Waving Voltmeter

[Captain Obvious]

Well I'm not really sure what would be the best way to proceed here... I've got two proposed ways to go forward:

1) We can go over the exact three wires would be needed (in excruciating detail) to confirm or deny your waving voltmeter phenomenon on the bench.

2) We could skip the above step and I could document (again in excruciating detail) what each of the wires on the back of the gauge SHOULD connect to and leave it to you to find the corresponding wires on your car (because your dash harness is non-stock and I don't have reliable documentation for what's where.

Those are my thoughts. What do you think would be best?   

[heyitsrama]

I think option 2 might be the better option..... The positive wire on the voltmeter and both wires on the charge light are really the only two things in question.....

I think hope by isolating the positives for the volt/fuel meter the waving will go away.....

I'm waiting on a potentiometer extension shaft to correctly wire in the PWM dimmer for the LEDs, so we have some time to think about it 🙂

Edited October 4, 2021 by heyitsrama

[Captain Obvious]

Cool. I think I have a loose volt/fuel gauge around here to snap some pics. Just to make sure what you have there is the same as what I have here.

I think that with the mix-match of years/harnesses/parts, etc, we need a solid point of reference. I'll see what I can find.  

[Captain Obvious]

OK, I found a spare volt/fuel gauge here and it's the same as the one in your pic, so I think we have a solid point of reference.

I added descriptions to the wires in your pic trying to show the functions. Important to note that the wire colors here ARE THE WIRE COLORS RIGHT AT THE BACK OF THE GAUGE. The reason that's important to note is that Datsun SOMETIMES CHANGED THE WIRE COLORS AT THE DASH CONNECTORS, and I believe the volt/fuel gauge is one of those situations. So be careful when referencing wire colors on the wiring diagrams since they sometimes changed colors on the gauge side of the connectors.

In other words, the colors on the diagrams are the colors that exist ON THE HARNESS SIDE, and not always on the gauge side.

But all that said... Here's what the wires do right at the back of the gauge:

Most of the connections are unique and should be relatively easy to identify on the dash side (assuming things aren't TOO hacked up), but there are two connections that are not. There are a) two ground connections, and there are b) two "hot in ON and START" connections.

a) The two ground connections just go to ground and there's no real requirement for them to be kept separate. If you wanted to for some reason, you could just connect those two to the exact same ground location.

b) The two "hot in ON and START" connections originally went to two different fuses in the block, but if you get really stuck and just can't deal with it anymore, then it probably doesn't matter much which fuse get's which function. But if you really want to figure out the real-real correct fuses, we can work through that too.

So with the different years and different PO's in there, I think it's going to have to be up to you to find the correct connections on the dash side, but that's what they should be connected to.

Is that enough excruciating detail? Hope that helps!

[Captain Obvious]

Oh, and while you have the gauge on the bench... The two voltmeter connections (Black and black with red tape) should be completely isolated from everything else about the gauge, including the metal housing. Take a resistance reading between those two wires and gauge housing metal and it should show no connection.

And the other ground (the one for the fuel gauge) should be the opposite and should be tied hard to the gauge metal housing. Check that one too.

I'm just still a little bugged by your waving voltmeter test that started this thread. I still think the problem was either simply just the wrong wires used or a source impedance issue with your motorcycle battery getting pulled down by the additional load of the fuel gauge, but I'm just looking into other possibilities while you're in there.