Actually, low or no refrigerant *will* cause the compressor to not engage. There is a hi/lo pressure cutoff switch screwed into the receiver/dryer. If refrigerant pressure falls below two atmospheres it disables the compressor.
Common causes of the low pressure cutout switch triggering include 1) It's cold out (below ~35F) 2) Low refrigerant charge.
Evaporator leaks are relatively common on Mercedes Benz cars. Dunno why. Because of the expense associated with removing & replacing the evaporator (>$1,000) I decided to gamble and use a leak sealer. The sealer is activated by moisture in the air. It is carried through the a/c system by the oil which circulates with the refrigerant. As the oil leaks out of the system, the epoxy sealer is activated and (in theory) seals the leak.
My 124 wagon would leak down from a full charge to the point where the a/c was useless in about 2 months. I installed the sealer early last November. When I checked the a/c in April, it was about 6oz down on refrigerant. Much improved! Since the beginning of April it has not leaked at all! The a/c works extremely well, at least for a 124 car...
Total cost was about $300 for refrigerant, oil, a new receiver-dryer, the sealant, and vacuum pump rental. I did all the work myself on a Saturday morning.
If you do have an evaporator leak you may wish to investigate an epoxy sealer. The participants over at the air conditioning bulletin board ( http://www.aircondition.com
) seem quite impressed by their performance. It saves having the evaporator removed from the dash, about 12 hours labor on a 124 chassis car.