Factory tach indicates 2X actual

[Dale Lucy]

So I finally got the in dash factory tach in my '69 GTX connected to the MSD ignition system, I added a MSD 8920 tach adapter, and sure enough the tach now works, but reads 2X (ish) the actual rpms. Any ideas?

Dale

 

[kiwigtx]

So I finally got the in dash factory tach in my '69 GTX connected to the MSD ignition system, I added a MSD 8920 tach adapter, and sure enough the tach now works, but reads 2X (ish) the actual rpms. Any ideas?

Dale

Which wiring diagram did you follow on the Instruction sheet? I have installed 3 of those, and all of them worked fine - Mopar's and a Ford.

 

[kiwigtx]

You should be using this one...

 

[Dale Lucy]

Yep, thats how its wired.

I wired in another aftermarket tach, comparison is as follows

Factory tach Aftermarket
1500 1000
3500 2000
5000 3000

Maybe just a calibration issue?
Dale