Alternator working, Tach Not.

[99gmccrew]

Everything has been working fine, suddenly noticed while driving 65 mph tach is redlined.
At idle fine. once over 2k rpm suddenly jumps. Does same thing while reving in neutral. Any ideas. Alternator pumping out 14 volts while driving.

[Hye]

My '94 did the same thing just before the alt. bearings went out. Take the belt off and see how much bearing play you have.

[99gmccrew]

I checked for play after removing the serpentine belt. There was no play. The connections all appeared to be OK. Has any one else had this problem?

[99gmccrew]

Alternator finally died yesterday. I replaced it (extremely easy to do!} and that cured my tachometer problem! Good call Hye. This site really rocks.