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