I have an w203 C280 luxury, and sometimes in the dashboard appears an indicator "Coolant, go to workshop", immediately my temperature needle jumps from 84 to 90 or above (it will increase to 120), and when this happens, my AC blows only hot air. No codes thrown!!!! What should i do??

pressure test the cooling system and the cap, check for leaks in the cooling system and around the engine, make sure the coolant reservoir is full

No leaks, not even coolant smell; and coolant and wiper deposit are topped.

have you actually scanned the car for fault codes or are you just not seeing any check engine light warnings on in the cluster? The fact that the needle is bouncing around like you say would lead me to believe there is an air pocket in the engine cooling passages or possibly the coolant is not moving through the engine properly and is boiling inside the engine. You can always try to warm the engine up from cold and check the radiator hoses at the top and bottom and see whether they are about the same temperatures or whether they are cold on one side and hot on the other after the car is warmed up.

Any other guesses would include a faulty water pump or a sticking thermostat.

Yes, i've scanned the vehicle, not with DAS/STAR, but with another pro scanner; no codes thrown. And i don't think it's Water pump o thermostat, because there's movement in the water (the hoses are fully warm at operational temperature) I'm considering Front SAM or a short, but i don't know where to search at.

check your radiator cooling fan when this is happening; it usually would have stopped working and can be many things causing it--anywhere from just a fan that went bad to shorted wiring to the fan; the control unit for the fan is in the housing itself