Hi. MB Doc. Thanks for your reply, That is what we expected, (for the air con to work at any time). But this one has got us beat, after fitting a new thermostat, the engine now warms up whilst driving which it struggled to do before, and the aircon works as soon as it gets to about 75 degrees, I was checking it using a Bosch kts 650 which would not give live data, and a snapon scanner which gives live data but the temperatures seem to be wrong. ( tested on 2 different Mercs) . no fault codes listed in either air con or engine management. We also wondered if it was designed not to work from cold start as the glow plugs could still be on, and if the aircon was turned on at the same time it could possibly cause the car to struggle to pull away from juncyions etc. (we have had a few customers with various cars complain that if the aircon is tuned on the car struggles to pull away). ??