Tach Jumping at Idle?

[LWJ]

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

[awdbandito]

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.

[rgmrgm]

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

[304065]

Vr

[Joe Bob]

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.

[Bob Kontak]

Originally Posted by Joe Bob

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.

[wwest]

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

[LWJ]

Thanks! Makes a lot more sense than the distributor.

Larry

[dshepp806]

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

[Targalid]

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.

[Joe Bob]

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......%^)

[rusnak]

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

[Joe Bob]

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

[rusnak]

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.