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MG MGA - Fuel gauge - odd behaviour

Hi
I have read the useful & comprehensive information on Barney's site but still cant understand whats going on.
The fuel guage had been saying empty even when tank was full, and I think that is a sender fault. However I took the guage out to have a look around and now it shows full permanently, whilst all connected up. But the strange thing is, even when I switch off the ignition, the needle stays on "Full". I left ignition off for 4 or 5 hours, and it is stayed there on full.
But if I earth the sender wire, the guage very obediently returns, as it should to empty, and stays there when ignition is off, just as it should.
What is causing it to stay on full with ignition off,
am I missing something? - Thanks
Graham
g Victors

You have it wired incorrectly - it is hard to see the tracer color on the wire but if it is not correct it doesn't work !
dominic clancy

Thanks Dominic but I have checked the wiring and it is correct. It does move to empty when it should (sender cable earthed) and I don't think it would remain on full (when not earthed) with all power shut off.
g Victors

Is the tank grounded? The tank normally grounds through the fuel line, but sometimes when the fuel system is modified, a piece of rubber hose gets plumbed inline, which breaks the ground. The gauge won't work right if the tank is not grounded to the chassis.

-Del
D Rawlins

Have done some further research, and with the help of the MGA Guru's site, have narrowed it down.

Disconnected the sender wire at the tank, and connected it in turn via 34ohm and 68 ohm resistors, to earth. At 34 ohms it registered just under half a tank on the gauge, so in the right ball park. This also confirms that all other wirings are ok.
When I switch off ignition, the gauge returns to empty, as it should.
BUT with the 68 ohm resistor, whilst it correctly showed full, when I switch off ignition, it stays on full, regardless of any tapping (or banging) of the gauge.
With the 68 ohm resistor connected, and gauge showing full, I then introduced a second resistor to bring total down to 34 ohms. The gauge obediently went down to half, although sometimes it needed a "tap".

SO why does it stay on full, when I switch off power after it reading full? Is it possible that there is some sort of induced magnetism holding the needle that side? But it doesn't move back after leaving several hours. very strange.

At least I now know that it should behave itself reasonably well once I get the float unit changed (in normal use it is still showing full after driving 100 miles, so I now know that is not a gauge fault).
Graham
Graham V

Graham,

Your last sentence is interesting. On my gauge, which is original to the car in 56 i believe, the needle does not move off full for the first 80 to 100 miles. Then it goes down gradually to empty over the last 150 miles. I refill around 250 miles or sooner.

Peter.
P. Tilbury

Graham, just stating the obvious but is the needle just sticking slightly on full?
Neil MG

Peter, have a read of Barneys site. You may just need to calibrate the gauge.

Neil. Yes it is definitely sticking on full, but not "slightly sticking". When I use the resistors to emulate a "half full" state, the needle will come off full with a tap or two. But if I am on full and switch off power, it just won't budge off full.
I guess in the half full state, the left magnet is pulling it away from full, but there is no pull other than gravity with power off. But that should mean it will be usable once I change the sender unit.
I am just interested why it should defy gravity with power off

Graham
g Victors

I have found that the usual cause for the gauge needle sticking is physical, in other words, the needle is probably catching on the back of the glass.

The glass is really simple to remove if you want to check this out.

Colyn
c firth

This thread was discussed between 06/05/2015 and 09/05/2015

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