Tach jumps around

[jerryb1]

My friend has a 1985 Tiara with twin 5.7L V8's with thunderbolt ignition. One tach bounces around +- 500-1000 RPM's. I thought we would try switching the Tachs but cant get at the wiring easily to do it. Is this usually the guage or the signal from the ignition?
Thanks

 

[Chris1956]

Re: Tach jumps around

Jerry, It is usually the connections for +12V, ground and sender. Is there no back panel (in the head) for that dashboard?