Fuel sender float

[Sparkydave]

What was the float made from?  I got my tank out to address some rust inside it, and when I took the sender out, I could not find the float!  Not even pieces of it.  The sender will need to be replaced (thank goodness Boos-Herrel has them), the float arm rusted through and fell off inside the tank.  Even moving what's left of the float arm, the sender resistance is all over the place, so I don't think I can save this one.  There's some loose rust sliding around in the tank, I can only assume the float rotted away.

[19tom40]

The float should be a brass cylinder closed off at both ends.

 

The current replacement sending units are hard to calibrate as they use a variable resistor. If your sending unit is not rusted out, I would replace the arm and use it. If the sending unit is rusted out, try to find a sending unit from a 1936-1955 Ford. You will have to replace the arm. The P/N for your sending unit is 21C-9275-A which is the same as 42-47 Ford truck. Here is a chart for bending the arm.

 

 

You can check out your sending unit by connecting an ohm meter, set on the Rx1 scale,  between the sending wire post and the case flange. Move the arm through the full range while looking at the ohm meter. The reading should stay around 0 Ohms. If it shows open or a higher resistance, try operating it through the range many times to see if the wiping action of the contact point will remove the oxidation. If you can't get a good reading, you can pry off the top and spray the contact points with electronic cleaner.

[Sparkydave]

I don't follow what "The current replacement sending units are hard to calibrate as they use a variable resistor." means, since the original sender is a variable resistor.  They don't measure zero ohms, but change resistance as the arm moves.  The replacement just arrived and measures 10-100 ohms between full and empty.

[19tom40]

The original sending unit was a King-Seeley unit that was a bi-metal strip heated by the current flowing in the circuit with the dash unit. The dash unit has a similar bi-metal strip that moves the needle. Here is a photo of the interior of an original sending unit.

[Ray500]

On 7/18/2024 at 3:51 AM, Sparkydave said:

What was the float made from?  I got my tank out to address some rust inside it, and when I took the sender out, I could not find the float!  Not even pieces of it.  The sender will need to be replaced (thank goodness Boos-Herrel has them), the float arm rusted through and fell off inside the tank.  Even moving what's left of the float arm, the sender resistance is all over the place, so I don't think I can save this one.  There's some loose rust sliding around in the tank, I can only assume the float rotted away.

That's the reason I had a stainless steel tank made as I got tired of the junk that collects in the old tank.  Clean fuel is important, and I replaced the fuel line with 1/4"  stainless steel tubing and a new 6 volt electric Carter fuel pump to use as needed.  Remember these cars were never meant to last 80+ years, but we showed ole Henry we can do it!  I did save my sending unit and that works fine.  And it's not just a variable resister, the resistance of course will change with the movement of the float, but it's almost a log-arithmatic device.  If it were just a variable resistor, the fuel gauge would bounce all over the place as you drive  around!   

[Sparkydave]

The fuel gauge doesn't bounce around because the heater and bi-metal in the gauge are slow to respond.  The sender and fuel gauge in my MG work the same way; variable resistor in the sender, heater in the gauge, but going from empty to full takes several seconds because it responds slowly.

[Sparkydave]

Well, it seems I was mistaken on how the old sender worked.  I found a description of how the King-Seeley works on the Ford Barn and I understand now that it's not resistive but it's a pulsing bi-metal switch where the duty cycle changes with the fuel level.  Mine unfortunately is very rusty and the float arm rotted away, but I'll hang on to it if it can be repaired.  I got a replacement resistive sender made by Drake from Boos-Herrel, but I had to make some adjustments to it.  The gauge seems to respond to about 15 ohms being full and 60 ohms being empty.  Glad I checked the sender out before I put the tank back in; it was measuring about 34 ohms with the tank bone dry because the arm has to come up quite a ways.  When I hooked it up, the gauge read about 1/2 full.  Some posts on the Ford Barn indicated similar results with the Drake sender.  So, I bent the arm up a bit so it was measuring about 65 ohms, and the gauge read nearly empty.  I e-mailed Chris Herrel to let him know that sender needs some re-calibrating to be in the ballpark, but I can use it.

[19tom40]

If the bellows is not rusted, the sending unit can be repaired. Did you check out the sending unit as I said in my previous post?

[Sparkydave]

It's measuring open circuit throughout the range as I'm moving what's left of the float arm.  I couldn't pop the cover off easily to see inside, so I'm not sure what the condition is.