s class diesel tick over

[markheaney2]

hi there i have a 2007 s320 cdi s class v6 diesel, the tick over is 800 all the time even when warm. I have a 2007 e280 cdi with the same engine that lowers its tick over after about 5 minutes to 650, what sensor controls the tick over its almost like it thinks its cold all time .

 

[Lord-WYBMAnE220]

Aircon ?
To obviate stalliness when the compressor's compressing ?