The fans come on low speed (through a resistor) when the AC is at operating pressure (on at 20 bar, off at 15 bar, for what thats worth). They come on high when the coolant is at about 100 (watch your fingers.) If they are acting funny below 100 the AC might be low on pressure meaning low on freon. Search around here, its that time of the year for fan problems/solutions.
Do you have one of two fans, and if two are they both working (they are wired in parallel.)