Waving Voltmeter

[heyitsrama]

I have a voltmeter that reads pretty stable if the circuit is isolated, but when the ground for the fuel level is connected it starts to be flutter around. Anyone seen something like this before?

 

https://youtube.com/shorts/YMjUsnmcalg?feature=share

[heyitsrama]

Actually I think I fixed it, there are 2 standoffs on the back side of the volt meter, the looks like 2 long hex shafts. They were loose from the volt meter. I opened it up and it looks like the voltage is stable now.

i shot some electronics cleaner into the potentiometer for good measure. It stopped waving at me. 

[heyitsrama]

For reference, these. 

[heyitsrama]

Ugh. Assembled it and the wiggle is back!

[dutchzcarguy]

I assume you have it on the bench just connected to a battery? and when you connect the fuel meter it starts bouncing? then that meter is faulty.. does the fuelmeter normally work when in the car?   (i assume your fuelmeter is not connected to anything when on the bench?)

I would look for another set of meters and test it..

[Captain Obvious]

That wiggle is caused by the temperature/voltage compensation circuit built into the fuel gauge portion of the meter. But that said... the wiggle should not be happening, so let's explore a little.

It looks like a source impedance issue where the amount of current you're pulling from your source is dragging the supply voltage down. The question is "But why?"

How many wires are you connecting on the gauge? Two for the voltmeter, and a third when you bring the fuel gauge into the picture?

[Captain Obvious]

Forget all that stuff I wrote here. It's wrong.

Oh, and by the way... The ground side of the voltmeter and the ground side for the fuel gauge are already connected inside the gauge assembly. So when you say "you're connecting the fuel gauge ground"...  I'm not really sure what you mean by that.

In other words, there is no fuel gauge ground accessible outside of the assembly. There is the fuel lever sending unit connection, but no ground.

So if I had to shoot from the hip here, I would guess you're connecting the wrong wires somehow.

Edited October 3, 2021 by Captain Obvious
My bad

[heyitsrama]

@Captain Obvious I was reading the FSM in bed (everyone’s second favorite activity after eating ice cream in bed) last night and I think your conclusion about the wiring being off is correct.

There are 3 positive connectors for the volt meter, fuel meter, and charge warning LED all feeding off the positive white/red wire on the underside of the dash harness. This 260z has a alternator upgrade, so it’s wiring is a little different, I think the voltmeter goes to the white wire (positive always on), the fuel meter goes to w/r (on with key) and the charge light goes to the L sense wire which is on the back of the alternator/ where the modified voltage regulator plug is.

@dutchzcarguy I pulled my dash out a couple months ago to do the EPAS modification to the steering column, I was cleaning the dash up a little and wanted to see if there was something wrong with the voltmeter because it always waves at me. I don’t like it. Yeah there’s a motorcycle battery on the bench that I used to power things up. I’m going to try fixing the potential wiring issue, and (fingers crossed) won’t have to order a new one…..

 

BTW anyone reading this have amber dash lights? I can’t decide between amber and white. Not really digging the green anymore. 
 

Edited October 2, 2021 by heyitsrama

[heyitsrama]

@Captain Obvious On the gauge that is in front of me I have accessibility to each wire individually. When I connect the positive/negative wire for the fuel gauge to the same power supply source as the volt meter, that’s when the waving comes back.

the black wires at the top are for the voltmeter. 
the wires were joined together by shoving one behind the other inside the plastic connector. Then taping around….

[Captain Obvious]

So it sounds like this is a non-stock application where you're putting a 280 gauge into a 260? Do you know what year is the gauge?

And by the way, forget that stuff I said earlier about the grounds for the voltmeter and the fuel gauge being tied together inside the gauge... They're not. I was not reading the FSM diagrams correctly. 

The two black wires at the top of the gauge (one solid black and the other is black but has a red wrap of tape on it). Those two are for the voltmeter. I assume the one with the red tape on it is positive and the other is negative. Those two come out of the back of the gauge at 12:00 ish when looking at the back.

And then the voltmeter ground is the other black wire that comes out of the back of the gauge at 8:00-ish

Those are the three wires you're working with?

[Captain Obvious]

And about your dash gauge color... I changed my Z over to white and never looked back.

I had amber in my late eighties Pontiac Fiero and it was cool. For about a week.

[heyitsrama]

2 hours ago, Captain Obvious said:

So it sounds like this is a non-stock application where you're putting a 280 gauge into a 260? Do you know what year is the gauge?

And by the way, forget that stuff I said earlier about the grounds for the voltmeter and the fuel gauge being tied together inside the gauge... They're not. I was not reading the FSM diagrams correctly. 

The two black wires at the top of the gauge (one solid black and the other is black but has a red wrap of tape on it). Those two are for the voltmeter. I assume the one with the red tape on it is positive and the other is negative. Those two come out of the back of the gauge at 12:00 ish when looking at the back.

And then the voltmeter ground is the other black wire that comes out of the back of the gauge at 8:00-ish

Those are the three wires you're working with?

I think the gauge is off a 77 it's been a while I'm not sure actually.

That is correct those are the wires that I'm working with. I plugged positive wire for the voltmeter to the white/red plug on the dashboard. I plugged the positive wire for the fuel meter to the green wire under the dash, b/w on key+. I dont think that I can run the charge light with the wiring that I did in the engine bay, from what I understand it needs to connect to the L outlet on the alternator, but  I tucked the voltage regulator wires when i had to snip and add the diode for the alternator.

As i type this I realize I am THAT PO that hacked the wiring harness.... In my defense the engine harness was missing 😞

Here is a crude wiring diagram, the negative on the charge light is what is up in the air... I ran an extra line under the dash for it if I ever redo the engine harness it will be straight forward to add....

 

It seems like the waving starts if the positive is also shared from the bench top battery, I wonder if this will also be the case when the dash is back in the car and everything is plugged in, the positive source is eventually the battery/alternator......

Edited October 3, 2021 by heyitsrama