Speed monitoring tap wanted

As I recall, a hall effect sensor, mechanically connected to the transmission is used. I'd imagine it's squared up and appears at the engine control computer. As the cops get greedier and greedier, there might be a market for such an indicator...

In my 1976 VW Rabbit, I put in an aftermarket "Rev Counter" ;) and it tapped of the + side of the Ignition Coil. Definately and analogue input.

Dan

Or maybe mount a second ABS sensor by the ABS gear on one of the hubs. Then, there wouldn't be any chance of computer problems due to tapped sensor signals.

It's a piece of cake for anyone who has a rudimentary understanding of electronics to safely tap into a VSS signal without loading the circuit. The source impedance isn't particularly high, nor is the signal difficult to trigger off, since it is a nice clean square wave. So unless you put a fairly low impedance load on the signal, you're OK.

I once designed and built my own electronic speedometer for an '83 Olds that had a defective mechanical speedometer. I tapped into the VSS signal at the speedometer head, and used it to gate a counter that drove the address lines of an EPROM, the EPROM data was two BCD nybbles (?) to drive some TIL311 latched LED display units. It worked very well and had no effect on the ECM.

Nowadays, I would lean toward tapping the OBD-II bus instead, although it's a lot more work than just hooking up to a VSS signal, there's a lot more interesting data there that would come along for free, to be displayed on a multifunction LCD screen.