high speed cooling fans not running, refrigerant blows off at compressor

im having a problem with my 99 300m, the high speed cooling fans are not coming on. ive jumped the fans and they work, ive checked the relays and they are good as are all fuses. when the car is started cold and i run the air, the compressor makes a nasty noise immediately followed by a blow off at the compressor. when the car has run long enough to temp trigger the fans on (only the low speed run) this does not happen. the low speed fans keep the temp low enough to not trigger the high speed fans, there is some kind of switch , sending unit etc. on the a/c high pressure side that should also trigger the high speed fans. i believe this is where the problem is but i dont know what im looking for. what triggers the high speed fans in relation to the a/c running? what is it called and where is it located?