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MG MGB Technical - Voltage stabiliser issue?

As I was driving up through France on Monday, returning from the Circuits Des Remparts weekend in Angouleme, I had a strange issue with my fuel and temperature gauges. (As my car is a Feb ’74 former US car it has an electric temperature gauge.)
The fuel gauge had been reading just above half and the temperature gauge had been just to the high side of N. Then I noticed the fuel was down near 1/4 and the temperature had dropped to a similar position. After about 15 minutes they returned to their previous positions.
A while after that when the fuel gauge was just about bang on half I stopped for more petrol and got 24 litres in which is just about half a tank.

Thinking about what had happened made me focus on the voltage stabiliser. However it is difficult to see what the problem is. It is unlikely that the stabiliser’s open phase would become extended and then rectify itself due to the way it works. An alternative is that the system voltage dropped below 10 volts and then recovered. However, the was no sign of the alternator light coming on.

Any ideas?
David Witham

I think you would have noticed running problems from insufficient voltage for the ignition if it had been system voltage, I've never measured it but I'd say it would have to drop quite a bit below 10v for the gauges to drop from 1/2 to 1/4, and that may well be enough to affect the tach as well.

The voltage stabiliser has a contact that opens and closes, connecting system voltage to the stabiliser heating coil and gauges while it is closed. It's theoretically possible that instead of making a clean connection when it closed one time, it made a resistive connection, which would drop the voltage to the gauges and heating coil, and with the gauges dropping that much the heating coil wouldn't get enough energy to make the bi-metal strip move again until maybe vibration made a clean connection again.

Alternatively it could have been an intermittent bad connection anywhere in the green circuit feeding the stabiliser and from there to the first gauge in the chain.
PaulH Solihull

Thanks for the reply Paul.
I guess I need to go through the circuit cleaning all the connections.
There is nothing like a long drive to generate a job list.
David Witham

THE STABILISER IS A PRETTY CRUDE MAKE AND BREAK DEVICE TO REDUCE THE BATTERY VOLTAGE TO AN AVERAGE 10 VOLTS FOR THOSE TWO INSTRUMENTS. INTERNAL CONTACTS ARE THE MOST LKELY PROBLEM, BUT IT IS SENSITIVE TO IT'S MOUNTING POSITION. i.e., it must be the right way up and near vertical!!
Allan Reeling

Buy a solid state stabiliser off Ebay. Around £10 and it works accurately and consistently.
Mike Howlett

Mine is 44 years old and has never caused a problem. They may be primitive, but they were built to last. RAY
rjm RAY

Fuse to the green wire circuit? That also has an effect on the temperature and fuel gauges.

Les
Les Bengtson

The benefit of the original stabiliser is that it supplies full voltage for a few seconds, until it 'warms up' which means the fuel gauge rises to it's correct level quicker than it does with the solid-state stabiliser.

If the problem was at the green-circuit fuse operation of brakes or indicators or selecting reverse would kill the supply to the tach and fuel gauges altogether.
PaulH Solihull

This thread was discussed between 21/09/2011 and 27/09/2011

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