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MG MGB Technical - Fuel Gauge

Hi,
The fuel gauge on my '75 bgt has been acting up.When I fill the tank it registers 'full' and then registers correctly down to 3/4's.At this point,it drops to 1/2 and remains there until about another gallon has been used when it drops to the bottom of the gauge where it remains until I fill the tank.The cycle then begins again.
Now the warmer weather is coming I must try to fix it.Any suggestions would be much appreciated.
Thanks Peter
Peter Jones

Most likely your sending unit is the problem.
Kimberly

I've had a sender unit pack up and it registered empty all the time. The tank is quite shallow and any slight slope can change what it registers dramatically. Try filling it up at a quiet a station where you can check the needle two gallons at a time.
If you've got to change it, it's not a difficult job.
c cummins

Peter,
Some years ago I also had a very variable fuel reading and also changed the sender, but to no avail. The reason for the problem was the 12V regulator, the mounting for which which had become so loose that the body of the device was no longer earthed. This then provided a variable voltage to the fuel sender etc. A firm tightening of the mounting, complete with crinkle washer to add some bite into the bodywork, sorted the problem. The device, a small rectangular can rather like a relay, is usually mounted on the bulkhead under the dash, near to the steering column.
Might be worth a check. Good luck.
Regards
Richard
Richard Coombs

Sounds like the resistance wire on the sender has broken 3/4 the way down, which is exactly how all mine (4 of them on two cars) have failed.

Make sure you get a new sealing ring with the sender.

Make sure the new sender has a smooth action from limit to limit with no snags or roughness. If you have an ohmeter confirm its resistance changes from about 30 and 300 ohms (although it seems to vary quite a bit from sender to sender, more on this below), one of my replacements wasn't working at all until I tweaked the slider.

You can change the sender with nearly a 1/4 of a tank of fuel if you raise the right rear corner.

The sender is held in place with a locking ring with three tapered tang that wedge under three flanges on the tank. You can slacken the ring by tapping on each tang in turn (make sure you correctly identify which part belongs to the locking ring and which to the tank or you can damage the tank), it only has to move a few degrees before out, the sender and old sealing ring come free.

Put the sealing ring against the tank and the new sender on top of the sealing ring (NOT the other way round), making sure you get the sender the right way up, there should be a cut-out and notch, then replace the sealing ring tapping it home using each tang in turn. If the area has got wet with fuel you might like to use non-ferrous drift and driver.

When I replaced my senders the first two new ones varied wildly from the originals, such that I promptly ran out in one of them whilst still showing 1/8th tank. It's easy enough to adjust the gauge, do it with a full tank first. The back of the gauge should have two holes, possibly covered by cork plugs, containing slotted plates. Use a close-fitting screwdriver ( a small screwdriver may 'round-out' the holes as the plates are only thin and can be stiff) in the hole nearest the F mark to adjust the needle. Then enjoy your tankful of petrol, and when you get down to about 1/4 make sure you have a spare gallon with you. Run out, then adjust the gauge for E. I chose to put the spare gallon in first, so I had a 'reserve'.

Neither of the first two replacements lasted very long (probably a bad batch) and were a similar big distance from the E and F marks of the old senders. The next two replacements have (so far!) lasted well and although one needed a further small adjustment to the gauge the other didn't.
Paul Hunt 2

Hi,
Thank you all for the help and advice.I'll let you know when the problem is sorted.
Again,Thanks Peter
Peter Jones

This thread was discussed between 09/03/2007 and 13/03/2007

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