[s-cars] tapping the speed sensor signal

[Mike Fitton]

I have a special project in the works and I need a reliable source for 
speed information.  I've messed around with magnets and hall sensors 
over the last few years and have been less than impressed.  I'm betting 
Audi's idea of a speed sensor is also a hall sensor, but there seems to 
be a crucial difference between me putting it together and Audi putting 
it together.  In my car, Audi's setup is still working after 215k 
miles.  Mine haven't lasted 100.

So I've been strongly considering tapping off the factory speed sensor.  
Internal accuracy isn't very important, but consistency is, so running 
off the transmission's output shaft seems to me like it would be very 
tolerant of individual wheels' variances with respect to actual vehicle 
speed.

Here's the catch.  I need to pipe it into a PC parallel port.  I know 
how to do that (sort of, like I said) with radio shack parts, but I have 
no idea what the signal from the factory sensor looks like 
electrically.  Questions like...is the voltage across that line going to 
blow up my port?  Is it a square wave or a sine wave?  I don't even know 
which freakin wires it is.

Thoughts, comments, suggestions, hard data?

For the curious, the main thrust of the idea can be found here:  
http://www.rvscc.com/  I'd love to see some more Audi guys this year.  
Historically it's been about 50% Subaru kids and 50% other.  Sadly, I 
suspect I'm the only person in recent memory to take an urQ to PBX...

-Cheers!
Mike