Wacky Tach

[TillamookB2]

I don't know when it happened... But not too long ago, my tachometer told me that at an idle, it was going just over Zero RPM. I became suspicious and shut off the truck. My RPM dropped to somewhere around the fuel gauge. The tach needle moves normally, its just way off. What might have happened and how can I correct it?
BTW, its a 1988 B2 2.9 5sp 4x4.

 

[Beef52751]

my 86 ranger does the exact same thing!!!!

 

[Bronco_Chuck]

My guess would be an added resistance in the signal wire (from the coil I think?) to the tach.