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MG MGB Technical - Fuel Gauge

Back in July 2004 I asked if anyone could help me solve a problem with my fuel gauge, and received several helpful replies.

Since then, I have fitted a new sender unit and get the same result as before.

Since birth, my 80 LE has had a fuel gauge which limps up to just about half full, even when the gas tank is full.

Thinking it was the sender this was replaced and no change.

Wondering if the new sender was faulty, tried a new third, smae lazy reading.

The voltage stabilizer feeds the temp gauge, and that is OK.


Any further thoughts would be welcome, its just one of those things that sends you craaaaaaaazy!



Jack

If the sender is mechanically sound (sounds like it should be ) it must be electrical. Try shorting the two leads to the sender unit to see if the gauge reads full then. That at least confirms electrical. The ground for the sender is exposed under the car next to the tank, and so gets road spray and debris, and would probably benefit from a cleanup. Next for me would be to hook a voltmeter up to the new ground and the feed to the sender, to check the voltage arriving. If that's OK, I suppose it's possible the gauge itself is flawed, and just not moving as much as it should under a given current.

Steve

Steve Aichele

Jack....check out Paul Hunt's web site www.mgb-stuff.org.uk or is it uk.org ..not sure.

There is a diagnostic section on gauges which has an extensive diagnostic path for the fuel gauge. Follow it exactly and it will diagnose your problem. Anything else is just opinion and we all have one of those like other things....
J.T. Bamford

Jack, Back in May, I went through four sending units before I found one that worked and it was purchased from NAPA.
I also agree that you check out Paul Hunt's web site.
Robert Browning

Everything is new, the gas tank, sender etc., the only bits which are original are the fuel gauge and the wiring.

Almost certain we hooked the latest sender up to the wire, earthed of course, and manually moved the sender arm, but we never achieved a full tank of gas, so to speak.

Sure is a real puzzle, I thought it might be the voltage stabilizer, but the fact that it feeds the other gauge OK made me discount that thought.
Jack

I have a similar problem...was planning to adjust the float in the sending unit so that it reads correctly. Are these floats pre-set? Or do you sometimes have to adjust the float to read on the guage correctly?
Frank Sperling

Hi Jack....

This is VERY familiar, but there seems to be a cure. After replacing the sender unit in my '81 LE, the same thing happened. I checked ALL the wiring, the sender, everything.

Then it was suggested to me that I should adjust the gauge....

There are two little screws on the back of the gauge, one sets the 'full' mark, and the other sets the'empty' mark. SO: fill the tank, set the 'full' one. Then (carrying a can of petrol around!!) drive till the tank is empty, and set the 'empty' adjustment. This sometimes throws out the full adjustment a but, so watch it when you fill up again.

I can't definitively say that this is your problem, but if you've replaced the sender then it's likely.

Let us know how you get on....
Happy Driving
Bruce
B. Mann

I had similar issues with my fuel gauge. I would get no reading or a lazy reading. The gauge would read full when I shorted out the wiring - replaced the ground lead, works fine now.
Good luck.
Hilary

The floats should be preset, but having to change both of mine I found the new ones were way off in calibration, fortunately not a difficult thing to adjust at the gauge. Subsequent replacements to both cars didn't need any further recalibration.

If linking the two wires on the sender (only do this briefly) causes the gauge to read F then the problem *is* the sender. Don;t forget two or more new senders from the same source could have the same problem, and I have heard of a new part having the same fault as an old, previously good one.

But late models didn't have a ground wire anyway, relying on the mechanical fixings. So get a wire from a known good grounding point and use that. This will also help diagnose a bad ground on cars *with* a ground wire. If this doesn't result in an F reading either then you need to investigate the connection where the green/black in the rear loom joins the main loom by the fusebox, with your ground wire as well as checking the connections are good. If still not good then go onto the green/black spade right on the back of the gauge. If still bad then it is the gauge or the stabiliser or connections between them. But if the temp gauge is electric and not the dual temp/oil gauge, and that reads as it should, the stabiliser is almost certainly OK.
Paul Hunt

It's known that the replacement senders are not calibrated the same as originals, resulting in the problem you've got. Pauls web site is excellent at describing how to adjust the guage.

I found that I couldn't get enough adjustment at the guage, and so took apart the sender to adjust the potentiometer. If you do this, a foreward of advice: Before starting, have the sender in the car and fill up the tank. Then measure the resistance in the sender unit. This way when you get the sender out of the car, you'll have a way to figure out where the float is when the guage should read full.
Matt Kulka

Thanks for all the responses. I was going to ask this question but found some good answers before asking the question. Great Stuff!!

Jerry
JH Wheeler

This thread was discussed between 09/09/2004 and 16/09/2004

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