The Stovebolt Site Forums The Shop Area The Electrical Bay Gas gauge question

In my 61 Apache, the previous owner mounted a different gas tank under the truck. He does have wires running to the sending unit. I do not know what this gas tank came out of. I do not think the gas gauge is working correctly so I think the sending unit does not match the Ohms resistance of the original gauge. I can not get to the sending unit because the bolts for the straps are on top of the tank. I think it was mounted with the bed off. Is there a way that I can tell the resistance of the sending unit with out taking the take down?
If they do not match , is there a way I can change the resistance of the original gauge to match the sending unit? Is it best for me just to purchase an after market fuel gauge and use the sending unit that is in the tank?
Thank you

I'm a '40s and '50s guy, so take this advice with a grain of salt.

You say "wires" running to the unit? Plural? Usually, there's only one wire going to an insulated terminal on the sender, which is controlling the ground resistance for the gauge. A second wire could be just a separate ground - my '50 AD has two wires, the second just grounding out on one of the mounting screws. A second wire could also be power for a fuel pump IN the tank . . . you'd probably be able to hear that pumping when you turn the ignition on.

Mismatched senders and gauges will usually read like offset scales: It will show FULL-FULL-FULL-3/4THS when the tank is full-3/4ths-half-1/4th, OR, mismatched the other way, It will read EMPTY-EMPTY-EMPTY-1/4TH when the tank is empty-1/4th-half-3/4ths.

But the only way I know of to find out the sender resistance is to have the unit in-hand and test it with a multi-meter. Then you can meter it and read resistance while you move the arm from empty to full and back.

You could: buy a universal sender unit at the parts store (you'll then know the resistance), brake the wire(s) going to your sender, and hook them up to the store sender to see how the gauge reads then. (don't forget to ground the sender body to the frame with a jumper wire.) That will tell you whether THAT particular resistance range is correct for your guage. Most common is, I think, 0-90 Ohms; other popular one is like 30-240 Ohms or something like it.

Then, I guess, you could put in a gauge of the other resistance range and see if it behaves with your tank sender.(and return the sender you bought).

OK . . .this seems complicated. How bad is it really to unbolt the bed of the truck?

-Michael

My 64 has two wires, of which one is a ground. You can measure the resistance across these wires with the tank full, half full, empty, etc. That's the only way without removing anything. Various years/models have different ohm ranges. Yours may be 0-30 ohms.

There are two wires going to the gas gauge, a pink one and a brown one. A brown wire runs back to the sending unit. I hooked my ohms meter up to each one of these wires to the gauge individually and grounding the gauge and my meter does not move from "1" which is on the display as soon as you switch to ohms. I tried this test with the ignition on and off. What am I doing wrong?

usually meters read 0 when they're not hooked to anything - if it's an original gauge the sender should be 0-30 ohms, so the meter needs to be on a low range .... not really clear what you're measuring, how are you "grounding the gauge"?

with ignition off you should be putting one of the meter leads to ground, then the other lead to the brown wire [sender], that should read the sender resistance - if there's no reading check at the tank, if you can't weasel the meter probe in to the sender terminal, use a safety pin to poke into the wire near the tank and put the meter probe on that, if you get a reading there the wire is bad, sender to gauge .... if you can't prove the sender works and check what the resistance range is, you won't have any idea what gauge to buy

Bill

Bill, I am not getting any reading at the brown wire behind the gauge. I will try to probe the brown wire closer to the sending unit.

Actually an Ohmmeter not hooked up to anything will read infinity. That is open. If it reads 0 that is a short or, too high resistance setting. Scale the meter to what you expect to see, 30 Ohms will require a 100 Ohms setting on some meters and 200 Ohms on others. Too big a resistance to read at 2, or 20 Ohms. Most meters are scaled 2,20,200,2K,20K, and 200K. So for a head light, 2 Ohms might work. For a sending unit 200 will be best. For continuity, wire connections, 2 Ohms would be what you want.

I finally found some strapping to replace my gas tank straps, so I lowered the tank and removed the sending unit and it is marked 30 ohms. When I connect the two leads to the sending unit(black to ground and red to the screw terminal on the unit) and the float is in the empty position the meter reads 00.6 as soon as I move the float up the meter reads infinity. Is this correct or should it be slowly moving up to 30 ohms to indicate a full tank

the sender is a 30 ohm variable resistor, it should read about 0-30 ohms [or 1-28 or 2-34, etc] as you move it, your wire winding may have a break along the way or the wiper contact might be corroded or not be tight against the resistor

Bill

I got the sending unit to work, at least temporarily. With the sending unit hooked up to the trucks wiring and the float is in the empty position, when I turn on the key, the gauge needle pegs to full and does not move at all when moving the float. When I hook up my meter to the wire coming from the sender going into the fuse panel, the readings are normal 0.5-32 ohms. I will pull out the gauge, but do you think the problem is in the wiring from the fuse panel to the gauge or the gauge itself?

When the wire coming from the gauge is not connected...the gauge will peg to the full side. If that's what is happening, you have a bad connection or a broken wire. If you touch that wire to ground the gauge should go all the way to empty.

Well I really screwed something up this time. I used a test light at the terminals going to the fuel gauge and shorted something out because I have no juice to the fuse box at all and the truck wont start. Is there a fusible link or something like that where I should be looking?

I found an inline 30 amp fuse that blew, and a bad connection at the gauge. At the moment all seems to be working.