How hard can this gas gauge wiring really be?

So you guys have never steered me wrong on the wizardry required to deal with the electrons....looking for advice on how to troubleshoot this. I get a new gas gauge for my 69 Case 580 12 volt negative ground with an keyed ignition. The tank sending unit has a single wire in the center and the ground wire mounted to the frame. The new gas gauge ordered from the Case dealer has 3 terminals; one marked S (sender?) one marked I (ignition?) and a G (ground).

So, being a product of a public school, I connect the wire from the sending unit to S, a wire from the ignition terminal of the switch to I. and ground wire to the G. Turn the key and bingo it pegs the gauge past full.....problem is that the tank is only 1/4 to 1/2 full. No instructions or diagram so I am convinced it is so simple as the gauge is already marked but maybe not.

The sending unit with float looks good but I do not know how to determine if it actually works. I assume the float mechanism acts on a metal strip creating a varied resistance out the sole wire which in turn would be reflected in the voltage through the gauge? Is there a method to test the sending unit? I also tried hooking up a small 6 volt battery to the gas gauge thinking reduced voltage would perhaps only show 1/2 full? Wrong.... it still pegged past full with 6 volts? So now I have exhausted my "sparkie" abilities and hope you can explain how this style of gauge actually works in theory and how to test a sending unit. Then I can do some troubleshooting I hope.

Thanks

I hope someone comes along to do a better job than I in explaining this, but I always thought of a gas gauge as an ohmmeter measuring ohms to ground. If you hook it up and it pegs, then you either have to much or too little resistance to ground, depending on how it is wired. Pull the float and check resistance from the ground and the post. Swing the float and make sure the resistance changes. You may find that you have a bad wire between the sender and the gauge or that you need to bend the float arm to get it to read on the gauge.

You can test the gauge by putting power (and ground)to it. It will read full or empty. Then ground the sending unit terminal. It should swing all the way the other direction.

HJope this helps,

Aaron

With most gauges the sending unit grounds the wire going to the gauge. It sending unit varies the resistance to ground and that's how it makes the gauge needle go up or down. The higher the float is in the tank, the more resistance the sender makes and the higher the gauge goes. Remove the wire from the sender and see what the gauge does. If the gauge goes to empty with the wire off the sender the sender is shorted out. If you touch the wire to a ground it should peg the gauge. If the gauge still pegs with the wire off the sender the wire might possibly be shorted somewhere between the sender and gauge. If the wire to the sender is good, someone else on this forum is gonna have to help you out. Jim

Several thoughts:

1 - Your gage wiring sounds right (at least I'd wire it up the same way you did...)

2 - If you disconnect the wire at the sender and turn on the ignition the gage should read "E". If it still pegs at "F", the wire between the gage and the sender has a short to ground.

3 - The gage sender works like a variable resistor exactly as you describe. You can quickly check the resistance of the sender with a multimeter. And the electrical manual for your machine should give the range of resistance readings (full and empty resistance) for a good sender. If you find the sender resistance is too low, it's time for a new sender!

4 - Most automotive gages are "voltage compensated". Ie the gage reading does not change with changes in system voltage. Typically a gage will remain accurate with system voltage anywhere between about 5 and 16 volts.

Can you hook up the guage to the original power wire? Reason I ask is Ford uses an "Instrument Voltage Regulator" that "pulses" the current. This same pulsed current also powers the temperature guage and oil pressure guage, if they aren't mechanical. I know it's not Ford, but they may be similar in construction.

I believe (though am not certain...) Case equipment of that era uses 2 pole, magnetic movement gages. They were manufactured by either Stewart-Warner or AC (General Motors). There is no "instrument voltage regulator" required for this type of gage - they are by design self-compensating for voltage.

Thermal gages (which use a pulsing instrument regulator you describe) were used by Ford, Chrysler, IHC and others.

I'd say you probably have it wired correctly.. but..

Try unplugging the wire you have going to the ignition switch from the sender and give that a shot.. If the original didn't have that I terminal, maybe you still don't need it.. If it's like my 930 Case is, it has a wire from the ign. switch to the gauge, then a wire from gauge to the sender, and a ground. Possibly having 2 HOT wires is creating the false reading. Surely the sender is made to fit many models, some may require the extra post and some may not.

The other possibility is that the new sender sends out the "improper" resistance.. I know, being it is supposed to be a replacement part it should work the same, but sometimes..

Let us know what you figure out, as my 930 needs a new sender, so I may be going through the same stuff soon.

Brad

sender ohms and guage have to match, may not have correct sender to match guage.

I had about the same problem with one for an 830 Case, I even called the maker, they said I needed a certain amount of resistance so I got a resistor with the required ohm rating from Radio Shack, I even bought another sender and never did get it working. It stays on full until 2/3's of the fuel is gone and then the gauge flickers between F and E, I have bought senders for 70 series tractors from Case made by the same co. and they work fine.

S-W and GM gauge systems are NOT compatible. The SW sender has a ohm range of 33 to 240 OHMS. GM senders range from 90 to ZERO. You need to know which you have if both systems were used by Case. You also need to be sure the fuel tank is grounded to the frame of the tractor. It does no good to ground the ground wire of the sender to the fuel tank IF the fuel tank is not grounded. Rust and age can compromise grounds.

Kent

Thanks Bob. I was just throwing out this idea as it seemed to fit his problem.

Okay most of what has been posted I understand is is useful. I am going to determine if I can which sending unit I have and will then see if the gauge is a match.

So if I understand the gauge/sender premise.... WITHOUT the sending unit in the equation, if I hook up the keyed ignition wire to I and a ground wire to G on the gauge only, the gauge should remain on empty (no sender in the cicuit...no resistance) ... and if I have a working sending unit that MATCHES the new gauge, the resistance of that sending unit will provide contolled grounding of the gauge moving the needle towards full (grounded).

Do I have that concept or are you shaking your head and hoping I keep a fire extinquisher in the shop? I thought it would be handy to at least check the mechanical to electronic aspect of the sending unit by using a small meter. Will the simple ohms test sending unit wire to ground reflect varying ohms depending on float level? Mine seems to go right to pegged continuity with some gas......If not, how would I check the sending unit? How do you measure the resistance? Perhaps my little multi tester is not up to it? Also wondering if that is the only way to determine the type I have as no numbers are evident.

Appreciate the help. It seems like a little thing but it is such a pain to move the machine into a spot get it working on a stump or hole and then run out of gas because I forgot to climb up and peer in. I WANT a gas gauge!

We had a similar problem with our Massey 285. Had a new sender put in the tank as it never worked. The new sender was always off, and the dealer couldn't figure it out. After they called AGCO and did some digging, the answer was given to put in a new guage. Apparantly the old guage and new sender weren't compatable, even though they should have been.

Donovan from Wisconsin