So my tach was acting weird ever since i bought my 1987 6.9 dulley. It would start out fine but after driving a while it would cut out and drop down to 0 looked at sensor and wires everything was fine so i was stumped until my battery light came on. If you havent figured it out yet my altinator was only working speraticly. So the tac also worked speraticly when the altinator stoped all togeather i found the issue, but what i think is great is i now have an altinator malfunction warning gauge. If your tach is jumping around your altinator is about to go

 

That or if it squeaks. Mine squeaks after being run for a while. If I take the belt off and spin it, it squeaks. I'm just waiting for it to go any day. I'm short on cash right now, so it better wait.

Good to know about the tach, though.

 

Was the O/D light flashing when the tach went dead?

 

no light came on till the altinator went dead and the batteries were drained then just the batterie light came on