Speed Sensor #1

[b3lha]

Here's a question for the gearbox gurus: What does speed sensor #1 actually measure? It must be counting the number of rotations of some shaft over a period of time.

I know that notionally VSS1 reports the rear wheel speed. But it can't count rear axle rotations because it is located in the gearbox, not the rear diff. To report rear wheel speed, the TCU would have to count rotations of the prop shaft and divide this by the diff ratio to work out the rear wheel speed. In this case, the TCU must know the diff ratio.

If this is how it works then VSS1 would read incorrectly on cars with a swapped diff ratio while VSS2 in the front diff would read correctly. So there would be a discrepancy between the front and rear speed sensors. Can anyone confirm this?

Looking at the program in the TCU, it seems to look at what gear you are in and multiply by the gear ratio. Does this mean that VSS1 is on the input side of the gearbox? Or is the program possibly multiplying rpm by gear ratio in order to fake a VSS1 signal in the event of a sensor failure?

Any ideas?