Fuel Level Sender Unit Query

[summerinmaine]

I've pulled the fuel level sender unit as a step in my tank cleaning, as when I had the tank filled with rinse water there was a leak from around the "live" contact screw. I've pulled the unit apart, and it clearly needs a thorough cleaning (and yes, I'll be very careful with the rheostat windings). And I believe that I can re-seal the live contact screw to the backing plate and cure the leak. But two questions now arise:

1. Should the unit housing form a liquid tight seal with the backing plate? There would have been no leak from the contact screw hole if the liquid had not been bathing the interior of the unit. If it should be liquid tight, it should be a simple matter to lay down a bead of some fuel-resistant sealer before I re-assemble.

2. What are the resistance values for the unit at dead empty and full? I'd like to test it once it's back together, and I can't recall whether it was all that accurate when I laid the car up awaiting repairs. I expect Brian's book might have these, but I'm afraid I'm flying blind until I can place my order.

[paddy]

2. What are the resistance values for the unit at dead empty and full? I'd like to test it once it's back together, and I can't recall whether it was all that accurate when I laid the car up awaiting repairs. I expect Brian's book might have these, but I'm afraid I'm flying blind until I can place my order.

There are two sorts, depending on the type of gauge you have.

If the gauge has a number starting with FG (probably FG2530/71) then the sender is 0 ohms empty and 90 ohms full. The FG number is printed at the top of the face.

If the gauge has a number starting with BF then the sender is 230 ohms empty to 30 ohms full. The BF number is hidden but can b seen if you look "under" the top half of the face. (see here: http://www.mgb-stuff.org.uk/images/fg1.jpg)

Paddy

[prezoom]

Jim, If you have a problem with the sending unit, I had mine repaired at Foreign Speedo, just off University on Illinois. Works great. Had an open in the winding. Had them go through the gauge and the sending unit.

Rob Walker
26-4889

[summerinmaine]

If the gauge has a number starting with FG (probably FG2530/71) then the sender is 0 ohms empty and 90 ohms full. The FG number is printed at the top of the face.

Paddy

Thanks Paddy! That's the one.

Any thoughts on the liquid seal issue? I'm concerned that if it's supposed to be dry inside, then it has been allowing the fuel (and vapors at <1/2 tank) to be exposed to electric current (no spark, but still). Electricity and gasoline contact skeers me!

[summerinmaine]

Jim, If you have a problem with the sending unit, I had mine repaired at Foreign Speedo, just off University on Illinois. Works great. Had an open in the winding. Had them go through the gauge and the sending unit.

Rob Walker
26-4889

Thanks Rob. If I can get Q1 answered, and the gauge still functions, I'll save that task for the next go-round. It cleaned up okay inside, but the "live" lead brace has come adrift from it's rivet point on the body, so it obviously needs a bit of work.

[billwill]

I was worried about fuel or vapour in the sender too.

But I did find that they are not sealed. Indeed in some other units on other cars the winding is fully exposed to the fuel. Petrol doesn't conduct electricity.

Do some tests in a jar of petrol if you are worried. (a small jar on long wires from the wires in the boot so that the gauge is in circuit), so that it is well away from the car).


I do wish Lotus had used a top mounted sender though, the side seal is a PITA, especially if the thin threads in the tank, strip the thread. Next time I might make two brass semicircles (like horseshoes) to fit behind the existing bolt holes and fasten them inside the tank each with two EXTRA countersunk screws. And of course make the brass thicker than the existing feeble threads and tap them with the right thread for the sender fixing screws.

[summerinmaine]

I was worried about fuel or vapour in the sender too.

But I did find that they are not sealed. Indeed in some other units on other cars the winding is fully exposed to the fuel. Petrol doesn't conduct electricity.

Do some tests in a jar of petrol if you are worried. (a small jar on long wires from the wires in the boot so that the gauge is in circuit), so that it is well away from the car).


I do wish Lotus had used a top mounted sender though, the side seal is a PITA, especially if the thin threads in the tank, strip the thread. Next time I might make two brass semicircles (like horseshoes) to fit behind the existing bolt holes and fasten them inside the tank each with two EXTRA countersunk screws. And of course make the brass thicker than the existing feeble threads and tap them with the right thread for the sender fixing screws.

Thanks Bill. I'm not particularly worried, unless it was not the way it was supposed to be. As a young guy, having heard that liquid gasoline would not ignite, I threw more than my share of lit matches into jars of gasoline (to impress the ladies, of course). Ignition requires vapors (at the proper A/F Ratio, of course ) and a spark. Neither of which should be present in the sender unit enclosure.

So I'll rebuild it as it is, and if there's a problem, well I usual rely more on my trip meter anyway.

I also agree with your other observations. But my threads are pretty pristine, and I always chase threads with the proper tap/die before reassembly. Something I learned so early it's now a matter of instinct.

[summerinmaine]

Haven't reassembled the unit yet, but I measured the R values and they ranged from 12 to 132 ?.

[paddy]

Haven't reassembled the unit yet, but I measured the R values and they ranged from 12 to 132 ?.

That sounds like 0-90 with a bit of oxidation on the contacts

Paddy

[summerinmaine]

Haven't reassembled the unit yet, but I measured the R values and they ranged from 12 to 132 ?.

That sounds like 0-90 with a bit of oxidation on the contacts

Paddy

That was my thought as well. I'll give it another good cleaning with electric contact cleaner, and then re-assemble and call it good.

I'm pretty amazed at how the post for the live contact is insulated (or not) where it passes through the housing base plate. I think I'm going to put on a small tube of heat shrink tubing for good measure, and then bond the contact plate to the base plate to prevent any leaking.

[alaric]

Hi. I share your concerns re the windings being immersed in fuel. I bought a replacement new unit from Mattys (I think), and it differed from my original unit in that the original appeared to be sealed inside the tank, but the new one clearly isn't as the wires are visible and the can around the windings is open at the ends. So, although it goes against my instinct, I've installed the new unit and will use it as is. I would be reassured if others can verify that they are using one like the new one that I bought - I think from Mattys. Actually they would be the people to contact wouldn't they.

All the best.

Sean.

[summerinmaine]

Thanks Sean.

I can't see how immersion of the windings would be a problem 99.999% of the time, but reading the "official explanation" for the demise of TWA Flt 800 should give anyone pause about immersing electrics into fuel tanks.

Not that I subscribe to the "official explanation" you understand.

[billwill]

This doesn't mention electrics immersion, but doesn't exclude it.
http://auto.howstuffworks.com/fuel-gauge1.htm

The writers of this wiki article consider it a fire risk.
http://en.wikipedia.org/wiki/Fuel_gauge

[paddy]

The writers of this wiki article consider it a fire risk.
http://en.wikipedia.org/wiki/Fuel_gauge

I think the comment they're making there isn't so much about the electrics being submerged, but the heating caused by the resistor in the sender unit.

Paddy

[Rusty]

Good morning all. I just purchased my second Elan, the first one I owned was in 1967, a 66 FHC. I am installing a new Lotus tank, aluminum, which came with a new sender and no screws. Question: what is the proper thread pitch and screw size? 6-32 seemed to be close, but not quite smooth enough for me. Any help appreciated. The car came with a Fuel Safe fuel cell so sender totally different. Thanks to all.