Fuel tank sending unit

[WPVT]

I removed the malfunctioning fuel tank sending unit from my 1954 C-1 truck. Sure enough, on opening it up, there were loose parts that fell out. Included in this was a cone shaped piece wound with resistance wire. I carefully put things back together. This is a single wire unit. The wiper contactor is grounded, one end of the cone is connected to the hot terminal. Reassembling, I tested and found that it did range from 9 oms to 90 ohms, just what the spec calls for. Unfortunately, the ohms are 90 when full and 9 when empty.   So I must have inadvertently wired or installed the cone upside down. 

 

Does anyone have a photo of the sending unit interior showing the cone shaped resistor ?

 

 

Edited November 26, 2020 by WPVT

[Dodgeb4ya]

A pic of the cone (broken one at that) ...wide part of cone down.... a very techie item to repair that works and lasts.

[WPVT]

7 hours ago, Dodgeb4ya said:

A pic of the cone (broken one at that) ...wide part of cone down.... a very techie item to repair that works and lasts.

Thanks so much. There's a little tab brass at the top of the coil, and something similar at the bottom. I think that a brass rivet held the coil in place at the bottom and that's what breaks. 

What's puzzling me is where and how the coil was connected electrically. Somehow, the top of the coil was wired to the hot terminal. Are there any clues on this piece ?

Thanks again.

[Dodgeb4ya]

It is a long thin brass rivet that fastens the cone.

I have never attempted to repair one.

I Always found good or new old stock ones.

[WPVT]

5 hours ago, WPVT said:

 

Thanks. I figured it out with an ohmmeter. The top wire of the coil is connected to a brass rod that passes through the center of the cone, and ends at the bottom rivet. So when it is riveted in place, the terminal is electrically connected to the top of the coil. The bottom of the coil winding is connected to nothing. 

I think this original old style has some advantages over the reproductions.  The cone shaped winding means that the ohm variation isn't entirely linear, the way it would be if the winding were on a cylinder. Full and empty would still be accurate, but in between, less so. The original engineers were pretty good at their job, and weren't satisfied with "good enough". That's why I enjoy getting into the inner workings of these old vehicles.  

[Dodgeb4ya]

That's great you could fix your sender.?

I couldn't do that...tried gave up a couple times.

 

[WPVT]

5 hours ago, Dodgeb4ya said:

That's great you could fix your sender.?

I couldn't do that...tried gave up a couple times.

 

Fingers crossed. I haven't fixed it yet, but I have a plan.

[JBNeal]

I tried to fix a couple of these and they fell all to pieces...bought several Roberts units over the years, and they would eventually fail with the same failure modes...so I opted to use the zinc plated flange from the Roberts unit to make Tod Fitch's modification using a Tanks Inc sending unit, complete with a ground stud for improved continuity...hasn't failed yet

[TodFitch]

1 hour ago, JBNeal said:

I tried to fix a couple of these and they fell all to pieces...bought several Roberts units over the years, and they would eventually fail with the same failure modes...so I opted to use the zinc plated flange from the Roberts unit to make Tod Fitch's modification using a Tanks Inc sending unit, complete with a ground stud for improved continuity...hasn't failed yet

I published it, but it was written by Bob Amos who should be the one getting the credit. Bob is/was a member of the Plymouth Owners Club and has an automotive electrical repair business in Southern California so he knows his stuff.

[WPVT]

Thanks. I drilled and tapped the bottom of the brass rod inside the cone, tapped the plate where it had broken off, and inserted a brass threaded stud #4-40. I added some epoxy  because it wasn't as neat a job as it might sound. It works and the cone seems pretty rigid. I wouldn't bet my life on it, but I didn't want to give up. If I had known ahead of time how it all worked and went together, I think I could've have done a neater job. If it fails, at least I know who to blame.