Fixing fuel gauge, found electrical confusion

[Reggler]

So on my '72 Suburban Ambulance, the gas gauge is pegged at 3 o'clock. Reading on here says ground issue, but when I tried to ground out the gauge, that's where things get confusing. I read that you ground the wire that goes to the fuse box, where it says fuel gauge. But my truck might not have the right wires going there.
I have an orange wire that was plugged into fuel gauge on the box, that also has a pink wire piggy backed to a ground at the voltage regulator. But when tracing the orange, it goes into the headlight switch harness. Then, the cluster lights plug, has a grey wire, that has a light tan wire tapped into it also going up to the cluster but in a different harness than the cluster lights one. With both of those disconnected from the fuse box the only thing not working is the right bulb in the heater control, everything else is still functioning[aside from broken fuel gauge].


Attached are pics of the fuse box and wires that I'm dealing with. I'm trying to fix this, but it's really a struggle without pulling the whole harness apart.
BTW truck is a C10, 350/auto, No gauges, A/C, column shift. Mostly original, also aside from ambulance roof.

[Bigdav160]

The terminal in the box goes goes to the gas gauge and the sending unit wire plugs into it. GM did this because the rear pickup harness is different than the Suburban/panel rear harness.

Orange is power. I can't remember if it is battery or ignition. The grey wires, I am pretty sure are lights (steering column indicator, dash).

Off of the top of my head I don't remember the white. And my Suburban is at my work place. The door jam switches are white. The wire goig to the sending unit could be white.

edit: on another monitor I can see the wire in your pic is tan not white. That could be the one from the fuel sending unit. You could test by unplugging it and checking resistance to ground.

[Bigdav160]

OK, the fuel gauge is the terminal at the top (or the far left in your picture).

I have a tan wire there.

The empty terminal that you have circled in red is also empty in my fuse box. I didn't check to see if it was fused battery power or ignition.

[Reggler]

Ok. So someone plugged the tan wire into the grey wires, that lead to positive then. So I should unplug that, run the tan into the fuel gu spot on the box, and then test again. I have a feeling the resistor is going to be blown out since it had power on either side of it.
I do have an aftermarket gauge I'm willing to install if the factory one is junk. I have tested it with a sending unit on the shelf and it works, but could figure out where to get the sending unit wire from under the dash since everything was so messed up.

Thanks

[Reggler]

Just glanced under there a quick second. Does the Orange plug go into the cig lighter port on the fuse box? Was going to undo those 2 connectors, but needed a screw driver. I'll try again later today

[LockDoc]

Originally Posted by Reggler

Just glanced under there a quick second. Does the Orange plug go into the cig lighter port on the fuse box? Was going to undo those 2 connectors, but needed a screw driver. I'll try again later today

The wire for the lighter is a Red wire with a double connector on the fuse box end. I'm not sure where the orange wire would go. It looks like a good sized wire. I know Suburbans have different wire looms than the pickups and I'm sure the ambulances have even more wiring.

LockDoc

[Reggler]

Well I just got in from tinkering, and I got somewhere at least. Turns out, at the cluster the tan wire was on the wrong pin. Switched it, and now when I ground it out at the back it goes to 0. Tried to clip a lead onto the sending unit to ground, and the gauge did flinch when I did it, but still stuck at 3 o'clock. I have a plastic tank and can see a ground coming from the top, but the floor is in the way to get my hand in there. So progress was made, just need to either swap the unit out, or maybe try a cleaner ground.
But the orange wire, runs up the left side of the dash, taped together with the grey "cluster lights" wire,that only powers up the right heater control bulb, and then joins into the harness above the cluster where I lose it. I was digging around the dash for over an hour but couldn't see where it goes. Looks like a newer wire at least.
Oh well, there's always tomorrow.

[LockDoc]

-
There is a wire with a ground clip on it that clips to the dash and goes to the cluster plug. (see picture) Make sure it has a good connection.

LockDoc

[Reggler]

Hmm... I have not seen anything like that in there. I'll have to search harder. Also, the 3rd post on the fuel gauge, is that supposed to have a ground on it?

[LockDoc]

Originally Posted by Reggler

Hmm... I have not seen anything like that in there. I'll have to search harder. Also, the 3rd post on the fuel gauge, is that supposed to have a ground on it?

That wire is the ground for the whole cluster. The 3rd post grounds through the cluster tin. Have you seen this thread at all?

http://67-72chevytrucks.com/vboard/s...d.php?t=640615

LockDoc

[Reggler]

Yes I have read that one and 2 other posts that had some good info. I think I've read too much, and a lot of the info gets scrambled in the ol noggin of useless wonder. But I've gotten to the point that the gauge works correctly when tested now. Next step is to drop the tank and either make a new ground from sending unit or replace it as well.
Thank you guys so much for the help!

[LockDoc]

Originally Posted by Reggler

Yes I have read that one and 2 other posts that had some good info. I think I've read too much, and a lot of the info gets scrambled in the ol noggin of useless wonder. But I've gotten to the point that the gauge works correctly when tested now. Next step is to drop the tank and either make a new ground from sending unit or replace it as well.
Thank you guys so much for the help!

No problem. Let us know if you have any more questions.

LockDoc