Sending Unit Question

1945 - 196*, Willys CJ series, questions, discussions, regarding anything related to the post war jeep.®
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Stingray
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Sending Unit Question

Post by Stingray » Wed Nov 09, 2016 11:03 am

After opening up our sending unit and cleaning it we have it working again. A question came up about how does the 6volts and rheostat keep from igniting the fuel? Thanks
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Fortyeight2A
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Re: Sending Unit Question

Post by Fortyeight2A » Wed Nov 09, 2016 7:13 pm

There's no open spark and probably only gas vapor with no free oxygen available inside the sending unit.
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Wolfman
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Re: Sending Unit Question

Post by Wolfman » Thu Nov 10, 2016 6:31 am

Low voltage and a lot of resistance to ground equals low current flow. Not enough to generate a spark, plus a low amp. breaker protecting the circuit. Good reason not to replace the 2 (or 5) amp breaker with a 20 or 30 amp breaker, in case something inside the unit did short.
I have wondered the same thing about having the fuel pump inside the gas tank of new cars and truck. I always thought that was a receipt for disaster, but they don't blow up.
I think, in this case, the motor is an induction, permanent magnet motor. No brushes to generate a spark.
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Stingray
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Re: Sending Unit Question

Post by Stingray » Thu Nov 10, 2016 8:28 am

Thanks for the info. We noticed there is not much at keeping the fuel out of the sending unit so we assume it still functions if fuel does get in.
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Wolfman
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Re: Sending Unit Question

Post by Wolfman » Sat Nov 12, 2016 5:16 am

That unit has been used in just about everything with an electric fuel gauge since the beginning and still is. I have never seen an effort to keep fuel out.
Today, instead of a wire winding, the arm moves across a resistance strip. Works the same, just using 21 century material.
The only other type of electrical sending unit I have run across is a capacitance type. Instead of an arm sweeping back and forth on a winding or resistance strip, it uses two rods running parallel from the top to the bottom of the fuel tank. Current actually flows through the fuel between the two rods. The more fuel in the tank, more of the rods are exposed to fuel, and there is more current flow. The gauge reads higher. Less fuel. Less current flow. The gauge reads lower. This all happens inside a tube with plastic separators so the rods never touch each other or the outside tube.
Mike Wolford
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Comm./Inst. SEL
AOPA ( 50 yrs)
EAA ( 49 yrs)
4th Inf. Div. - 5th Inf. Div. - 2nd Armor Div. - CIB


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