[Gmecm] VSS

[Burntkat@sc.rr.com]

Just a note-

When finding "known good" mile for measurement-- don't depend necessarily on
highway mile markers, and especially not Google Earth.

Had to hoof it to Advance a few weeks back to get a starter- Google Earth
said it was a 3.4 mile round trip. I got curious and took my GPS and by the
time I got back home I'd walked exactly 5.7 miles. So while cool, Google
Earth isn't something I'd depend on- cheap GPS unit is MUCH more dependable.



-----Original Message-----
From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf
Of bcroe at juno.com
Sent: Monday, June 19, 2006 12:10 AM
To: gmecm at diy-efi.org
Subject: [Gmecm] VSS

I would suggest this to check accuracy.  Spin the speedo 
cable at a constant speed.  I use the induction motor (don't 
use a brush motor) on my drill press to spin a spare cable, 
motor reversed.  The pulleys let me pick several relevant 
speeds.  Use a stopwatch to time exactly how many 
seconds for the odometer to roll 1 mile.  Divide the seconds 
into 3600, and you have the exact MPH the speedo should 
be reading (as compared to what its really reading).  

If the speedo is adjusted to match the odometer as above, 
its then a matter of comparing your odometer against 
some good mile markers, change trans speedo gears to 
get a match.  Go 10 miles since individual mile markers 
may be off a bit.  There is no particular speed required 
to do this.  

Bruce Roe

18 Jun 2006  Jared Ryan <jryan at caminofx.org> writes:
> 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
> 
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