case fuel gauge

[scf84]

the fuel gauge on my 970 case has never worked
since i have had it(around 6 months)
thought i would work on that today, and sofar
i have cleaned all the ends and ground wires
with no help, gauge just sits on empty with switch on and off. i had an extra gauge in the barn and put it on and the gauge jumps to full
when the switch in on. only 3 gal in tank.
i ran a new wire from gauge to sending unit
and it still jumps to full.
what is the problem?

 

[saltbranch]

If it was a car/truck I would be checking the float to see if it was stuck/froze up. I am not sure how tractors are set up, but I would think they have float in the tank for the fuel gauge.

 

[lavacarancher]

Fuel gauges are pretty simple devices, at least in older designs. I wish I could draw you a picture (schematic) but I'll try to explain how they work and you can troubleshoot it.

+12v from the ignition switch is applied to one side of the gauge. The other connection on the gauge goes to the sending unit located inside the tank. This device is a variable resistor that changes resistance when the float moves up and down because of fuel level changes. This change in resistance causes more or less current to flow through the gauge and indicates a proportional amount of fuel. Oh yea, the sending unit has one wire attached to it coming from the gauge. The other side of the sending unit is ground.

To troubleshoot start by disconnecting the wire from the gauge to the sending unit. Gauge should drop to "empty". If this works, take the sending unit out of the tank (carefully), connect the wire from the gauge to the connection on the sending unit and add a jumper wire from the metal sending unit to tractor ground. Gauge should respond. Move the float up and down and gauge should respond accordingly. If all this works then the sending unit was shorted to the tractor ground. Maybe you can see where this short is and fix it. If it doesn't work there are two possibilities. One, the sending unit is defective (shorted) or two, the gauge and sending unit are not matched.

Good luck

 

[scf84]

i unhooked the wire goin to the sending unit and it still shows full when switch is on turn switch off and it goes to empty. makes no difference if the sending unit wire is hooked to the gauge or not.

 

[lavacarancher]

84, I've looked at wiring diagrams for several tractors and all of them are as I described earlier. Something is mis-wired.

Look at the back of your fuel gauge. There should be only two connections (maybe three if the gauge is lighted). One connection will say "BAT or +" This connection goes to the switched battery somewhere in your tractor wiring. The other connection will be labeled "SEND" or it may not have any labeling. This connection goes to the fuel tank sending unit. You may wind up having to run new wires if you can't figure out where the wires go in the wiring loom of the tractor.

Without going into the calculations for determining the gauge sensitivity (in millamps), shunt resistance and sending unit resistance so that the gauge and sending unit are matched I would think that a mis-wire is your culprit.

One other thing. Make sure the tank and the sending unit is grounded. There should be an electrical terminal on the flange of the sending unit that has a wire connected to it that goes to the tractor ground. The other electrical connection on the sending unit goes to the gauge. If these two connection are reversed the gauge will respond exactly as you described.

PM me if you have any other problems.