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I got a '68 Mustang fastback at the beginning of the summer; when I first bought it, when I would fill it up the needle would go past full, take forever to move to the 1/2 full mark, and then would go really fast from the 1/2 full to empty mark. After pulling the engine this summer, putting a Mass-Flo EFI kit on the car, and having an AN fitting welded onto the end of the sending unit, it does almost the opposite; when the tank is completely full, it will only read 3/4 full, although it seems to decrease much more steadily now. Does anyone know anything about this? Is my gauge broken, or is the sending unit bad? Does that even have anything to do with it? By the way, in case it matters, it's a 16 gal. tank. Thanks!
 

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You'll probably have to pull the sending unit and bend the float arm so full is full and empty is about 3 gallons left.
 

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Fuel sending units work off of resistance. Since you do not want a spark in your fuel tank, the float arm acts like a variable resistor. I can't remeber which way the resistance goes, but as you empty your tank the resistance either gets higher or lower which in turn moves you needle on the gauge. You can pull your sending unit out of the car and put a multi-meter on it and make sure it's set you read ohm. (the little horse shoe) You put one lead of the meter on the connector for the sending unit and one on the round part that the seal touches. Then move you float arm up and down, and your meter should reflect the movement with higher and lower resistance. Check you readings of you sending unit with a new one. And also check to see if any radical jumps in resistance throught the sweep of the float arm. Hope this helps.
 
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