tach on my 91 Mark VII is sporadic, malfunctioning
the tach on my 91 markVII is acting up it reads right for about 5 seconds the jumps to around 4 grand for another 5 seconds then goes back to the right reading and continues this cycle the whole time its running, just wondering if there is a ballast or relay for the tach that may have went bad or is it just the cmplete tach unit itself any input would b appreciated thanx