tveltman

I've been over the wiring diagrams many times (I bet you're all surprised that a 928 has electrical problems), and I've noticed in passing that there is a sensor marked "vehicle speed sensor" on the diagram. Does anyone know where it is, what sort of sensor it is, what output signal it offers, and what the ECU uses it for? Seems like it could be an interesting device, especially since it would appear that the speedometer is electronically-controlled (at least in my '88 S4). It looks like there is an electrical motor that gets variable voltage that drives the worm gear for the odometer, and the speedometer looks like a simple voltmeter. The reason I ask is that my speedometer is about 4% too low, and I was wondering if there was a variable resistor or some such in the circuit that could be re-calibrated. Also, that speed sensor would be super-useful for creating a fuel-economy gauge. Thanks for tolerating my curiosity!

dr bob

It's pretty much a reed switch, actuated by magnets, in the side cover of the differential where the axle shaft exits. The wiring, IIRC, is shared in a harness with the heutral safety and gear selector switches if you happen to have an automatic. Wiring runs into the battery well, conneector, then to the harness to the dash.

Later cars, '90+, use a front wheel ABS/PSD sensor for speed input.

Vilhuer

Its at rear cover, not side cover. Inch wide round thing held by small wire which is bolted to cover.

tveltman

Cool, thanks