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MG Midget and Sprite Technical - Fuel gauge adjustment - and a cautionary tale!

My Frogeye fuel gauge has never registered much above half so I removed the tank and checked the sender. At full tank position, gauge was still half so I thought I'd attempt adjustment - this is very well covered in mgaguru but, in case you should try this please read the following as it could save you a lot of time.

The gauge works by magnetism whereby there is an EMPTY position electromagnet and a FULL position electromagnet. The needle mechanism is pulled between the two depending on sender resistance. There are two adjustment nuts on the back of the gauge (see picture). When loosened these enable the two electromagnets to be moved thus changing the needle position. The adjustment range is considerable. HOWEVER, if you are tempted to try this be VERY careful. If the screws are undone too much then the magnets are free to rotate as you will have released a shallow plastic key on the base of the magnets which locks them in the slot (see picture). This will then probably snap the magnet wire which is 38AWG or 0.1mm.

It gets worse... My "friend" found one of the nuts to be very tight so applied more force. The nut turned but the entire magnet turned with it. It was obvious there was internal damage so the gauge was dismantled. Disaster. The EMPTY magnet had rotated but had also broken part of its plastic base where the key was holding it.

I removed the magnet (nut was seized) and discovered that some turns had been snagged and broken. These parts are unobtainium so I decided to rewind the magnet repairing the breaks with solder as I progressed. This was painstaking work. However, I did it (see picture), reassembled the gauge and calibrated it (using mgaguru info) and it works! Several hours work that I could have avoided.

So, be VERY careful if you attempt to adjust one of these older gauges. It is worth noting that the FULL magnet earths via its adjustment nut and it may be that a resistance develops here which affects the full reading.

Pictures to follow:
Bill Bretherton

Inside gauge showing electromagnets

Bill Bretherton

Gauge rear showing terminals (lower) and the two adjustment nuts (higher).

Bill Bretherton

Rewound magnet. Could be fewer turns than original but it works. I would ideally have used new wire but couldn't find length I needed other than with long postage time from China. Soldering this wire is tricky. Heat will burn through the varnish.

Bill Bretherton

Full reading in car (using resistor to emulate full tank).

Bill Bretherton

Thanks Bill

Very interesting. I have a spare gauge that has a similar problem. I was thinking of repairing it as a winter project so this advice is very timely!!
Bob Beaumont

Bob
It's worth noting that the EMPTY coil (magnet) is wired in parallel with a wirewound resistor/coil that lies across the top of the gauge. Combined resistance (between B and T terminals) is about 60 ohms. If the EMPTY coil is damaged you'll get the other coil's resistance only which is about 150 ohms. The two main magnet resistances are about 100 ohms each.
Bill Bretherton

Thanks Bill. I am going to carefully soak the terminals in plus gas before trying to undo anything. Hopefully it just needs a bit of adjustment!!
Bob Beaumont

This thread was discussed on 10/11/2022

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