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MG MGB Technical - Fuel Gauge

Why has the fuel gauge on my 71 GT started reading very high. When the tank is full, the needle is hard over past the F mark. When it comes down to the F mark, the tank will take about 20 lts.. Owen
O McNeill

Owen

Is your temp gauge still reading ok? If that's high too it'll be the voltage regulator. Failing that the sender is failing in what I would have thought is an unusual way or, more likely, the gauge is starting to fail or its calibration has slipped.

Bob
R.A Davis

If the instrument voltage stabiliser has failed it will either cause the gauge to read high or not at all. The factory stabiliser it actually a thermo-electric device that switches system voltage on and off about once per second. The ratio of on to off varies according to system voltage, a high voltage means a relatively long off period, a low voltage a relatively short off period. This takes 2 or 3 seconds to start pulsing on and off when you first turn on the ignition, if you see a constant 12v on the light-green/green terminal of the fuel gauge after this period, then the stabiliser is probably the cause.

A dual oil/temp gauge won't be affected by the instrument voltage stabiliser, only the separate temp gauge fitted to Mk2 North American spec cars and 77 and later UK cars.
PaulH Solihull

Have you recently replaced the sender unit or gauge?
Allan Reeling

It could be the resistance windings, on the sender assembly, have worn through and are now giving you false readings. Not uncommon for a sender that's 42 years old. RAY
rjm RAY

Paul

You're correct about the temp gauge, for some reason I had my midget setup in mind.

Owen

I'd still check the voltage regulator first as it's the easiest item to check.

Failing that try the link below.

http://www.chicagolandmgclub.com/techtips/mgb/f-gauge_cal.html

Bob
R.A Davis

The usual symptom of the sender windings having worn through, or broken, is reading normally down to a certain point, then dropping back altogether i.e. past E as if you had turned off the ignition.
PaulH Solihull

Paul, yes that is what you would normally expect to see with worn winds, but after 45 years working on various cars, trucks, boats and motorcycles nothing surprises me anymore. Every week seems to bring something new and odd to my door. RAY
rjm RAY

This thread was discussed between 25/03/2013 and 26/03/2013

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