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MG MGB Technical - Fuel gauge / sender

Hi,

I have a problem with my fuel gauge, in my '66 B.
Firstly I have calibrated the sender to be full at 20 ohms and empty at 221 (couldn't quite get 222)ohms. Also calibrated the gauge to read full at 20ohms and empty at 221 (lucky that the standard 220 ohm resistor was a bit high). However, in the car a full tank only shows as a bit over half way and it sits on E even though the tank was still half full when dipped with a stick, plus I drove over 100 miles at this level. Checking, the gauge reads correctly with the sender out of the car, connected through the car's wiring harness and at its FULL - EMPTY limits. Thought that it might be due to a rotational misalignment with the tank, but the float sticks out (into?) the tank at right angles and would be quite insensitive to some rotational misalignment.
The only thing I can think of is that the float arm is at the wrong angle, however, I have found a quote that this angle should be "about 20 deg". Mine is 18.5 deg, which is about 20 deg. I doubt whether a 1.5 deg difference would make such a large difference in the reading.
Soooo does anyone know the exact angle this should be, or have any other suggestion as to the cause / fix?

Thanks

Herb
Herb Adler

Just a thought but are you sure you are using the correct voltage stabiliser for that gauge?

Colin
Colin Parkinson

Herb -
Clamp the sender in the position it would be in the tank, and measure drop and lift positions. Then figure out the limit positions imposed by the tank.
Sounds to me like the float arm is out of "clock" with the resistance coil. Might be the wrong sender, or somebody bent it. As long as you have the right variation over the travel, you should be able to futzy the arm until it all comes out correct.

FRM
Fletcher R Millmore

The float doesn't have a hole in it so it's filling with fuel when in the tank does it? I once has a holed float on a sender and it did things a bit like you describe.
Simon Jansen

Hi,

Colin,

The regulator is an all electronic one that I built, giving 9.87 volts. Note also that, as said, the gauge reads correctly with the sender out of the tank and at its stops.

Fletcher,

Thanks, that's an option I thought about but a lot of fuel and time to get any where near right. I was hoping that someone could tell me the correct angle of the arm.

Simon,

No hole.

Herb
Herb Adler

Herb -
Why? The tank is say 6" deep. If the sender fits in the middle vertically, then the float has to drop 3" and rise 3". So clamp the sender to a board and see where it is.. You could clamp two boards to the first to represent the top and bottom of the tank. I think you need to put a bend in the arm so the resistor is more "up" while the arm stays where it is, assuming the arm now actually goes up 3 and down 3". Or whatever the numbers are. Angle only comes into it if you are comparing to another (straight) arm of the same length. Many arms are all squiggley, and squiggles change the clocking.

FRM
Fletcher R Millmore

Is it a new sender unit? My new one has a stiff bearing and doesn't move properly. It gets stuck at the bottom of its movement. Perhaps yours gets stuck at the other end.

Mike
Mike Standring

It's far easier to calibrate the gauge to match whatever the sender is doing, than try to tweak both to meet some notional resistance values. That is, once the sender is able to move through its full arc when fitted in the tank. Having had three senders in each of two cars they can vary one to another quite considerably.
Paul Hunt 2010

Hi All,

Back after my eye op and with some time to spare I tackled the gauge sender unit, the way Filmore suggested, see attached photo.
What I found was that the sender float was about 1/2" off the "bottom". I played with the connecting arm and wound up straightening it to get the float to touch "bottom".
Once re installed and ignition turned on the gauge read somewhere near where I expected it to, ie just off empty. I still need to fill it up and see how close it comes to full.

Thanks for all your ideas.

Herb

Herb Adler

If you bend the arm to get the float lower you will prevent it going as far as it was before, so it will stick at its highest point (which may not be F) for ages. Mind you, my Celica and ZS both stayed/stay on F for about 60-70 miles.
Paul Hunt 2010

Simon Jansen, New Zealand
The float doesn't have a hole in it so it's filling with fuel when in the tank does it? I once has a holed float on a sender and it did things a bit like you describe.

i had thos problem to but also my fuel clock was buggered.
to be honest the senders are cheap enough to buy its not worth the hassle or head work trying to fix your old one.

s truman

Hi,

Firstly sorry to Fletcher for calling him Filmore, can't think where I got that from.

Paul

My original problem was that the gauge only went to about half full and sat on E for ages. Having bent the arm to give correct(? to be confirmed) reading at E the float will go higher and, hopefully give an F reading.
Even if as you say it will be stuck at F for a considerable time, I don't think this is a problem as I know the tank is full, having just filled it. What I do need to know fairly accurately is when I am running low, so that the gauge's accuracy around E is much more important than at F.

S.Truman

Yes I could invest in a new sender, but it could well have the same issue with the shape of the arm and therefore not solve my problem.
Also I'm the type of guy that hates throwing anything away, because its stopped working, without trying to fix it first. You'd be surprised at how minor a fault can "kill" a device. Years ago my Dad had a battery operated drill that stopped. He took it to a shop to have it repaired, and after testing was told that it was permanently dead. In those days these drills were very expensive, not like today with cheap Chinese stuff. Anyway he brought it around to me and I took it apart and found that a wire had come off the battery. Soldered it back on and many more years of use. Total time 20 minutes.

Herb
Herb Adler

A holed i.e. sunk float will show E all the time, not be stuck up at half when the tank is empty.

Sounds like your sender is just plain faulty, Herb, if you can only get it to work at *either* E or F. Whilst I've had very variable results from senders (two replacements on each of two cars in quick succession) neither were anywhere near as innacurate as that, although I did run out once when mine showed about an eighth of a tank left. I suddenly got concerned when I realised the needle hadn't moved for a while, started hypermiling, but ran out, and even though I managed to coast about 1/4 mile towards a junction with a filling station it was the incline just a few hundred yards before it that did for me. I agree accuracy at E is more important than at F, which is when calibrating the gauge I say to do F first, then E. If altering E affects F that doesn't matter, the other way round would.
Paul Hunt 2010

Paul,
I found when I calibrated my fuel gauge and sender together on the bench that it wasn't a question of bending the arm but of adjusting the stops to allow a full range of movement. I just used a 12v battery and a voltage regulator. Like you though I had run out of petrol before that. Pity someone doesnt make a 15gall fuel tank (apart from Frontlines very expensive ally one) that will fit round my Panhard rod!
bob p

Hi All,

Well I've just filled her up and from a gauge reading of E it now shows F, therefore the problem has been solved, bending the arm did the trick. Thanks Fletcher.

Herb
Herb Adler

Herb,

"...from a gauge reading of E it now shows F, therefore the problem has been solved..."

Hopefully so - providing it again reads E when it should? Keep an eye on mileage for a few runs.

Regards
Roger
Roger T

This thread was discussed between 18/03/2010 and 19/04/2010

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