Tach Jumping at Idle?
 

Hello,
I have an 80 SC. Runs great above idle. I have an intermittent issue where the tach jumps around at idle. There is a corresponding RPM change / surge but not to the extent that the tach shows. I am pretty certain this is electrical due to the tach. I suspect the distributor.

First question - agree or disagree?
Second question - what am I looking for if it is the distributor? I am failing to grasp what kind of fault would cause this.

Thanks,
Larry

I had similar problem on a '75. Jumped around at idle but once the car was good and warm the problem went away. Troubleshot all the electricals with no solution. I finally swapped in another tach and that solved the problem. I'm guessing something internal to the tach was on its way out.

i had a tach jumping problem and it was caused by a bad voltage regulator

Vr

Put a meter on the battery posts. Look for voltage spikes when the tach jumps. Spikes could be over 14v will indicate a bad regulator. High spikes will boil the battery.

+1

The lazy man's check of the VR is to idle the car at night with lights on and see if the headlights vary in intensity.

Symptom of battery being overcharged, sometimes (often?) caused by intermittent shorting of alternator stator windings.

Thanks! Makes a lot more sense than the distributor.

Larry

Joe Bob's on with getting out the voltmeter. I'd be much more concerned about an overcharge condition, as several things can take a hit...and you don't want the DME to be included in that list.

BEST!

Doyle

You can watch one of these to see if your voltage regulator is failing.
Equus 3721 Battery and Charging System Monitor : Amazon.com : Automotive

Also lift the engine compartment in a dark garage and look for arcs in the wiring.

Just checked the alt on the blonde with big boobs two houses over....she has 15+v at the battery. Got a hug with schmooch.....and a reach around.....might get more when I swap out her alt.

Stay tuned....

a DVM can be a lifelong tool......%^)

JB, just be sure to explain that the rotton egg smell is her battery, not you. And wear clean underwear.

That was the pickled pig's feet....and Mongo runs Komando....just sayin'.....

yeah, but you can blame her battery for the off-gassing.

I gotta tell you kids everything. I'm like Cicero giving you pickup lines whilst hiding in the bushes.