Code 24: Vehicle Speed Sensor (VSS) (Integral Buffer): Notes
WARNING: This page does not describe the selected car, but rather 28 other vehicles, including the 1984 Pontiac Phoenix, 1984 Pontiac Parisienne, 1984 Pontiac Grand Prix, 1984 Pontiac Firebird, and 1984 Pontiac Bonneville. However, it is still accessible from the selected car via links, so may be relevant.
Vehicle speed is sensed by a Vehicle Speed Sensor (VSS) located behind the instrument panel (I.P.) cluster. The VSS is supplied with 12 volts from the ignition and the ECM. The output from the VSS to the ECM is a "toggling" of the voltage. As the speedometer turns, the 12 volt signal from the ECM is turned "ON" and "OFF". The faster the vehicle speed, the more rapidly the signal toggles. 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 condition can keep the Transmission Clutch Converter (TCC) from applying and affect idle speed control motor operation on deceleration.
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- This test checks for a VSS signal to ECM while turning drive wheel. Normal voltage will vary from under 3 volts to over 6 volts as wheel is turned. The faster the wheel is spun, the less the variation will be. If the voltage varies while the wheel is turned, the problem is not in the VSS circuit. Fault could be an ECM connection, the ECM or TPS setting.
- Checking voltage at the VSS connector, determines which part of the circuit is at fault. Term. "A" is a buffered 12 volt source from the ECM and term. "C" is a 12 volt ignition source. Term. "B" is ground.