Hot Stuff Disco

My Disco TDI300 runs great loses a little water, not much but if I switch on the air con it overheats in a few miles and fills the expansion tank to the top.
Think I might just dissconnect the compressor what you think guys?

The air con is not connected to the engine cooling system in that way

Is engaging your air con pump effecting the drive of your water pump in any way or is it a separate belt
Have you an electric fan fitted to help the cooling through the Rad and air con rad (a lot of vehicles have these) and is it working

Usually when the ac is on the cooling fans are switched on. Check the condition of the radiator and feel if it is hot all over.
Hope this helps.

The air con was n.f.g so I took it off anyway, turned out the water pump impeller had come loose so any extra load just pushed the water temperature up too high.