tach problam on a 1983 70 horse johnson outboard

[Mr woody]

HI , need help my tach is doing funny things , when motor is running tach junps all over the place, I checked the batterie and its getting a charge when running. When motor is off and key is in the on position and I tap the tach it will spin all the way around, also I looked on the back of the tach, I read on one of the forums it should be on 6 on the back of tach , on the back of mine it has A B C ??? SO is it a bad tach , or other problams?? Thanks..

[boats.net]

Sounds like bad tach.