pawoSD - my 81 SD tach has not worked since about 86; when it does it exhibits the same strange pegged or zero reading or uncontrolled bouncing. Then a few months ago I got a rebuilt tranny installed. The tranny would not shift but the tach now worked perfectly. I drove it about five miles back to the tranny shop with the tach working fine. They installed another rebuilt tranny and tach has not worked since. Except for a sporatic 100% peg or a few bounces. So what happened between the first replacement tranny and the second; why did it work with the first replacement and not the second? Almost sounds to me like whatever attaches to the tranny could need tightening up, loosening, or something.


Ideas?