It was just something Lummox mentioned a while ago on a similiar thread...

With all this hot weather and the air con on max the engines are under a lot of load and the diesels seem to generate alot of heat. So why hasnt the coolant temperature needle moved? Ive even heard the fans kick in a few times so theres got to be an increase in temperature. Does the needle have any bearing on the actual temperature of the coolant?

Middle position is a range of normal coolant temperatures so if it goes past middle, something is wrong so it never should go past.

Yeah, IIRC the computer keeps the guage dead still in the centre when the cars up to temp to make you feel nice and safe The actual temp can be a way above the reading.

fan will cut in and out because aircon is on ,this is normal