Vehicle Speed Sensor Questions

Questions About a Vehicle Speed Sensor

Jun 10, 2007 Vincent Ciulla

Some Questions Concerning the Operation of the Vehicle Speed Sensor

Question: Vehicle Speed Sensor Questions

My Dad e-mailed you earlier this week regarding this, but basically here is the problem. Both my ABS and cruise control are not working. This happened about a year ago, after an off-roading excursion. I am assuming water or something damaged the speed sensor. I am also assuming that the cruise control piggybacks its speed check from the ABS unit.

My questions are:

1. Do my assumptions make sense?
2. How does this Vehicle Speed Sensor work electrically? Does it output a voltage that is proportional to the speed, or does it send out pulses for each revolution?
3. How could I go about testing my current one to make sure it is actually broken.
4. How does the other Vehicle Speed Sensor work, electrically? (The one connected to the transmission).

Thanks,

Chris

Answer:

1. Makes perfect sense to me, Chris, except the cruise control gets its signal from the Vehicle Speed Sensor (VSS).
2. There are a couple of different types of VSS but they all operate on the same basic principle.The vehicle's speed sensor, which is mounted on the output shaft of the transmission (some later models with mechanical speedometers have the speed sensor mounted behind the speedometer) sends electrical pulses to the computer, pulses which are generated through a magnet that spin a sensor coil. When the vehicle's speed increases, the frequency of the pulses also increases. Note that for any given speed of the vehicle, there is a corresponding pulse frequency. It is this pulse frequency which the cruise control tries to maintain as a constant. Speed sensors are usually made up of a toothed metal disk mounted on a rotating shaft and a stationary detector covering a magnetic coil.If the vehicle speed sensor fails, the cruise control system will not get a speed signal. If this is the case, most probably, the speedometer will usually stop working as well.
3. To test the Vehicle Speed Sensor:
   • Remove the Vehicle Speed Sensor from the transmission.
   • Connect an AC voltmeter set to the 0 to 1 volt scale to the connector pins (a) and (b).
   • Turn the Vehicle Speed Sensor pinion quickly with fingers and measure the voltage across pins (a) and (b). Reading should be approximately 0.5 volts AC.
4. The one mounted on the transmission is the Vehicle Speed Sensor. The ones mounted on the wheels are Wheel Speed Sensors (WSS). The Wheel Speed Sensors sends individual wheel speeds to the Anti Lock Brake System Control Unit to control the Anti Lock Brake System activation.

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