I'm lost, gas gauge pegged passed full

[VeryTangled]

Originally Posted by old guy rich

VeryTangled, thank you, I'll give it a try.

old guy rich for the win!!

[old guy rich]

Originally Posted by 19Fordy

Rich, Thanks for posting the photo of your sender.
Based on what you say: "I used the two normal wires that it would use if it was in the tank. (feed wire and ground wire) I moved the sending unit arm to the empty position and the gauge went to empty, moved to full and gauge went to full, went to half way gauge went to half full. Installed the sending back in the tank and the gauge pegged. The float is empty and if it was full the gauge should read empty as it would be at the bottom of the tank."

I too am puzzled that the sending unit and gauge reads accurately when the sending unit is outside the tank but, inaccurately when reinstalled.

I don't have a solution. As a last resort I would run a new wire from the gauge to the sending unit and a new ground wire from the sending unit to the frame. But, before doing that, check the continuity of the original wires. Perhaps there is a short.

Also, loosen and clean the connects of any wires on the sending unit that are held by screws. Make sure the soldered joints are good.

I am still surprised that the after market gas tank sending unit gives correct readings on your OEM gauge.

19Fordy, I will do what you have suggested tomorrow. I will report my findings. Thanks. rich

[old guy rich]

Originally Posted by VeryTangled

old guy rich for the win!!

VeryTangled, I can't lie, my wife helped me. Thanks. rich

[40cpe]

Maybe the sending unit is shorted internally and shorts the gauge when installed in the grounded tank.

[old guy rich]

Originally Posted by 40cpe

Maybe the sending unit is shorted internally and shorts the gauge when installed in the grounded tank.

I never thought about that, that gives me an idea. Thanks for your comment.rich

[Veeder]

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This sending unit needs to be modified when first installed in a tank. As previously mentioned is the float arm traveling smoothly when in the tank?

This is purely a resistive device connect an ohmmeter to it and measure the resistance from htf to the center point empty tank mid range and then full

Then install it back in the tank and take the same measurements

[old guy rich]

OK, thank you all for the comments and help, very much appreciated.
Here is what I did today. Read ohms/unit in tank wires disconnected= 14.5. Wires attached key off=9.4 wires attached key on= no ohms/0. bench tested arm at full=12.1 arm at empty=100.3
I submerged the unit in a bucket of water up to the flange and used the normal wires from in the trunk. Gauge works correctly. I used 2 jumper wires to get the readings while next to the car. The tank is almost full, so I'll drive it for a while and use enough gas to get down to 1/4 tank and see what happens. Thanks once again. rich

[19Fordy]

Rich, After driving your car for a while, how accurate were the gas tank readings?

[Flathead Fever]

If has that metal adaptor sticking out with the wire screwed to it. If that wire is the power to the sending unit and not the ground, and that piece of metal were to contact the tank the gauge would read full but when it's out of the tank with a ground wire hooked to the sending units flange it would work normally because that piece of meal would no longer be grounded. Wrap a piece of electrical tape around that piece of metal and see if that stops the problem. You can't leave the tape on because the gas will dissolve it. If that fixes it, grind down that piece of metal. It looks like it sticks down low enough to clear the tank but it's something you can try.

They used those same brass floats on the Ford F-600s-F800s with the 40-gallon step side tanks up until at least the early 1980s. They would always be filling up with fuel, sinking and reading empty. Ford sold them in packs of two and probably still do. We would take the ones completely full of fuel (no air in them), stick them on the bench and move a match aground to find the pin hole. They made the coolest little burning blow torches until the gas was gone. Stupid, idea, yes but it was funny. The old time WWII mechanics passed on that wisdom. I don't know which one of them was dumb enough to try it first. I bet we lit 50 of them over the years and we all retired with full set of fingers. Besides, how else would you find the hole that let the fuel inside? Don't do it!

[Lawson Cox]

With gas prices being what they are today, who's complaining about your tank always being full??? (The devil made me do it.)