Yes, you are most likely low on refrigerant. There is a low pressure switch that interrupts the clutch circuit because there is no oil circulating when refrigerant is low. This saves the compressor.
You need to find the leak and recharge with R12. MB's do not have the reserve capacity to convert well. To properly convert you should completely dismantle the system, THOROUGHLY flush out the mineral oil, replace the filter drier, pour the old oil out of the compressor, put in the correct amount of ester oil, put on 134 adapter fittings, evacuate and charge.
There are kits at wal-mart to convert at home, but these are called death kits by those familiar with auto air systems. Not to death to the person but death to the a/c. They will often make cold air, but usually not for long.