Honda S2000 Vehicle Speed Sensor Readout

Question

I'm attempting to figure out how the Vehicle Speed Sensor on my 2001 (AP1) Honda S2000 works. Unfortunately, I don't have access to a proper oscilliscope, just a multimeter.

I was hoping it would be a straight voltage (0 volts and you're not moving, voltage increases the faster you're going), but it appears to jump up to around 5 volts whenever I'm moving. This makes me think it must be a square wave and the speed is determined by the frequency (since it's just a coil spinning around somewhere and being read by a magnetosensor everytime it goes by).

Can anyone confirm this (or explain how it actually works), and is there a table somewhere where I can figure out what frequency corresponds to what speed?

Answer 1

That's exactly what it is. Typically a VR sensor for ABS systems. This outputs an analog wave signal that's translated to a square wave pulse signal. The frequency is then correlated to speed.

Some cars used Hall Effect sensors as well, but they don't hold up as well in nasty environments. I'm pretty sure all modern (~2000+) cars use VR sensors.

For the actual speed you'd have to consult the manufacturer docs, like a Factory Service Manual. I suppose reverse engineering it is probably not difficult. I'm thinking you could count teeth on the sensor wheel, and your wheel and tire diameter to figure it out.