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MG MGB Technical - fuel gauge not working

fuel gauge on my 1970 car has not worked for a whiles now.
today i have tried to replace it with a used item that i bought with a petrol tank that i was told was in good working order?

anyway new sender ive tried is still the same gauge moves a couple of mill but that's it.

i noticed when i roved the old gauge the float was full of what i guess to be petrol?
cant see how this would have got into it or if its supposed to be in it?

if i hold the 2 wires of the sender together the gauge shows full?

s truman

Definitely the sender unit faulty if when you hold the 2 wires that attach to the sender unit together and the gauge then shows full

The float could be catching in the tank if the new sender is also not working. Try removing the sender and leaving the wires attached then moving the float by hand to see if the gauge moves

cheers
Steve D

ok been out and tried that, the gauge only moves to 3/4 full when i move the float to where it would be when full.
didn't move at all when fitted in the tank.
so i expect the new sender is also faulty?
the float on the old sender is full im guessing with petrol hence it not floating in the tank when fitted.

one last question why would it only show 3/4 full?
s truman

ok cant seem to get any sense here should the like to the sender flick on and off as the fuel pump pumps

ive put a tester on the live to the sender on the light on the tester flicks on and off quite fast seems like it may well be switching on and off as the pumps pumping.

are the pump and the sender worked off the same live?
s truman

Having replaced senders on two cars twice each I can with some authority that replacement senders are very different to the originals, and vary from sender to sender. Not only did I have to recalibrate the gauge for the first replacement on each car, but had to again for the second replacement on one car although the other was OK.

Fortunately the gauge is relatively easy to adjust, see http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Gauges' and 'Fuel Gauge'.

Other than both being powered by the battery or alternator there is no connection between the fuel pump and the sender. Or *should* be no connection, they both share an earth (number plate bolt) so if that is bad you could see voltage pulses from the fuel pump at the sender, but you are more likely to have problems with fuel starvation. What I suspect you are seeing is the operation of the instrument voltage stabiliser, which despite its name actually switches 12v on and off about once per second. You can verify this by turning the ignition on but not starting the engine. You may get one or two clicks from the pump initially but then it shouldn't click more than about once every 30 secs at most, whereas the stabiliser will be switching on and off about once per second.
Paul Hunt

that sounds about right with the light flashing.
didn't have a clue that you had to adjust the fuel gauge!
ive ordered a new one now so i,l try that again tomorrow
s truman

This thread was discussed between 02/08/2009 and 03/08/2009

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