulty. If voltage remains less than 10 volts, then circuit 437 wire is grounded. If circuit 437 is not grounded, check for a faulty ECM connector ECM.
- A steady 8-12 volts at the ECM connector indicates circuit 437 is open or a faulty VSS.
- This is normal voltage which indicates a possible intermittent problem.
- If SCAN displays vehicle speed, check P/N switch chart C1A or vehicle with auto trans. If switch is okay check for intermittents.
NOTE:
To prevent misdiagnosis, technician should identify the type of vehicle speed sensor used, prior to using this chart. Disregard Code 24 if set when drive wheels are not turning.