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I edited this opening as it contains other things at the end than about the PCM... hope it is helpful!

Tachometer at the end added 11/22/09 ------------

I have been working on getting rid of the PCM in a 1999 Saturn Sl2. Bypassed the Tachometer on atzi's 1998 Saturn...

http://www.saturn.offical.net/node/326
So now it is on to the speedometer. I used a Heathkit (yep, I have one!) function generator

to send a pure sine

Wave to the PCM... I settled on 60 MPH as this is the most important one to use - as it represents 1 mile per min or according to the output in all the manuals I could find the Speedometer = 5000 pulses per mile (thus per min) .. What is strange is all the information I can find says Saturn VSS has 10000 pulses per mile (ppm) so at 60 mph I should have 10000 = 1 min and 10000/60 = 166.66666 HZ... right?

But I get 230 HZ for 60 mph !! that equates to 13800 ppm (VSS sine

Wave NOT square

Wave to speedometer) . SO if that is true then there is a strange number to divide by to get the square

Waves for the speedometer ! 13800 / 5000 = 2.76 .. not the best number to digitally divide by !!

So it does a bit of strange processing NOT the simple divide by 2 that 10000ppm would get me !

The formula I use to get MPH from the sine

Wave is ---[ MPH x 13800 / 60 /60 = HZ] or 60 x 13800 = 828000 /60 = 13800 /60 = 230 Hz..

Easier to see a different one:

30 (MPH) x 13800 (PPM) = 414000 /60 (min) = 6900 /60 (sec) = 115 HZ (or cycles per sec)

AND YES 115 hz gives me 30 mph!!

I thought I would throw this out to see what others think! Perhaps they phase lock it at some frequency but not sure!

Dave