Solved! Aux tank sending unit interference?

[Bad137]

I’m having an issue with my aux tank sending unit reading the fuel level in the tank. I think there may be interference with the fill tube blocking the sending unit float. I’ve searched the forums and have not found anyone with a similar issue.

My problem: Aux tank instrument cluster gauge was reading 5/8 full when the tank was full. It would eventually drop to Empty as the fuel level would drop. I diagnosed a bad sending unit and replaced it with a new TBP sending unit (aux tank part number, not main, with slightly shorter tube after the bend). With the new sending unit in the tank, the level reads Empty regardless of fuel level.

I do not believe it is a gauge/voltage/wiring issue:

- All other instrument cluster gauges work
- Main tank reads Full to Empty fine
- Aux tank reads dead Empty when pigtail unplugged (same for main tank)
- Aux tank reads pegged Full when pigtail short circuited (same for main tank)
- Aux tank reads across the full range with a sending unit plugged into pigtail but out of the tank

I do not think it is a sending unit issue (but I first thought the original sending unit was bad as it would not read above 5/8 when full—hence why I pulled it):

- New (TBP) sending unit benched tested fine on ohms (7.5-76.5)
- New sending unit made Aux tank gauge read Empty to Full when plugged into pigtail but not placed in tank. (But new sending unit in the tank is currently stuck on Empty—my current problem.)
- Original sending unit benched tested almost good after being pulled out (14.5-77.7)
- Original sending unit made Aux tank gauge read Empty to 7/8 when plugged into pigtail but not placed in tank. (Makes sense as it reads max 7/8 full when manipulated out of the tank).
- With the new sending unit in the tank, the gauge moves from dead empty with no power to reading at the Empty mark when power is applied, so I know the sending unit is “working” as far as wiring.

When I looked into the tank, I was surprised to see how the fill tube and vent tube look to be in the sending unit’s way. Both are close to the centerline of the sending unit opening, and both the vent and fill tubes “drop” down into the tank more than I imagined (they terminate at the 1/2 full level. I tried to get a tape measure in to see how far the fill tube protruded into the tank. It’s on the float’s side (towards vehicle center) and what would likely hit the float. I think I hit the end of the fill tube with the tape measure at about 9”. The float is also about 9” from the face of the sending unit. I was disappointed but not surprised when my installed gauge did not read anything, even when the tank had 7.5 gallons in it. Good news is I have no leaks after replacing both aux and main sending units.

Before I pull the aux sending unit again and start bending things, breaking things, and irrecoverably breaking seals, I figured I better talk to people smarter than me on this. It is a poly tank, but I don’t know of what manufacturer or how old. It looks to be in fantastic shape if original, so I doubt it. Restoration was done 15 years ago, I’ve owned the Bronco for 5.

Pic of the vent and fill tubes is looking straight in through the sending unit hole.

 

[DirtDonk]

Just for reference, can you take a picture of the new sending unit next to the old sending unit? Or is it still installed at this time?

My first thought is that the float is leaking, which is a semi-common issue even with brand new units. My second thought was just to bend the arm to clear any obstructions.
Because in theory the fuel level should not get too much above the lowest point of the top of the filler neck and vent tubes, the fact that it does not go up past them should not be a factor. They can still potentially impede the movement, but they should not be keeping it from reading at least almost full.
In theory at least…

I think it was an auxiliary tank that Viperwolf1 took a few years ago and cut the filler neck shorter to gain an extra gallon or two of capacity. Not sure if it was a stock tank or not, but I think it was.

Seems to me though, that the bottom line is that the two tubes being above the sending unit should not be causing this problem.
But there is nothing against you tweaking the float arm slightly to keep it out of the way of any obstacles.

Good luck!

 

[Bad137]

Thanks Paul, I didn’t think about the float leaking.
I do not have two side by side pics. The new one is installed, and looks like a new TBP unit. I’ll obviously be pulling it out in a couple days. I have two other floats from the original units I pulled and can swap if needed. Here is a pic of the old aux unit I pulled. I was worried it didn’t have the filtered pick up on the end and tried to see if it was in the tank (I didn’t see it). I’m guessing the previous owner removed it to make it a shorter (aux) pick up?

