The Stovebolt Site Forums The Shop Area The Electrical Bay SOLVED...I let the smoke out of the wires

I had my instrument cluster on the floor of my cab, testing the fuel gauge before i installed it back into the dashboard. I shorted the fuel gauge (by accident) when my jumper wire touched the back of the instrument panel (metal). It sparked, but I caught it fairly quickly. The needle on the gauge went off the scale but returned to a normal position a while later.

Today I installed the instrument panel & carefully wired it up (thinking my fuel gauge was fine) Turned the key on & let the smoke out
I believe the gauge was not fine & was possibly shorted, thus melting the pink wire (+) when I turned the key on.

1. Are my assumptions correct?
2.How shall I proceed? Buy new gauge & replace the damaged wires? (the pink wire is only a few inches long & goes from the ignition switch to the fuel gauge)

I would start by looking real close at the rest of the wires in there. Especially from the ignition switch to the battery (along whatever path that takes) to make sure there are no other damaged wires upstream.

That much magic smoke indicates pretty close to a dead short. I would also unhook all the wires from the gauge and test it for being shorted to ground. That will tell you if the short is in the gauge itself or in how it is hooked up.

With that information you can decide how to proceed. Replacing the burned section of wire is a given, but depending on what you find there could be different options for what else you need to do.

Thanks for the input Fibonachu..... I'll do some testing tomorrow & post my findings

If you created a direct short to ground across the fuel gauge it is possible you ruined it. The fuel gauge is what's known as a balanced electromagnetic scheme and that means it has two electromagnets (both wound with different sized wire) and a permanent magnet pivoting in the middle. Those electromagnets are wound with very small wire and that wire is coated with shellac or varnish. You can see an example below. Any short to ground will either burn out the wires causing them to just melt or get them hot enough to melt off their shellac/varnish insulation and that renders them useless as electromagnets. Good luck!

I was hoping to find a fried gauge but it appears fine. Nothing melted or charred. All the soldering is fine & even all 3 windings are still intact.

The only wire that was toast was the 12 volt + from the ignition switch to the fuel gauge.

Fyi.... power goes from starter to Amp gauge ( blackwire). Then 2 red wires go from Amp gauge to light switch & other goes to ignition switch.

I have another gauge on the way but need to find my error before installing new one.

Help

Edit 2: I figured out this is the alignment pin on thr mounting plate hitting the PCB.
Also, did you measure the resistance across the coils? Heat damage will occur in the center first, so it might be fused internally even though there is no visible damage.

Edit: Also, this gap concerns me as well. Depending on how it shifts when you tighten the nuts, that pin might be shorted to ground when installed. If the alignment pin on the right was what kept it from mounting flat, that would have pulled the whole gauge to the side. And potentially closed that gap/shorted it to the case.

Tangentially related question: do you know the history of that gauge? We saw someone else with a similar one recently. His died when he hit a big bump, but the design was unfamiliar to Jon G and me.

I would be interested to know where it came from.

The gauge is a 3- years old from classic parts.

https://www.classicparts.com/mobile/1955-59-Fuel-Gauge/productinfo/24-721/

I'll test the resistance later today 🙂👍

Not real familiar with that gauge, but I think the two small holes are for locating pins to keep the two terminals from touching the housing. The pin on the left in the attached picture is keeping the gauge from seating flat, and possibly letting the terminal short against the mounting housing. If the gauge coil resistances are OK, you probably dodged a bullet. Get it reinstalled correctly so the terminals are isolated from ground. See my notes at the red arrows.

I believe the we found the problem. You guys are correct,

.... since the gauge WASN'T mounted correctly ( flat) The post grounded out the the metal instrument panel

Do you guys agree?


The other problem is my eye sight aint what it used to be

Based on that measurement and the closer pictures, I concur.

I would still recommend checking the wiring "upstream" from the ignition switch to make sure nothing between there and the battery was damaged in the process.

Sounds like good news. I was backwards, thinking that the pins were on the gauge itself, but they're on the housing instead of the gauge. I think you're on the road to success.
Like Jason said, check that there's no damage to the wire from the starter terminal to the ignition switch. There's probably no fuse to protect it.

Thanks for everyone's input it really helped 👍 I'll get a new guage & fresh roll of 20g. Pink wire in a few days & have it back together .

Have a great Thanksgiving everyone

I'm pretty sure those gauge studs need insulators- - -even something as simple as a short spool cut from a heavy duty plastic soda straw would prevent a short to the gauge cluster, or a couple of layers of heat shrink tubing around the studs would work. I'd also suggest adding a short piece of fusible link wire at the battery cable connector in series with the wire going to the ammeter. It is pretty effective at preventing a positive smoke check of the entire wiring harness in case of a serious high current short.
Jerry

As long as the locating pins are in the slot of the gauge circuit board, the studs will be isolated, but they also need a fiber washer under the nuts holding the gauge in place.

That piece of fusible link wire is a good idea. I'll probably do that when I get the harness installed in my truck.