Abingdon Spares fuel sender

[bergxu]

Installed my new fuel sending unit I bought from Abingdon Spares last fall on TC5771 today, was wondering if this paper tab under the electrical stud needs to be removed? My low fuel light was always illuminated and upon removing the old sender, discovered it pretty rusty and seized solid, but now with the new sender in, my light won’t illuminate when I moved the float to the “empty” position before I installed the sender on the tank,
So I thought maybe that tab under the stud had something to do with it but didn’t want to risk damaging the sender and it was too late to call them today to ask so figured maybe someone here has seen this too…see pic..

Regards,
Aaron

[Steve Simmons]

The sender needs a ground path, so you can either install it loosely to test or just connect a wire from the body of the sender unit to a ground point on the car. Then it should (hopefully) work.

[bergxu]

Steve,

Thanks, just use one of the screws on the sender body perimeter and run a wire to the frame then? Weird it wasn’t there before but I figured I must be missing a ground (or maybe I’m missing the power wire!). I need to see what color that wire is underneath the harness wrap.

[Steve Simmons]

Whoops, I read your post wrong and now I see that you've already installed it. Ignore what I said. But you could temporarily run a wire just to make sure the ground path is good.

[Duncan M]

Interesting that the new low level sender has 12V stamped onto the cover. I wonder what that supposed to mean?

[bergxu]

Well, I added a ground wire from one of the perimeter screws to the frame and the light still doesn’t illuminate and I’m fairly sure the fuel level should be low enough for it to (should be approx 3 gal in the tank IIRC) but maybe it triggers with two gallons?

The weird thing is that the fuel light was always on with the old sender in there (and no additional ground wire) but the old sender was seized up with the float arm near the top so???

Guess it’s time to get the multimeter out..

[Steve Simmons]

The fuel level is probably too high. Try draining a little out, or even better go for a nice Wednesday drive.

The old one was likely frozen down low, which seems the most likely position since the mechanism would not be submerged in fresh fuel.

[Rob Reilly]

Do not remove the paper tab. It is there as an insulator, to prevent the wire from accidentally touching ground.
I've never seen one stamped for 12 Volts, but perhaps the manufacturer has other versions for 6 and 24 Volt vehicles. Remember Ohm's Law, V=IR.

[Duncan M]

Original low level sender and matching dash warning light operates at 2.5V. The ignition warning light originally also used the same 2.5V bulb as the fuel level light. The idea of using low voltage in the original fuel level sender involved a good understanding of fuel combustability. `

[bergxu]

Steve,

I was expecting to see the arm frozen down low since the light was on but alas when I removed it, the arm was frozen up high which is baffling, so??

Rob; thanks, I did leave the paper tab in place. I assumed it acted as an insulator and a call with Abingdon Spares this AM also confirmed that.

I’ll run the fuel level down a bit more and see what happens. I did realize today that I need to drain my tank and flush it and the line because holy cow the fuel filter was full of rust and debris, and in fact, I’m amazed the car still runs as well as it does after I saw what was inside the filter

[i.thomson]

I just want to correct the idea that the sender unit has kess than 12V on it. It hasn't. 12V is switched to ground when the fuel level is low but is reduced to 2.5V by the winding around the original dash lamp. The use of 12V or less at the tank unit has nothing to do with an understanding of fuel combustibility, indeed the fuel level contacts and thus the 12V on them spend most of its life submerged in fuel perfectly safely.
Ian Thomson

[Duncan M]

I was hoping someone familiar with the history of replacement 12V fuel sender circuitry would chime in, as my TC has a fully operational original fuel sender and warning light. Sender contacts are soaked in fuel and 2.5V goes to the sender and warning light. The warning light unit receives 12V (usually from the fog light switch) but before the current gets to the warning light bulb or fuel sender it is reduced through copper wire windings to 2.5V. Not 12V.

The replacement 12V units are not soaked in fuel, and the warning light must also be replaced with 12V units.

I am sure some people have replaced that warning light with the commonly available 12V unit, and unknowingly run 12V through the original type (fuel soaked) sending unit. I would not suggest it. The reason the original circuit was 2.5V is at least partly due to fuel combustibility. The merits of the original 2.5V circuit has been widely talked and written about for decades.

[i.thomson]

OK. Let's dig deeper into this 12V/2.5V controversy. Forgive me if the following sound patronising but I feel there is a need to be precise here. Firstly don't confuse current and voltage. They are different, but related, things. Voltage drives current through a circuit but the amount of current depends on the resistance in the circuit. The relationship is determined by Ohms law: voltage = current X resistance. The fuel low light is one such circuit whereby 12V is supplied from the A4 terminal on the control box, either RF 91 or 95, via the fog lamp switch. Thus the lamp unit has 12V on it. So far no controversy. The lamp unit, at least the original lamp unit, consists of a coil of resistance wire and a 2.5V bulb before being connected to the tank unit at the rear of the car by a green or purple/black wire. This unit is nothing more than a float which opens and closes a contact dependent on the fuel level. When the fuel is above a certain level, which can vary from car to car depending on whether and how much the arm has been bent, the contacts are open thus keeping the light extinguished. With the contacts open the circuit has not got the continuity it needs in order to bring the light on. Thus no current flows while the fuel level is high and therfore there will be 12v on these contacts, or at least the one which is not connected to earth. Once the contacts close both of them are now at earth potential but as current is now flowing all of the 12Volts is now dropped across the wire resistor/bulb combination on the dashboard. What actually happens is that 2.5Volts is dropped across the actual bulb and the rest, 9.5Volts is dropped across the resistance wire around the bulb holder. Ohms law determines this for this who want to do the math(s). Basically while no current is flowing all points on the circuit have 12Volts on them, including the tank unit. This is true for the majority of the time, as I previously said as we normally have enough fuel in the tank. When fuel is low there is no Voltage on the tank unit at all, not even the 2.5Volts which only exists across the bulb.

