off the wall ?? mercury tach

[marcdups]

where does a mercury 250 hp efi 1999 model get it's tach signal from, is it the alternator or the ecu??or the flywheel sensor?? I cannot think about outboards as we are installing twin Cummins in a dive boat right now and after having John Deers in it modifications are quite intensive and he has two charters Saturday, :shocking: :shocking: :party: :outtahere

[Fish boy]

DO you have a smartcraft gauge or are you using a standard gauge?

[marcdups]

standard Fish, therefore I thought it would be the alt, as I could not find an external regulator/ rectifier, but voltage output is okay at all rpm's, just tack jumping all over the place, so ???

[MOP]

This may work try rotating the switch back and forth in the back for cylinder changes, sometime that helps by cleaning its contacts. I have resurrected a few condemned tachs doing that. Also check grounds.

[marcdups]

MOP tried new tach with same results ??