should the ran be running continously when the ac is on

Home / C-Class This can be a number of issues such as faulty A/C pressure/temperature switch, damaged dash contol unit, up to & including the fan controller module. The first thing to check is that you have a full & correct refrigerant gas charge in the system. You obviously have some gas or the A/C would not start at all. It has a protection circuit to protect the compressor. Check refrigerant first. Shortly after a cold start, with ambient coolant temperature and the HVAC system switched on (not in ‘economy’ mode), the radiator/condenser fan is promptly energized. Otherwise, it’ll only activate when coolant temperature substantially exceeds the thermostat’s nominal (excluding those on M272's) 87° Celsius opening threshold. There’s a switch on the high pressure side of the air conditioning compressor circuit (cattywampus from the radiator’s drain petcock) that signals fan operation -independent of coolant temperature- when refrigerant pressure exceeds ~12 bar. It’s a PWM signal, so full fan speed typically occurs only when more than 20 bar/290 psi has been achieved.