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Old 11-07-2009, 10:47 AM
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fuel gauge reads full

I have a 54 Ford that I changed over to 12v neg. I put in a new tank and float unit from Mac's. The float could be used for 6 or 12v it stated. I put runtz resistors on the stock gauges and the fuel gauge reads full with only about 5 gals in the tank. The temp gauge is perfect. Any ideas? Thanks Joe

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Old 11-07-2009, 11:20 AM
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Gas gauge

As a '54 Ford owner I had a gas gauge problem too,first with a volt meter check the runtz to make sure voltage has dropped from 12 to 6 volts.The problem I had was lack of a good ground,I ran another ground wire from a tank sender mounting screw to the frame and it was fixed.If that does not fix it the float may be hung up in the tank it is important that the float points the same direction as the one you took out.
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Old 11-07-2009, 12:09 PM
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I am working on a 1947 CJ2A Jeep. It is still 6 volt.
The fuel tank sender is just a variable resistor. It does not care what voltage the battery is. Your fuel gauge does care.
I went to parts store for a "universal" sender, and the parts man showed me the book he had.
There were five different senders in this book. Each sender had a different range of resistance, and two senders read low resistance for full, and three senders read high resistance for full.
You need to match the sender to the gauge.
On my Jeep, when I put in a new sender wire, I also put in a separate ground wire that goes from the sender mount on the tank, to the bracket that holds the gauge to the dashboard. I am not trusting the body to tank connection for a ground.
Depending on how your gauge is set up, either high resistance from the sender terminal on the gauge to ground, or low resistance from that terminal to ground causes the gauge to go to full. Disconnect the sender wire from the tank. See what the gauge does. If the gauge drops to empty, with the wire disconnected, your gauge wants low resistance to ground to read full. If your gauge goes full with the sender wire disconnected, you want a sender with a high resistance for full.
Temporarily shorting the sender wire to a known good ground should cause the gauge to go immediately to the other reading.
By the way, on my Jeep, if I connect a small test light across the ground and sender wire at the gauge, the gauge reads about half full. If I connect the tesl light across the temp sender on the engine head, tha temp gauge reads in the mid range.

Last edited by DanielC; 11-07-2009 at 12:24 PM.
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Old 11-07-2009, 06:04 PM
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Thanks for the info. Thinking about it, the sending unit is probably not getting a good ground. The tank has rubber pads on the top between the body and rubber insulation on the straps, no metal to metal anywhere. I'll drop the tank and run a ground wire to the chassis and see how that works. Thanks again guys. Joe
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