Cancel the coach!!!
Moose I think you have it - When the engine is cold with handbrake on, on tick over
exhauster thingy cuts in and you can over ride it by switching switch which lights up
When engine is up to temperature all is normal nothing cuts in ect
Thanks for all your comments guys good , bad and funny
Yes defiantly what it is . I think the switch has the picture with an arrow because the arrow maybe pointing to the exhaust/engine brake which is always to the right of it and they are linked in a way. Thats the only theory that seems remotely likely
Dan Punchard:
Does it turn the tacho off !! Some naughty previous owner may have used any old switch ,it’s how most get caught out ,like a f12 with 2 exhaust brake warm up switches .
I wasn’t far out then !
Another trucking mystery solved 
DAFMAD:
lucemm:
after ticking over for about 10 seconds the engine gets a load on it
if you rev the engine over say 1000 rpm or press this switch which lights up
it removes whatever was loading it possibly by air
sometimes when the switch is on the engine will load again and the light will go out and you have to reset the switch againSounds like a cold start load thing
I remember the F717 had a cold start device load thing
Uht hum 