 

[SHX669]

Try bending the float arm/rod AWAY from the pickup tube and sending unit. My fuel gage was sticking on 1/4 then after sloshing around a bit would go to 1/2 ( but never over 1/2) . While I was adjusting the arm/rod ( there is a set screw to do this on the TBPs units) I noticed there was enough slop in the arm pivot point to allow the arm to bind against the sending unit.

 

[Bad137]

I pulled the TBP aux sending unit out again and it bench tested good and also tested good in the instrument cluster. I must have internal interference with the float.

Here are units side by side. Left Is TBP from the main with longer/deeper pickup and longer float arm. Center is TBP aux. Right is the orig aux I pulled when I thought it was a sending unit problem.

 

[Bad137]

I pulled the TBP aux sending unit out again and it bench tested good and also tested good in the instrument cluster. I must have internal interference with the float.

Here are units side by side. Left Is TBP from the main with longer/deeper pickup and longer float arm. Center is TBP aux. Right is the orig aux I pulled when I thought it was a sending unit problem.

Oh, and the float is fine. No holes; still floats.

 

[DirtDonk]

Is the unit keyed so that it fits in one position only? Or is there a little leeway that you can rotate it? Just wondering if it’s just so slightly off that you can rotate it a little bit so even if the float arm is working at a slight angle, the float will go up and down between the two tubes.

 

[Bad137]

Yes, the unit and tank are both keyed so it must go in "vertical." But as I was putting it in, I did tilt the whole unit up and down to see if there was contact. When vertical, there was a metallic "tink" as I moved it up and down, presumably as the float hit the fill tube. When not all the way in/keys not engaged, I rotated the unit 10-15 degrees counter-clockwise and in that plane I could rock the unit up and down without interference.

I tried a long bar down the fill tube and while it would bend the tube a bit, it just moved back were it was. I didn't want to gorilla it though. Since I now have two presumably good units, I thought it better to tinker with the unit than the tank. I have the opposite problem that SHX669 experienced (the screw he talked about is on the old unit, my new TBP unit does not have it). I thought I could bend the float closer to the pick up tube to clear the fill tube. That only bought me an extra 1/2 inch. Dry fitting back in the tank I was still getting the metallic tinking sound.

I opted for more extreme surgery, and bent the float 90 degrees, to be in line with the float wire. That gave me an extra inch of clearance that I did not have before. Stock configuration has the end of the float 3" from pick up centerline, my modification had the float 2" from centerline. I tried to keep the float's "center of buoyancy" the same compared to the original bend when both empty and full; it's not perfect but I think pretty close. Dry fitting and tilting, I did not feel or hear any interference. I pictured myself posting here "Solved!" and a happy ending. As I filled the aux tank that gauge slowly rose to 1/4 tank and I was enjoying a natural high. Then the tank read 3/8s, then 3/8s, then 3/8s. I filled that damn tank until gas came up the fill neck and the gauge read 3/8s. I'll save you the curse words.

Pumped the gas out and the gauge still reads 3/8s. I'll pull it again in a couple days and see what else I can do. Good news today was the old pick up filter that was not on the old sending unit when I pulled it (and I couldn't see it anywhere) decided to expose itself inside the tank. A coat hanger and some finger gymnastics and I was able to get it out. After some unsuccessfull fishing attempts, I was tempted to leave it in there but then I thought if the sending united didn't read correctly I wouldn't forgive myself. At least the old filter was out before all the trials above.

 

[Bad137]

Pulled the sending unit again today and finally got good measurements on the fill tube. It projects into the tank 7 7/8” from the sending unit mounting ring. I (now) know the fill tube is also too far towards the center of the hole, but didn’t figure out a good way to measure that accurately. My float’s arc is around 9” from the same reference point, so it is not tube/float interference. I figured it must be the float wire getting caught.

I took the small remaining bend out of the wire and made it as straight as I could while still giving clearance and movement around the pickup. Installed this second modification with great success! In the end I did not need the 90-degree-off float bend, just a straight float wire. There is a small “guard” around the wire where it mounts to the resistor (?), so this will keep a straight wire off the fill tube if the guard is pressed against it.
Lots of trial and error, but finally glad to figure it out. I assume my aux tank must be manufactured differently (poorly) than the norm.

 

[DirtDonk]

Nice detective work and a result!
Glad you got this frustrating problem figured out. Now on to the next three!

Paul