Anyone ever seen a pulsing voltmeter?

On our 72 Bronco, I just rebuilt the whole rig and put in a voltmeter in place of the old ammeter. We put very few miles on it with the ammeter after reviving it from its 10-year-slumber. I frankly don’t think I looked at the ammeter more than one or two times in the 5 times we drove it pre-tear down. 

So I don’t know if it did this before, but I noticed a very strange occurrence and wondered if anyone else had ever seen this:

As the speedo drive cable turns, the voltmeter needle pulses. At freeway speed it just vibrates. When going 5mph it slowly pulses. It also jumps with the turn signal which was unexpected as I pulled the signal wire from a wire I thought would be pretty isolated. I believe I tapped into (properly spliced into) a wire I ran under the dash directly from the battery. 

I have another voltmeter that is a little digital guy that is part of my usb plug outlet and it reads a little higher than the gauge but doesn’t seem to fluctuate. 

Has anyone ever seen this? I’m thinking about taping a metal shield around the cable at the back of the gauge to isolate any emi it might be generating but wondered if anyone else had ran into it.

The link to the turn signal is the interesting part to me.  I'd double check all your connections first.  

I wouldn't think the speedo cable turning would create any EMI unless it was somehow magnetized.

Sounds like a failing voltage regulator

Bad instrument panel ground. This is grounding through the speedo cable instead of where it should. Run a ground wire from the negative side of the gauge direct to battery negative, and a second one from the panel itself to the battery negative. let us know if this works too, I am WAGing it.

In reply to Mr_Asa :

The old mechanical speedometers are magnetized. They work by spinning a magnet around the needle shaft. The needle has a spring that is always trying to pull it back to zero, the spinning magnet trying to pull it away from zero, and the faster the magnet spins the farther it pulls the needle. But I've never heard of one effecting a voltmeter. Does the voltmeter ground through the body of the gauge, or a wire? Either way I would I use a jumper wire to a known good ground and see if that fixes it. Where are you getting your 12v+ for the gauge? Run a jumper from the battery to it and see if that fixes it.

TurnerX19 said:

Bad instrument panel ground. This is grounding through the speedo cable instead of where it should. Run a ground wire from the negative side of the gauge direct to battery negative, and a second one from the panel itself to the battery negative. let us know if this works too, I am WAGing it.

I like this post, it sounds right.

Good thought.  I'm pretty sure I grounded the gauge through the cluster housing.  The lights also ground through the casing and seem fine but I will run a jumper and see if that helps!

I've got nothing to offer on the voltmeter, but what's the story on the Morris Minor in your avatar?

What TurnerX19 said. 

I had a car fire in a trans am because the engine was trying to ground through the shifter cable.

TurnerX19 said:

Bad instrument panel ground. This is grounding through the speedo cable instead of where it should. Run a ground wire from the negative side of the gauge direct to battery negative, and a second one from the panel itself to the battery negative. let us know if this works too, I am WAGing it.

That is way too entirely plausible to just be a WAG.

I am think ground issues or you need a filter.

Its a ground issue.  Cluster is grounding through the speedo cable.

Seen it way more than once

I'm cautiously optimistic but I was very thorough in making sure there were bare metal grounding paths.  But I'll try it and report back. 
 

plus the behavior of the needle is more like a magnetic issue. 
 

but we will figure this out!

In reply to jfryjfry (Forum Supporter) :

I diag'ed an older VW that had instrument cluster issues that screamed bad ground, but I had a good ground when backprobing the ground wire.

Three hours of tracing later, I felt like a complete rookie because the problem was a cracked terminal in the connector, not making contact.  I depinned it and stole a less vital circuit's terminal.  Cluster operation restored.  And somewhere there is an '88 Quantum Syncro Wagon that will forevermore have an inoperative oxygen sensor warning light.

In reply to Pete. (l33t FS) :

It is also my initials....

ammeters and volt meters have different wiring.   Any use of the amp mete wiring ?   Just a thought.