Fuel gauge resistance

[John Cornforth]

Hi

The pictures show an early type G33 sender and matching gauge. The sender will be near zero ohms when full and about 30 ohms when empty.

Your new sender is 3 ohms when empty and about 150 ohms when full.

Putting a 40 ohm resistor in parallel will give you a gauge that reads backwards and will have a very non-linear scale. As the fuel level drops from full, the gauge will tend to always read nearly empty. Once you have about a gallon left, it will begin to climb fairly rapidly until it indicates 4 or 5 gallons when you are about to run out. You might find that you get used to it and it will therefore still be of use.

Even if you found a later type gauge of the opposite sense (or modified the original) you would still be stuck with the pronounced nonlinearity.

If you can find a way of refurbishing and re-using the original sender you will get a proper reading. They are pretty low-tech and are often repairable if the resistance wire is intact.

I will give some thought as to whether there is any simple discrete transistor circuit which could be used to better match your new type sender and existing gauge.

[Old Dave]

Thank you John. That is most helpful.

I am starting to lean towards using the original sender which as I have indicated does give some information. The fact that the tank has been leaking a couple of inches from the top means I have not filled up for some time.

If I fill to that level the gauge read up towards 4 gallons, and the gauge decreases with time as you would expect and does occasionally drop quickly. I chicken out and fill up hoping to stop before the leak. I am tempted to get the manufacturer to replicate the "hole" on the tank and have a go with the old sender. I guess thread compatibilty of the 6 bolts will be the issue.

A good earth might help

Thanks again.

[John Cornforth]

Hi

From memory, the sender has 6 regular spaced plain holes. The screws go into tapped holes in a ring shaped boss in the top of the tank, with a cork gasket to seal. The thread type and size has been argued about on this forum several times, and I can't remember the final outcome. I suspect there are several similar sized threads, hence the confusion. You can use whatever you see fit.

The sender relies on the tank being earthed. The tank U-bolt fixings were originally set in rubber strips, so I suspect earthing was actually via the copper petrol pipe back to the engine. This is fine as long as you don't put in a rubber pipe section ! You could also run back a dedicated earth wire from the sender body if you want to be certain. The gauge earthing is equally tenuous, and relies on the gauge body being connected to the instrument panel and that panel being connected to the body. Again, a dedicated earth wire to the gauge earth stud might be useful.

One thing which makes the senders unreliable is that the "hot" connection has a tufnol insulating sleeve where it goes through the body. Smiths had the bright idea of trapping the earthy end of the resistance wire in the gap between this sleeve and the mazak body. You can imagine how good that earth connection is after 80 years of corrosion, so dismantling, scraping and reassembly may be called for.

The resistance wire is wound on a thin former, and the 'finger' attached to the arm and spindle bears against it. Cleaning the surface with contact cleaner and ensuring that there is at least some finger spring tension at all points of the travel may eliminate any erratic behavior.

[Adrian Payne]

Hi Dave
To get the new gauge to read the right way round, is it possible to swap the float arm to the other side of the sender unit. Some universal units have the arm held to the sender spindle by a grub screw, undo and insert arm from other side. 
Hope that makes sense.
Adrian.

[Old Dave]

The modern sender that would be supplied with the new tank can be seen here.

https://www.compbrake.com/product/fuel-s...cba1185463

I am enquiring if the 6 hole spacing is the same as the Austin?? If it is then I could use either sender. If it isn't then I will ask for a match to my old tank which the manufacturer has as a template.

Thanks to all for your help - much appreciated.

[bob46320]

Stick with the original unit and save £35 - make sure that there won't be any baffles in the way of the float arm.

[Old Dave]

I think that is the right way forward Bob - thank you.

I have already left a message for the manufacturer to get the mounting in the new tank replicating the original tank.

Thanks again

["Slack Alice" Simon]

To save possible confusion (and a little money) I asked my manufacturer to leave the mounting plate blank, and drilled and tapped for my original sender.

I run a separate earth as a matter of course, I don't trust any original earth returns on a car as old as mine.

[Old Dave]

Thanks Simon. Yes defnitely an earth return. I might also rewire the feed from sender to gauge.

I hope there will be no confusion. The manufacturer has the old tank for template. I have told him that the thread of the new "holes" isn't important as long as they are in the right place and close to the same size. We will see....???!!!

[Old Dave]

Thanks All. I feel I have made the right decision (with moral support from you lot) to stick with the original sender unit. It is not as if it is dead, just a bit creaky (a bit like me really). Hopefully new wiring including the full return earth might be enough to get useful info from it.

Thanks again.