[Gmecm] Tachometer circuit question

[Ron Drew]

I have a 1988 Cutlas Cierra (2.8L) which the tachometer gauge has not worked 
since I purchased the car. The schematics that I have indicates that the ECM 
gets an RPM signal from the ignition module via white wire. Other than this, 
I can find no other reference in my manuals to the feed circuit for the 
tachometer gauge. I assume it would be similar to the VSS circuit (the pulse 
train is fed to the ECM which applies a scale factor and outputs the scaled 
signal to the tachometer).

This is what I have tried so far:

- Swapped the ECM.
- Swapped the tach (I purchased a spare instrument cluster from a junk 
yard).
- I did swap the ignition module for no other reason than I was running out 
of things to try.
- I have not tried to ohm the tach feed circuit since I am at a lost as to 
what circuit specifically supplies the signal to the tachometer gauge.

Can anyone offer any help?

Thanks
Ron Drew