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MG MGB Technical - Fuel gauge not right reading

The fuel gauge does not read true, I filled the tank up almost to the rim of the neck but the gauge only showed 3/4 full then after doing 100 miles it goes to empty. I have read that the gauge can be ajusted ,but how do I take the instrument fascia off to get to the gauge. Please make it simple to understand I'm not very good with cars. John T.
J E Turner

Remove 17&16

http://www.victoriabritish.com/icatalog/mg/0066.html
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and 15, there are two plugs with slots you turn with a flat scresdriver to adjust readings. Turn very little to keep from breaking anything, and make sure you are turning the correct side.
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You don't adjust the reading at the gauge, at least I don't.

To test:
There is a wire arm that extends from the sender. The float is attached to the other end. Remove the sender and move the arm to up as high as it will go. The gauge should read full. Do the same in the low position. The gauge should read empty. You may need to ground the sender. If the gauge does not read to its maximums, further investigation is needed. The sender is basically resistor. Inside is a finger attached to the wire arm. This finger slides up and down a shaft wound with wire. If you've ever played with slot cars, the fuel sender is just like the controler. The wire wound shaft in most of the senders I've seen is not uniform, meaning there are gaps between the coils. This is what causes the gauge needle to move as it should and suddenly jump an unusual distance. On fill-up, my MGA gauge moves gradually up to 3/4, then jumps to full as I continue to add a few more gallons. There is a gap between the wire wrapped around the shaft and the resistance jumps at that point. The MGA gauge is not dampened so it moves quite fast. You can try to adjust the wire spacing inside the sender, but I've never had much luck doing that. To test the gauge, you can apply voltage directly to it and it should read full. It's important to note that the gauge isn't 12V. I believe it's 10V on later cars. 12V doesn't usually damage the gauge, but I do not recommend prolonged exposure. I've had my share of wiring mishaps and I've yet to fry a gauge. In your case, I'd connect an Ohm meter to the sender and slowly move between its limits. If the meter shows open anywhere, fix that problem (new sender).

To adjust:
It's best to run the tank down to nearly empty, leaving a little for "reserve". With the car level, insert the sender and note the gauge reading. The wire arm can now be bent up or down to establish empty on the gauge. Since there is no way to adjust full on the gauge, you're done. Refit the sender and fill up. New senders and/or tanks almost always require a little adjustment. I've also seen debris in the tank that caused irregular gauge readings.

Sorry to be so long-winded, but I've chased this problem on various cars. As and Bs are easy because the sender is on the side of the tank. It's also easy to install the sender upside down on these cars! I've seen that a few times. midgets are a little more involved. If you have anymore questions, let me know.
Kemper

In case it wasn't clear, by sender I mean the fuel level sender in the fuel tank. It's on the right side of the tank. Remove the wheel and you're looking at it. Good luck.
Kemper

Do we really want somebody who admits to not being good with cars messing around with the fuel tank?
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You have to learn somehow and sometimes it involves singeing off your eyebrows.
Kemper

Does your fuel guage give a steady motion from full to empty or is it abruptly changing?

What year are we talking about, BTW?
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check out the spanners section on here www.mgb-stuff.org.uk , I did and it helped.

My gauge never went below 1/4 full, as I found when the B ran dry. After this I made up a solid state voltage regulator and calibrated the gauge as per the above site. I found that removing the sender unit and then moving it to the extremes worked when calibrating.
S Longstaff

The most common problem I've seen is that fuel eventually seeps inside the sender float --- then it doesn't float anymore.

When the odometer tells you that you are down to less than a quarter tank, jack up the starboard side quite a bit to slosh the fuel left, remove the wheel and then the sender to take a look.

'Doesn't take long.


Dan Robinson

Thanks for all your input, the car is a 1980 GT,it can go from 1/2 full to empty in a matter of miles than run like that for another 150 miles.John T.
J E Turner

Sounds like a sender issue to me. These are fairly cheap and easy to replace, just reassemble it in the proper order or you will have a gassy mess on your hands.
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I'm guessing the float has fuel in it too. I had an 18 month old one go bad and was very tempted to replace it with a cork. As it happens, all the ones I had to hand were synthetic and I wasn't confident they aren't soluble in petrol! Any better alternatives?
Steve Postins

This thread was discussed between 05/06/2006 and 06/06/2006

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