Fuel gauge scramble

[Green Machine 79]

1979 c20 pickup

Fuel gauge has only worked a few minutes in the last 4 years. Now that i've gotten everything running smooth and perty, my wife wants this detail taken care of.. It's been stuck on high.

I should know better but i assumed maybe it'd be an easy fix. I lowered the tank to find the sending unit destroyed by rust and the lines rotted to the point they came right off rather than surrendering the hoses. So i replaced the sending unit and hoses, but no change to the gauge.

Following the wire from the tank it seems to cross over to the drivers side before moving forward. There is another wire that comes from the same tube but runs up the passenger side and goes up by the starter.. I dont see any issues along the paths. Its appears well protected all the way.

Next i went behind the gauge cluster and found a brown w white stripe wire had been folded back on itself at some point and appears to have melted itself along with hardening its neighbors. I figured on soldering those few wires but after looking over the guides, the dark blue and light blue wires are still functioning fine.

I still have not been able to figure out why my temp gauge is also not functioning but figure it's probably related..

So yesterday i decided to try and test the gauge itself. I was getting a 12 volt reading from all the gauges ports (with gauges removed). Then i tried to ground the fuel gauge (inserted back in port) with a jump wire. I heard a tick and now my fuel gauge needle is not moving at all! When i test now i only get around 6 volts in the empty port.. I don't understand how I've cut the voltage in half??

The circuit on the gauge cluster looks ok. It's a bit green in spots but no brakes along the way.

Everything else on the dash works great. No trouble with lights or turn signals or anything else.

Should i just run a new wire up from the sending unit? Is it a direct connection?
Any help would be appreciated. I'm lost on it. And i feel like i need to take a class just to be able to make the diagrams useful

Not sure if these pics help but i got em..

[mongocanfly]

usually a fuel gauge stuck on full is a ground issue..

[Tiger Joe]

http://www.chevyclassicsclub.com/the...ry-1965-newer/

I used this site to troubleshoot my gas gauge in my 79

for me it ended up being a bad printed circuit. also check the metal prongs that hold the gauges in. they are the connection between the printed circuit and the gauge, and mine were rusty and had a poor connection.

being that you have 2 gauges not functioning I would be looking for a common area between the 2- that's the printed circuit. I pulled my cluster out and testing each circuit on the printed circuit with a continuity tester. your circuit could be good, but if one of the "fingers" where it plugs in gets bent, it can short out.

[Dead Parrot]

Full scale(3 o'clock) on a gas gauge is usually an open circuit. When you replaced the sender, did you connect a new ground from the tab on the sender to the frame?

Single tank?

You can test the temp gauge and wiring by pulling the wire off the sender and grounding. Gauge should go full scale. Yes, the fuel and temp gauges have reverse action. (High resistance on fuel = high reading, high resistance on temp = low reading) There are many threads on temp gauges/OEM vs aftermarket sender units/and differences between gauge years. Unless you know the truck's history, don't assume the temp gauge is the original. A PO may have swapped gauges trying to fix it. Also possible a PO trying to fix the gauge was handed a sender for an idiot light.

[Green Machine 79]

Thanks for your insights!

I cleaned the frame and secured the new ground wire when I replaced the sending unit.

This morning I pulled the gauge cluster again to find that I broke one of the circuits when I tried to ground the fuel gauge (while it was seated in the port on the cluster). So I will be ordering a new circuit board..

But I'm gonna guess that wasn't the issue until I created it yesterday. We'll see. I'll have to wait til that circuit board gets here to test the connection again. I'm also considering buying a new gauge tho they cost 60$ new.. I figure on going by the wrecking yard tomorrow just to see what's available tho they won't let people do any pulling themselves which makes it costly.

Anyhoo, I appreciate your thoughts in this and wanted to respond as best I can right now.

[Tiger Joe]

if you want to, you can test the gauge itself outside of the cluster.

3 terminals- power, ground, and signal wire. I've done it this way to eliminate the printed circuit as an issue.

not that I mess with a ton of gauge clusters, but of all the ones I have, I've only had 1 oil pressure gauge be bad out of all the gauges.

[Green Machine 79]

Just in case anyone comes along this with the same issues in the future, I'll admit my embarrassment..

After replacing a completely broken sending unit with no change to the gauge, I shorted my circuit board trying to ground the wrong prong of the fuel gauge. (Hint: it must not be the bottom one)..

After replacing the board, I still had no change to gauge. I was planning to rewire the sending unit to the firewall but was confused on how to do that. So in my few moments of considering how bad that could turn out, I noticed my tank selector switch was set on the driver side which has no tank.. I had let my youngins play around a bit some weeks ago while working on something else.

So, a hundred dollars late, my fuel gauge is great. In fact, I just filled it up and was surprised to see a full tank stops the gauge directly on the full line.. I didn't expect anything from oreillys to be so specific..

Thanks everyone for chiming in!