speedometer calibration

[just plain john]

I'm thinking of recalibrating my 81 El Camino speedo from 85 to 140 mph. I don't know much about it, but I do know the ECM needs road speed input to control various management functions. My stocker has what looks like 2 optical triggers on a wheel inside the speedometer. The overall diminsions are different than the Buick, so I cant just swap them. I also have been told I need a different VSS as mine outputs a different "wavelenght' than the Buick ECM recognizes. Bolting together mechanical parts is easy enough, but this is new territory. As I have zero experience with this, any and all tips would really help.

 

[spidereyes]

The VSS should use the same signal in both applications what is refered to as a 2 pulse system, meaning 2000 pulses from the VSS to the ecm per mile, via the optical triggers on the back of the speedo. Later cars such as 1990 up tuned port F bodys switched to a 4 pulse system meaning 4000 pulses from VSS to ecm per mile. Also you will have to have a speedo shop recalibrate your speedo head to read correctly with a 145mph face.

 

[just plain john]

Thanks for the reply. I mis-spoke when I said ;wavelenght', what I should have said is 'waveform'. At any rate, I still don't have a clue how to make it all work.