The tank unit arm passes through the rear of the compartment which houses the contacts but as it must be free to move, and there is no attempt at sealing, the contact compartment is normally full of fuel. I can't comment on the physics which prevents the fuel bursting into flames under these conditions but it doesn't. I believe it is something to do with the amount of oxygen present but I will leave that explanation to somebody else.

Once again my apologies to thos who already know how to suck eggs.

Ian

[Duncan M]

Ian-

Thanks for clarifying that the voltage in the original low fuel sender circuit is indeed battery voltage. All 170 milliamps or 0.17 amps of it. Or, enough to light up the 2.5W flashlight bulb! That is 2.25 Watts @ 0.17 amps @ 13 volts. 13V is my current battery voltage. 170 miliamps was measured with a meter. Ohm's Law calculators are easily available on the internet. https://ohmslawcalculator.com/ohms-law-calculator

Someone not familiar with electrical resistance might not realize by looking at a wiring diagram that the amount of current going to the (original) contacts soaked in gasoline has been greatly reduced before it gets there. First, the wire feeding the warning light unit is the smallest gauge insulated wire behind the dash. Second is the resistance winding on the original warning light units is right around 75 Ohms, as measured. Third is the type of electrical contacts that are bathed (usually) in gasoline. I am not sure what the sort of contacts used on the original units is called, and it is hard to describe. If you lace the finger tips of your right hand together with the finger tips of your left hand... that is similar. Using the 7 contact point fingertip analogy, you can see that the 170 miliamps would be distributed across the 7 contact points, so each contact point would carry about 24 milliamps when the fuel level gets low and the contacts meet-- to turn on the warning light.

People misunderstand about how the original TC sending unit fills with gasoline. It is entirely intentional, and it helps keep the contacts clean. There are 2 little holes drilled (or cast) into the back of the unit to allow fuel in. Problem is not when the area is full of fuel, rather the problem could occur when the tank is low and the fuel level is below those little holes. If the car sits for a few weeks with low fuel, the fuel in the unit will evaporate and allow air space to accumulate. Gasoline vapors can be explosive. The makers of the car knew that, and went out of their way to limit the size of any spark that could happen in that oxygen rich fuel vapor environment of a worst case scenario. My original warning light unit and original fuel sender are 72 years old and work perfectly.

We all saw the 747 that blew up many years ago from having low fuel in the tanks, and a spark. Gasoline vapors are much more volatile than kerosene vapors!

My entire point is that the original fuel sender system is well designed. The 24 milliamp sparks are not strong enough to cause an explosion, even under the worst set of circumstances. The whole point is that not using the original safety protocols in that circuit is not a good idea, IMO. For many years the usual suspects have sold a warning light assembly with NO resistance windings and a 12V bulb. For example. Now I see Moss has started to carry one with resistance windings. Good for them. I have never seen an aftermarket sender unit that was designed to be soaked in gasoline, and as a result, the contact life is dramatically shortened.

[Duncan M]

Pictures of original sender and warning light. Warning light feed wire is the white wire. Black wire with red shrink wrap goes to sender unit.

light.jpg (59.75 KiB) Viewed 3051 times

[i.thomson]

Duncan, all I set out to do, at great length so as to cover all bases, is to show that there is 12V on the contacts most if the time, and 2.5 Volts none of the time. I also declined to address the question of how this does not lead to an explosion. I could have attempted this but was aware I was on much shakier ground so said I would leave it to others. You have covered some of the points I would have made and I agree that the lack of combustion concerns the lack of power available in the circuit which is due to the limited current available (not voltage). I also agree that the danger period, if indeed there is one, is when there is air/fuel vapour surrounding the contacts. However, you have once again confused Voltage and Current. Current is indeed measured in milliamps, but Voltage is measured in Volts, or millivolts, but in your second sentance, you state: "All 170 milliamps or 0.17 amps of it. ". Referring back to the Voltage in the first sentance in milliamps is confusing the two. This is the same as you did previously when you stated: "but before the current gets to the warning light bulb or fuel sender it is reduced through copper wire windings to 2.5V. Not 12V". Current can never be reduced to a voltage, only voltage can. I expect you will think I am being pedantic here but many people do this. Electrics is sketchily enough understood by most people, and electronics hardly at all (another common naming confusion) so I feel it is better to be accurate when talking about such things.
Regards
Ian

[Duncan M]

Enough current to light up a 2.5V flashlight bulb.

Curious British factoid: After TWA flight 800 blew up off New York in 1996 (sparking fuel level indicator inside the tank) the airlines were required to use fuel tank "inerting systems" to fill the "airspace" in the fuel tank --with inert gas like nitrogen. But, the first aircraft fuel inerting systems were pioneered for use by 1944 in British bombers. See aircraft-
https://en.wikipedia.org/wiki/Inerting_system