Instrument Lighting

[Kittyhawk]

Yes that is still in place (I to gauge.)

[Bill_qaz]

So out of dash the fuel gauge is earthing via the sender unit in tank so working correctly but when in dash its earthing not via the sender so wrong reading is that what you mean?
So is the gauge terminal that goes to the sender shorting to the gauge body, so earthing via the speedo to car body when its refitted. Can you check continuity from the gauge sender terminal to the speedo housing if you have a meter.

[Kittyhawk]

So out of dash the fuel gauge is earthing via the sender unit in tank so working correctly but when in dash its earthing not via the sender so wrong reading is that what you mean?
So is the gauge terminal that goes to the sender shorting to the gauge body, so earthing via the speedo to car body when its refitted. Can you check continuity from the gauge sender terminal to the speedo housing if you have a meter.

Yes Bill that is correct. I will check continuity as you suggest. Thanks

[Kittyhawk]

I can confirm that the two terminals on the gauge are not earthing to the gauge body. (There are thin insulating washers between the terminals and the body). The gauge is now behaving the same whether in or out of the dash, i.e when ignition switched on, moves fron fully left up to empty and stays there.

I have anew voltage stabiliser on order, so will see if that sorts the problem.

[Bill_qaz]

It could also be the guauge itself thats faulty, I thought the voltage stabilisation was to just slow the gauge movement, a sort of damper. How far the needle moves is controlled by the resistance to ground created by the tank sender unit. Its strange that during the checks the behaviour has now changed.
Hope your new part fixes it, if not a new gauge maybe required.

[svenedin]

It could also be the guauge itself thats faulty, I thought the voltage stabilisation was to just slow the gauge movement, a sort of damper. How far the needle moves is controlled by the resistance to ground created by the tank sender unit. Its strange that during the checks the behaviour has now changed.
Hope your new part fixes it, if not a new gauge maybe required.

As I understand it, the purpose of the instrument voltage stabiliser is to provide a reliable voltage, 10 volts, that is lower than battery voltage. By doing this, the instruments read reliably through a wide range of states of charge of the battery from fully charged to almost flat. It is true that you can tell, just by looking, if the car has an instrument voltage stabiliser because the fuel tank needle creeps up slowly when the ignition is turned on. I had assumed this is due to the way an electromechanical voltage stabiliser works.

Stephen

[Bill_qaz]

Maybe so Stephen but on my car without it, I never notice gauge fluctuation with voltage. Maybe I don't get much voltage variation running an alternator. Or it could be witchcraft created by the devils negative earth

[svenedin]

Maybe so Stephen but on my car without it, I never notice gauge fluctuation with voltage. Maybe I don't get much voltage variation running an alternator. Or it could be witchcraft created by the devils negative earth

I don't know but it is only the later fuel gauge that is designed to operate using a voltage stabiliser. Smiths obviously thought it was needed.

Stephen

[myoldjalopy]

Interesting. I have the older type and, like Bill, never notice gauge fluctuation with voltage. And that's with a dynamo and positive earth!
One thing the older type gauge does do, and which I suspect the later ones don't, is when the tank is getting low, the needle tends to swing about a bit when cornering.

[svenedin]

For the older cars with no instrument voltage stabiliser, you say there are no fluctuations of the fuel gauge with supply voltage to gauge but how do you actually know that? You would have to actually set up an experiment whereby, say, the gauge read exactly half full at 12 volts and then see what happens at maximum charge and minimum charge. I don't know the answer.

The later cars have an instrument voltage stabiliser AND a different fuel gauge and sender so the set-up is not just the addition of the voltage stabiliser. In an ex-factory car there are no other gauges but Smiths obviously supplied almost every British motor manufacturer and many other cars had additional electrical gauges for oil pressure and coolant temperature for instance. The instrument voltage stabiliser can supply up to 3 gauges per unit. It may be that there is more obvious and critical error with differing voltage with say a temperature gauge. I don't know the answer, just speculation.

Stephen

[Bill_qaz]

The original fuel gauge is what is known as a moving iron gauge. It's immune to voltage variation, and acts quickly. It's also prone to register fuel slosh in the tank, during cornering or heavy braking, especially at lower fuel levels. After 1967, BMC changed the design of fuel gauges to a thermal type, which depends on heating of a bi-metallic beam and is voltage-sensitive.
Hence the voltage stabilisation. This type of gauge is less sensitive to fuel slosh as its slower moving/ reacting.

[svenedin]

The original fuel gauge is what is known as a moving iron gauge. It's immune to voltage variation, and acts quickly. It's also prone to register fuel slosh in the tank, during cornering or heavy braking, especially at lower fuel levels. After 1967, BMC changed the design of fuel gauges to a thermal type, which depends on heating of a bi-metallic beam and is voltage-sensitive.
Hence the voltage stabilisation. This type of gauge is less sensitive to fuel slosh as its slower moving/ reacting.

I disagree that the moving iron type gauge is "immune" to voltage fluctuation but agree it is much less affected than the later thermal type. Having a later car I have always rather enjoyed watching the fuel gauge creep up slowly and yes, it is not affected by fuel sloshing about.

So we got there in the end: the instrument voltage stabiliser is necessary with the later thermal type fuel gauge because this type of gauge is sensitive to voltage changes. The later gauge was introduced to reduce erratic gauge readings from fuel sloshing about.

Who needs AI when we have the MMOC hive mind?

Stephen