C220 cdi w203 air con not working . Dealer no idea !!

[mikesworkshop]

Hi all,Air con won't work on my 2003 c220 . Bin to dealer 3 times without success.
So far has had new condenser and pressure switch . New control module.lots of diagnostic but fault code still persists referring to low refrigerant although it is fully charged,
Pressure switch tested,wiring loom from control unit (Sam)to pressure switch and compressor all ok.
There is no output (12v)to compressor when air con is turned on.
Compressor tested with 12v (alternative) and works ok .
The fans also do not engage.
All fuses ok.
I've spent more than the car is worth but still no air-con .

Anyone any ideas or had similar problems.car is just to good to give up on .
If main dealers can't fix then who can !!
All suggestions greatly appreciated
Thanks