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Old 09-29-2021, 12:53 PM   #1
Clem Clement
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Default Gas gauge quit

My '40MERC's gas gauge quit recently. Reads nil. Reaching under the dash, I feel no loose wires. No problems in the last 4 years. I don't know if the sender is modern. Tank about 3/4full. Next step pickup the trunk mat and inspect the gauge top?
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Old 09-29-2021, 02:14 PM   #2
hotrodart
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Default Re: Gas gauge quit

CLEM - Most common failure seems to be the sending unit. I think your easiest first step is to pull out the sender (under the trunk mat) and determine if the float has failed......full of gas from a pin-hole, etc. If the float has gas in it, try moving the float/arm up and down, with the sender grounded with an alligator clip wire....to see if the dash gauge moves.


If no good results, then the sender or the gauge/wiring may be bad. Try a known good sender (borrowed fro Cliff Green or another club member), then you will know where to look next.......wiring or dash gauge.....neither of which is much fun.
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Old 09-29-2021, 06:01 PM   #3
51woodie
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Default Re: Gas gauge quit

Clem. I'm not familiar with a '40, but if it is the same fuel gauge system as my '46 coupe, I might be able to add some info. My fuel gauge was not reading full when the tank was full. One thing that I did while testing the gauge system, (you need the ignition on) was to disconnect the power to the coil to prevent damage to the point and/or coil. Another note, when the sender (in my case) is out of the tank, there will be some gas fumes in the area of the tank opening. If the disconnected fuel sender wire touches the tank with the ignition on, it will make a spark. I tested the circuit from the sender to the gauge by using a 2' lead with clips, so I could make the connection away from the tank opening. the gauge will read FULL when the gauge wire is connected to a ground. If you get a reading, the circuit is good, and the sender or float is likely at fault. In my case the float (aftermarket I think) was partially flooded.
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Old 09-30-2021, 11:39 AM   #4
Clem Clement
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Default Re: Gas gauge quit

Thanks for the important safety tip
Clem
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Old 09-30-2021, 12:40 PM   #5
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Default Re: Gas gauge quit

My original '39 sender has a cork float that is completely saturated with gas and stays sunk on the bottom of the tank. Jim in San Jose
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Old 10-12-2021, 01:03 PM   #6
Clem Clement
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Default Re: Gas gauge quit

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The builder put in a new tank. I'm very slow lately and will get to it when I can. Life is in the way.
Thanks
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Old 10-12-2021, 09:19 PM   #7
Zeke3
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Default Re: Gas gauge quit

Quote:
Originally Posted by v8nut View Post
My original '39 sender has a cork float that is completely saturated with gas and stays sunk on the bottom of the tank. Jim in San Jose
There is an old discussion on here that describes replacing the cork float with two replacement modern composite Model A floats glued together. I haven't tried it yet, but I have the same condition on my 1937 fuel sending unit as yours.
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Old 12-04-2021, 10:42 PM   #8
Clem Clement
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Default Re: Gas gauge quit

Mine system is supposedly all rebuilt.
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Old 12-04-2021, 10:58 PM   #9
petehoovie
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Default Re: Gas gauge quit

Quote:
Originally Posted by Zeke3 View Post
There is an old discussion on here that describes replacing the cork float with two replacement modern composite Model A floats glued together. I haven't tried it yet, but I have the same condition on my 1937 fuel sending unit as yours.
See this link for the fix > https://fordbarn.com/forum/showthrea...ght=cork+float


Photo shows the removed original cork float and the newly installed replacement float.

Buy replacement float here > https://www.snydersantiqueauto.com/p...9370&cat=41706
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Last edited by petehoovie; 12-04-2021 at 11:04 PM.
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Old 12-04-2021, 11:23 PM   #10
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Default Re: Gas gauge quit

Quote:
Originally Posted by v8nut View Post
My original '39 sender has a cork float that is completely saturated with gas and stays sunk on the bottom of the tank. Jim in San Jose
My solution at the time; The one piece that was useful on the repro sender was the float. I grafted it onto the original arm.
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Old 12-05-2021, 11:59 AM   #11
TJ
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Default Re: Gas gauge quit

Clem, supposed to be rebuilt and knowing it was rebuilt for sure are two different things. It sounds like a float issue, so pull the sender and check the float.
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Old 12-05-2021, 12:11 PM   #12
19Fordy
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Default Re: Gas gauge quit

Clem, Remove the sending unit from the tank and cover the opening with a rag to prevent fumes. Then, lay the sending unit on a piece of cardboard away from the tank opening and use an alligator clipped wire to connect the wire from the gauge to the top of the sending unit. Then run another alligator clipped wire from the body of the sending unit to ground (for instance, the trunk floor latch). Then, switch on the ignition and move the float arm thru its range of motion to see if it registers properly with the gauge. If your float is brass, check it for hairline cracks which allow gas to seep in and give false readings. Switch OFF the ignition before you reinstall the sending unit. A Fordbarn SEARCH for Gas Tank Sending unit will yield much more info. The 1940 Ford gas tank sending unit is shown below. 40 Merc is probably the same and both are made by King-Sealy. Neither operates on the OHMs Principle.
https://www.fordbarn.com/forum/searc...rchid=25890535
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File Type: jpg IMG_9101B.jpg (64.4 KB, 5 views)

Last edited by 19Fordy; 12-05-2021 at 01:03 PM.
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