Farmall 340 gas gauge

BIGR · Oct 30, 2011

I"m rewiring my 340 and repairing/replacing gauges, etc. My fuel gauge doesn"t work, but 1 wire goes up to the sender and 1 wire was disconnected. I"m assuming the disconnected wire might be power? Does it go to the switch? The tractor was converted to 12 volts, does the gauge need a resistor? The other gauges (oil, amp, and water) are original and they work.Is there a way to test the sending unit and gauge?

Pete7 · Oct 30, 2011

Hi...I also have a Farmalll 340 that was converted to 12 volts with negative ground. I have not been able to get the gas gauge to work. The 340 tractor was originally 6 volts with positive ground. The positive ground issue seems to be a problem for the gauge. My gauge works fine when I test it off the tractor with a separate 6 volt positive ground circuit. But it won"t work with negative ground regardless of how I switch the wires around. The gauge itself seems to be built just for positive ground polarity. The other thing you need to be aware of is that the sending unit is actually a 6 volt variable resistor and if you give it 12 volts it will burn out. Not a good thing to burn up a resistor inside a gas tank. So, you definitely want to add a resistor to supply just 6 volts, but the polarity issue would still need to be solved somehow.
Pete

mkirsch · Oct 31, 2011

The resistor has to be properly sized, based on the electrical characteristics of the fuel gauge, to provide 6 Volts.

You can't just hook up an ignition ballast resistor. That ballast resistor is properly sized to provide 6 Volts to an ignition coil based on how many Amps an ignition coil draws.

MN Mike · Oct 31, 2011

rewiring to 12v neg will probably require a new 12v gauge or an inline resistor to bring it back down around 6 volts.

rustyfarmall · Oct 31, 2011

(quoted from post at 16:03:22 10/30/11) I"m rewiring my 340 and repairing/replacing gauges, etc. My fuel gauge doesn"t work, but 1 wire goes up to the sender and 1 wire was disconnected. I"m assuming the disconnected wire might be power? Does it go to the switch? The tractor was converted to 12 volts, does the gauge need a resistor? The other gauges (oil, amp, and water) are original and they work.Is there a way to test the sending unit and gauge?

The oil pressure gauge and the water temperature gauge are both mechanical. They both operate without electricity.

If the wire from the gas gauge that goes to the fuel tank is still connected where it should be, then yes, the other wire from the gas gauge goes to the keyed switch.

If you have power to the gas gauge, you can test the gauge by removing the wire from the sending unit on the fuel tank, and then GROUND that wire. The gas gauge should then read either completely FULL, or completely EMPTY. This test will indicate a defective sending unit.

IF grounding the sending unit wire has NO effect on the gauge, the gauge itself is defective.

Farmall 340 gas gauge

BIGR

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Pete7

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mkirsch

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MN Mike

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rustyfarmall

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