Fuel Gauge Wiring Question
 

Hi Guys,

I thought I knew how this worked but I am questioning myself. So before I chase my tail I just want a consensus.

Trying to make my fuel gauge in my 62 Cat work. Currently, it goes straight to full when power is applied.

Two prongs on the back. One is yellow, which is confirmed hot. Other is a brown wire which then after going through some plugs goes to the sending unit.

Question. Is that brown wire that goes from the gauge to the sender supposed to be hot or is it a long ground wire?

I thought it was supposed to be hot. I always assumed that the wire was hot, and then the variable resistor in the arm of the sender reduced voltage back to the gauge. Am I wrong, and that wire is a long ground?

I think I always assumed it was hot because the sending unit usually goes have a separate ground wire that goes to the chassis back near the tank. So my brain says, "Two wires, one is ground and one is 12V"

I need to know if my gauge is bad because I am not getting power from one prong to the other on the back. Or if Im not supposed to get power to that second prong and I have a bad ground back at the gas tank.

Definitely not a hot wire. That wire is your wiper wire from your sending unit. It will vary from something like 0 to 90 ohms depending how much fuel is in the tank.

Okay cool. Now I know I just have to figure out if the ground back at the tank is bad, or if the wire is broken somewhere. Or maybe the sending unit is bad, which could be an issue. Not sure if a 60s wagon sending unit is going to be available with the strange side mount fuel tank.

I made up a easy tester to test the fuel gauge. All you need is a 100 ohm potentiometer which I think I got on Amazon. Attach 2 wires to the two wiper leads with alligator clips on the ends. Now attach one side to a good chassis ground and the other side to the wire going back to the fuel gauge, turn the key on and turn the potentiometer and you should see the fuel gauge going from empty to full.

You can test the sending unit with an ohm meter on the lowest setting. It should read between 0 and 90 ohm between the ground wire and the wiper wire.

I think mine being a 1962 is a 30 OHM, but yeah that's a good idea. Ill get back there over the 3 day and see what I can see. Hopefully its just a bad ground.

I need to drop the tank at some point and put some treatment in it. Normally I would just buy a new tank, but this isn't exactly an F body.

I believe 30 ohms when full, zero ohms when empty is correct for the '62. Jump the wire at the sending unit to ground and turn on the key. If the gauge still reads full the wire that runs from the gauge to the sending unit is bad. If the gauge reads empty either the sending unit is bad or it is not properly grounded.

It's not a "hot " wire per se, (not connected directly to the battery) but it is NOT a ground either. The sender is connected between this wire and a ground near the tank.

George

This is a good explanation of how AC fuel gauges work.
From Motor Auto Engines and Electrical Systems copyright 1977.

Played with it some today. It had multiple issues. Wire was severed. I fixed that but it seems the sender is bad.

If I unplugged the sense wire and jumped it to ground the gauge would go from full to empty. But no amount of sanding or jumping direct to another ground would do anything with the ground wire. So I have to think the sender isn’t working.

After many years inside of the tank an original sender will go bad. The 62 full size car should be a zero to 31 ohm sender.

Zero or one ohm = E and 30 or 31 ohms resistance = full.

If all else fails:

https://www.ebay.com/itm/35250124486...0AAOSwRLZT7MFq

I put this (and his new tank) in my 63 Safari and have no issues. I grounded the sender directly to the frame.

Good luck,

James Q