Can anyone point me where to look? Both of my tachs are dead? Seem odd that they both dead at the same time. I have Det Mar 6000 RPM tachs and Thunderbolt IV ignitions. Carb 454's.

I have 12.4 volt at the guage (Bat to ground)
and 12.4 signal to ground?

The tach doesn't move. How do I test the gauge?

Thanks for any help!

How do you tell if the tach is just dead. I also have "sincr" meter.

Thanks for the help

Hopefully you have solved your issue....

If not many voltmeters will do frequency as well. I think you could see if you're getting the signal at the tach then (at least that is what I just did......after checking every single other possible option). I haven't put in a new tach yet, but am fairly certain it is what is broken now.

There's info online on how to use a voltmeter as a home made tach which should help.

Check all grounds sounds like it's got to be something simple if they both went. I know it can be very frustrating. I just had one giving me fits I checked and checked ended up being bad connection in the cannon plug. Easy fix was just a headache.