The Vehicle Speed Sensor

 

The speed of the car as it appears on the dash speedometer is set by a sensor on the gearbox.

The sensor itself mates to gearing inside the gearbox with a screw clamp and provides a clean square wave output to the speedometer and to pin 4 on the EEC IV.

The speedometer operation is just a straight forward waveform count with the displayed speed being a function of the waveform PRF (Pulse Repetition Frequency)

The signal is also sent to the EEC for processing.

Speed Sensors can cause issues as they occasionally partially fail and put out scruffy signals that cause strange effects. See Below.

Operation

With the ignition on around 10v is applied to the VSS. As the sensors turns with the road wheels the Hall Effect signal output from the VSS is passed to the speedometer.

If you have access to an oscilloscope a typical, good waveform would look like this :-

VSS Waveform

This is the general schematic

VSS Wiring

The cable from the sensor runs under the gearbox tunnel and unto the cabin through a grommet sealed hole just by the drivers left knee.

The diagram shows the signal wire from pin 3 on the VSS to be inside a screened cable. A poor chassis ground here will cause problems with the radio reception as a swinging 12v pulse will radiate significantly. The gearbox tunnel itself is also screened by a wire mesh. On my car this was in a very poor state so I replaced it. See here.

The three pin connector is orientated as below.

VSS Plug

A Fault on my S3

My S3 had an issue that was ultimately resolved by replacing the VSS. The symptoms were irratic idling and over revving when coming to a halt.

Typically what would happen is that I'd draw the car up to a slow halt say, at the lights or a junction and the engine revs would rise to 2500 and the drop down to 900 rpm or so. I took a while for me to realise that if I crept along at a walking pace then the revs would stay high but as soon as the road wheels stopped completely the the engine would fall back.

After trying everything I could naturally think of (TPS, ISCV and AFMs) a post came up on Pistonheads that pointed me in the right direction. I added my own twopenneth for what it was worth.

A new VSS cured it straight away. Don't forget to clear out the EEC codes after you've fitted it.

This link explains all http://www.pistonheads.com/gassing/topic.asp?t=900752

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