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Discussion Starter · #1 ·
I have three of these trucks and all three broken sending units.

What really goes bad on these, really? Look simple enough. Just ordered one on eBay for $69, I can dissect the old one with no consequence.
Is there any info out there on trouble shooting the actual unit?
Very annoying on farm use bucket truck to not know fuel level (odometer works but means nothing on a farm, the hours matter). Also aggravating to have a part fail when all it needs is better contact or cleaning or something simple.
 

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It's a coil of wires around a board and a contractor that rubs on the wires. That changes the resistance as it moves or wipes on the wire. You can check the resistance on the wire by taking the sending unit out and moving the float. On vehicles that sit for extended periods or don't use a lot of fuel. Condensation will get on the coil of wires and cause corrosion where it is stationary for a long time. It's typically a low voltage low amperage signal to the + side of the unit. You might check for that voltage at the unit. Sometimes there is a resistor inline that reduces the voltage and it opens and goes out. At least that's how they were for a long time. Could be different now.
 

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Sending units were designed for mineral diesel. Most ULSD has a few percent biodiesel which is highly reactive to bare metals, like the variable resistance element in the sending unit. Sloshing fuel will move the wiper over contacts, exposing fresh metal which then reacts and forms an electrical insulator.

Oxygen and water are also needed so fresh fuel and full tank is the best way to prevent this problem.
 

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Discussion Starter · #4 ·
Sounds like there is nO happy ending for these sending units, just keep buying them....for 3 trucks.
 

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Short of keeping tanks full or installing an inerting system like on large aircraft, very little can be done.
Tanks cool at night and draw in air. Air which is humid can condense into the fuel, with an empty tank, it condenses on the sending unit.

Check the fuel cap and all hoses for leaks. Most diesel systems are just converted gasoline models and have all but the charcoal cannister and purge system so they can take +- 3 PSI. A leaking cap could be the problem.
 

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