Normally a tach is electronic and when the start to read incorrectly it is usually because of a dirty or loose connection. Check for these on the back of the Tach and correct as needed.
The other reason is the Tach is dirty inside-so if the wiring fix doesn't make it right, you should carefully take it out of the dash, take it into a work shop/clean working area, spread out news papers to catch small scews and look for dirty on the moving parts. IE There is usually a portion of the need that makes contact with a maget that gets dirty. Usually cleaning the dirt found on this and the other moving parts will make it as good as new.
Let us know how it turns out.
Kevin Smith, Richmond VA
1991 250/270 Weekender, 454CI, 330HP, Bravo II
