Find out if it is a mechanical Tach (Cable driven) or electrical.
If mechanical it could be one of Three things.
1) Tach guage (needle) / cable inlet stripped
2) Tach Cable, broken / stripped
3) The tourque/gear reducer on the transmission.
For trouble shooting the guage and the cable, disconect the cable from the trans. and attach a power drill to the cable. Making sure that the drill is rotating the right way, it should register a RPM reading. if you get a reading it is the gear reducer, if no reading, disconect the cable on the guage side and observe the cable ending, look for signs of stripping, and observe the rotation from the drill. If the cable is rotating and the cable end looks good your guage is the problem. If no cable rotation, cable is your problem.
In all of the Mechanical Speedo/Tach problems I've done it usually turns out to be the cable, or the gear reducer, but never rule out the tach.
As for the electric controled tach.......Well good luck, I would need a good electric troubleshooting manual
, a good test light / multi-meter, a six pack of a good barley beverage
, and a glossery of good swear words
.
Good luck
