The auxilary fans come on differently throughout MB models, but basically they come on with temperature of the hot side of the A/C and the temp of the motor.
Most fan operation comes with A/C temp. In recent models this actually is done with high side pressure. Somewhere around 300psi the switch turns on the low speed of the fan.
Normally with thermodynamic equilibrium there is an exact relationship between the temp of the refrigerant and the pressure. If the level of refrigerant is low then the pressure relationship changes and the system may never get hot enough to raise the pressure to fan cut in.
Bosch Master, ASE Master, L1
33 years MB technician