Tach diagnosis
 

My truck factory tach startre to bounce all over the place. I thought well, it is 32 years old. then I took a trip and it worked flawlessly! That was last week , now it is back to bouncing.

Is there some way to tell if my tach is good or bad? Or what would be the best way to find out what exactly is the problem.

Thanks to all help.

Check the connection at the coils, the plug at the fuse box... and make sure the dash cluster metal itself is grounded to the chassis dash.