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MG MGB Technical - fuel gauge reads 3/4 full

I just replaced the fuel tank in a 79 MGB.
I replaced the in tank sending unit also
The old tank is rusted and has holes
Tank and sending unit came form VB
It worked corectly with the old sending unit

Now its all back together when the tank is full the gauge only reads 3/4

Any ideas?

rsf fowler

I had a similar condition when I replaced the sender unit on my 70. You might find this handy:
Certainly worked for me.
B Anderson

First earth out the sender unit and the gauge should read full. If it does it's the sender unit at fault. Tweaking as described in will do the trick.
If it still reads inaccurately it could be the gauge or the voltage regulator. the latter is a crude make and break circuit designed to give an "average" 10 volts. They can be inaccurate and are position sensitive. If you have a 10volt supply wire it into the circuit with the 12v feed removed and regulator by-passed and see what readings you get. I have replaced these regulators on all my B's with solid state ones, which give a constant 10v. All the gauges have read faultlessly, the older one is now 4 years old.
Allan Reeling

I just grounded the wire between the gauge and the sending unit. The gauge went to full.
So it looks like its the sending unit. and this is a new unit.

I just filled the tank so the Ol'Lady will have to run it for a bit to use of some of the gas before I pull it and adjust the sending unit.

rsf fowler

It could also be the gauge that needs calibrating to the sender. See

Herb Adler

Very common, I've had to readjust my two gauges for for two replacement senders (the first ones only lasted a short while, their replacements have been fine). It's a lot easier to adjust the gauge than the sender. The slotted plates can be covered by cork sealing plugs, and be very stiff. They are only thin metal so easy to 'round out' the slots unless you use an implement that fills them.
PaulH Solihull

I actually found the sender easier. But then I did it before I fitted it (Insert smug grin) once I had managed to get it to the required values I checked it against the gauge before I fitted it so I knew the wiring was OK and the gauge was 'near enough' at both ends of the scale. I must admin the sender did feel a lot 'cheaper' than the one I took out I wonder how long it will last. I bet it will not be 42 years!

B Anderson

I will most likley not do this until next weekend.
I have to give the Ol'Lady time to run the gas out

Thanks for everyones sugestions and ideas

rsf fowler

I'd rather mangle my old gauge than my new sender in case it has to go back ...
PaulH Solihull

Just one further point. When adjusting the sender DO NOT be tempted to bend the float arm as that will put the range all over the place. just the small 'ears' on the plate. You will be surprised how much the resistance will change with even a minor 'tweak'
B Anderson

I recently replaced my sender, I had to remove the tank to do a cleanup and paint and decided to replace the sender while I had the tank out. I checked the sender and gauge on the bench prior to refitting using the method described in Bernie's link. To check the gauge I've made a test jig with a 10V regulator and a variable resistor calibrated as per the values shown in the link. As a final test I connected the sender and gauge to a 10V supply on the bench and was able to move the sender arm through its full range and see the gauge moving.

Once I'd replaced the gauge I put a couple of gallons in the tank and checked they registered on the gauge. I then drained the tank using the fuel pump and noted that when the tank was empty the gauge was also showing empty, in fact it was well below the empty mark. I consider the empty setting to be the important one as if it's above that I know I'm OK. There's nothing worse than a gauge that shows above empty when you're sitting on the side of the road with the pump ticking like crazy.

R.A Davis

"when the tank was empty the gauge was also showing empty, in fact it was well below the empty mark"

It's supposed to be, up to a point, in fact I deliberately calibrated mine that way by putting a gallon back after I had run out, and only then adjusting it to E. It gives one a bit of a psychological reserve.

That was after the other scenario you described, of sitting on the side of the road with a new sender and it showing 1/8th above E.

But better to 'bench test' with the actual gauge, than theoretical figures, you know it's right then.
PaulH Solihull


I checked the sender and gauge as individual items on the bench and then as a pair, also on the bench. The final check was as described, in situ, as a sanity check.

The empirical method of adjusting a gauge to suit a particular sender in situ obviously works but as you've found out if you then have to replace one of the items you have to repeat the procedure. Far better to ensure each item is working to spec before fitting. This was the same reason Ford when asked to build Rolls-Royce engines during WW2 said they couldn't build them to RR's spec. Not because the spec was too high but because RR were mixing and matching parts to get the tolerances correct. Ford on the other hand wanted to manufacture the parts to a tighter tolerance to ensure interchangeability of parts and ease of production.

"I'd rather mangle my old gauge than my new sender" Probably better to mangle neither.

R.A Davis

"Far better to ensure each item is working to spec before fitting."

And suppose something isn't? Keep sending (ho ho) things back until everything is to the same spec?

I say far better to ensure the sender you have just bought is going to work properly with the gauge you already have, whichever you choose to adjust to make them do just that.
PaulH Solihull

Had the same problem on my car.
Found the voltage stabiliser was faulty and only had an output of 7 volts instead of 10 required by the gauge.
Changed and fuel gauge now reads correctly.

P Le Bailly

This thread was discussed between 20/01/2013 and 25/01/2013

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