The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Guage ???????

I have a good one here. 1965 short step. When you plug the fuel guage in and its bolted in place it reads full tank all the time. When you unbolt it from the dash and hold it in your hand it reads correctly. If it grounds to the dash at all ,like to bolt it back in, it reads full tank. I have two different guages and they both do the same thing.

love2fixit,

Pretty strange. If you go with the odds, which isn't a bad way to go, it isn't the gauge. You have two gauges and they both do the same thing.

When the gauge is powered, it looks for a ground. The more ground it finds the more it swings to EMPTY. It compares the ground supplied by the sending unit with one attached to a coil in the gauge. This is influences the little pointer.

If it finds NO ground, it will read full. Your gauge swings to FULL when you bolt it back into the dash assembly...isn't this backwards to what it should do.

Before I condemned the gauges, I would prove the wires back to the sending unit and the sending unit itself. You indicate it reads correctly when you pull it from the dash. Do you know exactly how much fuel is in the tank.

Is the connector keyed so you can't plug it in backwards?

Do a little more looking and you might come up with the proverbial fly in the ointment!

Stuart

I added fuel so I could see a change on the guage. It is working correctly as long as it isnt grounded to the dash. I will do more checking. The plug is angled so it only plugs on one way, not so say someone hasnt switched them or something.

love2fixit,

I looked at a diagram of a '65 with gauges. The fuel gauge finds a common ground in the dash assembly with a whole bunch of other things...the temp sender and lots of the dash lights themselves.

I wonder if a dash light or some other device is shorted and powering the fuel gauge through its ground. I don't know if this would give you the goofy problems you are having, but it's a thought.

Is there some other item on the dash that doesn't work, maybe a light, or gauge. Does the problem change when you turn the headlights on?

Stuart

love2fixit,

I guess I must have OCD...I need to find the reason for this gremlin.

I drew a little cartoon, and if I understand all I think I know about the fuel gauge, which might be a stretch, it almost looks like the wire from gauge to sender or the sender to ground path might have a high resistance connection in it somewhere.

When you pull the gauge from the dash assembly, its ONLY path to ground is through the sender. This is a tough path, but it's the only path.

When you stick the gauge back into the dash assembly, the gauge case now has a much easier path to ground, which is saying it sees the connection to the sending unit as a OPEN and it goes to FULL.

The connection to the sending unit on your truck is made on a common terminal in the fuse box. Could this connection be full of fungus or almost ready to fall off?

I don't know if this theory is right or I need more meds...someone will show up and set me straight!

Stuart

anyone have a link to a schematic for this that I can look at?

This is a pretty good site. It's on a server that won't allow access all the time but you can't have everything. Good luck!

Stuart


http://rmcavoy.freeshell.org/Truck_gauges.html

love2fixit,

My previous post is way out in left field. There is a very simple procedure to check the dash gauge and the wiring back to the tank.

Put the gauge back in the dash and connect it properly..then, turn the key ON and pull the wire at the tank sending unit. The dash gauge should read FULL.

With the key still ON, ground that same wire you pulled at the tank to a known, good clean ground. The gauge should go to EMPTY.

If this happens, the gauge if good as well as the integrity of the wires back to the sending unit.

This points the fickle finger at the sending unit or the sending unit to ground path.

I think this is the best starting point. Either both your gauges ARE bad, or something is clouding the troubleshooting. The above test has been outlined dozens of times on this site and narrows the field as to which component is screwed up.

Stuart

Well its funny you said that. Yesterday I did just that. I connected everything like it should be. Grounded the sending wire at the tank to ground and the guage went down to empty. this led me to a sending unit problem. I did forget to mention that previous owner installed an under dash guage that seemed to work. Never thought that the sending unit would also be changed. I pulled the sending unit out of the tank and there it was, an aftermarket sending unit obviously with a different resistance rating than the factory guage likes. I connected wire to sending unit in my hand and ran float up and down and right at empty the factory guage just starts to drop from full. I am going to purchase a sending unit. I just wonder why the previous owner didn't do this. Seemes alot easier that replacing sending unit and mounting guage under dash. Thanks for the schematics. I found everything I needed to know on them.

Sounds like your fighting the 0-30 ohm vs 0-90 ohm gremlin. GM used 0-30 ohm gauges from 1960 - 1966. 1967 and later use 0-90 ohm. The aftermarket uses 0-90 ohm and call them 'GM' style. A one third full or more tank with a 0-90 ohm sender will drive the factory gauge to full. Either replace the sender or put a 43 ohm resistor in parallel with the sender. Works pretty good. You can get the resistor at Radio Shack.
Jim