[Gmecm] VSS

[Jared Ryan]

I mostly lurk on this list, trying to absorb what information I can, 
but I finally have a question, and I wouldn't be surprised if someone 
has covered this before.

My Chevy El Camino has a VSS in the speedometer.  It appears to rely on 
a bar that spins with the speedometer cable that breaks a beam of 
light.  It occurred to me that if I could have a computer (a desktop or 
laptop computer) input the signal from the VSS, I would have an easy 
way of measuring the speedometer's accuracy.

So my questions are two:

1. What signal does the VSS module send to the ECM?  Does it change 
voltage or the number of pulses in the signal it sends to the ECM?
2. What would be required to change the signal from the VSS module into 
something readable by a PC through an RS-232 or parallel port?

I would be surprised if someone hasn't already come up with a way to do 
this, so if someone has some hints, I'd really appreciate it!

Thanks, and have a good day.

  ---> Jared Ryan <---
jryan at caminofx.org | http://www.caminofx.org

1984 Chevrolet El Camino
350 roller camshaft, 300 HP
TH-200-4R transmission
TBI