Fuel tank sender unit conundrum

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John Montgomery
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Fuel tank sender unit conundrum

#1 Post by John Montgomery »

Recently fitted a Stoddard tank to my BT6 (with lots of guidance from a previous posting - thanks!). Sender unit and gauge were working perfectly when I removed the old tank. Now, new tank is installed, gauge reads 1/4 full all the time. I have removed the sender unit to check for damage and test it with Ohmmeter and seems to operate as intended. Power wire and earth attached as they should be. An interesting thing is that when all retaining screws are removed and the sender unit is resting in place, the gauge reads full (to the right of full actually) - as soon as I screw one of the fasteners in place, it reads 1/4 again. Any thoughts on what this could be or what I should check next?
Many thanks.

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John Brooks
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Re: Fuel tank sender unit conundrum

#2 Post by John Brooks »

The sender should be 200 ohms from the tab to the case with the float hanging, top horzontal. If you move the float it should go from 200 to zero and back as you move it up and down. Full is zero, empty is 200 ohms I would look to see if the screw is torching the cover or the tiny wire from the windings.

The power comes in, pass through the flat coil of fine wire to ground. ( these windings are the 200 ohms) The float wiper just removes parts of the coil to ground. The higher the resistance, the lower the gauge needle reflects. The gauge has the power and finds the ground at the sender. If the resistance is 200, no current flow, so the needle is to the left. As the float goes up, the resistance goes down, current flow goes up, needle goes up.

Easy fix, look for witness marks on the inside cover near the arm pivot.
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Re: Fuel tank sender unit conundrum

#3 Post by John Montgomery »

John Brooks wrote:The sender should be 200 ohms from the tab to the case with the float hanging, top horzontal. If you move the float it should go from 200 to zero and back as you move it up and down. Full is zero, empty is 200 ohms I would look to see if the screw is torching the cover or the tiny wire from the windings.

The power comes in, pass through the flat coil of fine wire to ground. ( these windings are the 200 ohms) The float wiper just removes parts of the coil to ground. The higher the resistance, the lower the gauge needle reflects. The gauge has the power and finds the ground at the sender. If the resistance is 200, no current flow, so the needle is to the left. As the float goes up, the resistance goes down, current flow goes up, needle goes up.

Easy fix, look for witness marks on the inside cover near the arm pivot.
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Thanks - will give this a shot this weekend.

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Re: Fuel tank sender unit conundrum

#4 Post by Ian William Stockwell »

Hi John

A couple of months ago I put a new tank in my B T5, which is made by Dansk, and I had the same issue when replacing the sender unit in the tank. What I discovered was that the cork Float was sticking on the base of the tank almost magnetic but of course not possible, and like your situation it worked when not clamped as it was free to move.
I rubbed the cork smooth but to no effect it still stuck.
In the end I had to bend the arm on the sender unit until it no longer stuck and then adjusted it until the gauge read zero leaving just the reserve amount in the tank.
It is now fine and works very reliably.

Ian
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Re: Fuel tank sender unit conundrum

#5 Post by Joe Gross »

Having a similar problem. I installed a new fuel sending unit after bench testing. It read 0 ohms down and 180 up in full position. Now in the car, a 356c, the gauge reads just above one quarter but tank is full. Tested gauge by grounding green sender wire and it read empty with key on which is correct as I read the other posts. Also disconnected green sending unit wire and it read three quarters full, not full, with key on. So maybe the gauge is bad?? Also tested the sender in the full tank and got a reading of 40 ohms. Don’t know if that’s relevant. Good ground on gauge. Any help on getting the gauge reading correct would be appreciated.

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Re: Fuel tank sender unit conundrum

#6 Post by Mike Wilson »

Perhaps the sending unit has to be matched to the gauge?

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Re: Fuel tank sender unit conundrum

#7 Post by Norm Miller »

Possibly the top of the sender is grounding to the aftermarket tank when tightened due to a different profile in the stamping.
 

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Re: Fuel tank sender unit conundrum

#8 Post by Joe Gross »

The tank that the aftermarket sending unit is attached to is the original 356c tank. It has a gasket between the original tank and the sending unit. When sending unit is loose in the tank it still reads quarter tank while full. So I don’t think the sender is touching the tank.

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Re: Fuel tank sender unit conundrum

#9 Post by Ron LaDow »

Is the float a float and not a sink?
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Re: Fuel tank sender unit conundrum

#10 Post by Mike Wilson »

Good point, Ron.

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Re: Fuel tank sender unit conundrum

#11 Post by Joe Gross »

The fuel sending unit I just installed in my 356c is a new one for a late 356 from Stoddard so I believe it floats fine and it bench tested 180 ohms and 0 ohms so I think it’s ok but the fuel gauge always reads about 1/4. The tank is full. I’m planning to use this sending unit until I can get the original one repaired somewhere. At this point I’m trying to decide if the combination gauge needs to come out for repair since it only read 3/4 , not full, when I disconnected the green sender wire with key on, which I understand from previous posts it should do. Since this car is new to me I’m not sure if any one previously messed with the combination gauge wiring so I guess I should check that first before pulling the combination gauge. I’d appreciate any comments or ideas about this plan and anything else I should look at. Thanks.

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Re: Fuel tank sender unit conundrum

#12 Post by Mike Wilson »

I believe Joris repairs the sending units. Also, I still believe the sending unit has to be calibrated with the gauge. Comments?

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Re: Fuel tank sender unit conundrum

#13 Post by Joris Koning »

Hi Mike,

I only restore the late pre-A to T-5 ones. Still need to look into doing the later ones but have had to many other projects on my plate.

On the earlier senders I bench test them with a gauge after assembly. Never have had any complaints about the readings from people installing them on their car with their gauge. As such, there does not appear to be a need for calibration based on my experience. I will however, happily defer if theory claims otherwise but this is what I have found based on rebuilding 75+ senders.

On the problem discussed above, I would hook up the sender out of the tank directly up to the gauge with some test leads to take away all other possible variables. If it still reads only 1/4 full and it indeed tests out to 180 ohm on the bench, you probably have an issues with your gauge somewhere.

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Re: Fuel tank sender unit conundrum

#14 Post by Mike Wilson »

Hi, Joris!

On the later senders, you can bend the tabs to adjust the ohm readings. I, too, have hooked up a sender out of the tank and adjusted the tabs to get the correct ohm readings but once back in the tank, it will read o.k. at full but reacts inconsistently when fuel is depleted and doesn't read correctly. I think you are right, the problem then rests with the gauge.

Stay well,

Mike
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Re: Fuel tank sender unit conundrum

#15 Post by Joe Gross »

Used Joris’s suggestion and hooked up test leads to new sender out of tank. Thanks Joris for tip. Both sender and gauge read correctly. Then checked green and brown sender wire conductivity which was good. Then reset sender in tank and sender ohms continued to read correct until I hooked up green wire . Then both sender and gauge read incorrectly at 1/4. Then did the quick gauge test and disconnected green wire and gauge read 1/2 instead of full. So my conclusion is a bad gauge. I’d welcome any thoughts. Thanks

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