I "believe" two different things can trigger the fan:

1.) The coolant temp in the radiator hits a certain point; or...

2.) You turn on the A/C (which it is true that the compresssor won't engage if the refrigerant charge is too low - so likely it wouldn't trigger the fan either).

So regardless of whether the A/C works or not, if the radiator hits a certain temp the fan should click on. You should be able to test this functionality by unpugging the wires connected to the radiator temp sensor and jumping the two contacts in the plug (with the engine running). Of course this will only verify the wiring, relay, fan motor, etc. It WON'T tell you if you a faulty sensor, which you won't know unless the radiator gets hot enough, or you could pull the sensor out and put it in the a cup of hot water and see if the circuit closes with a multi meter. I don't know at what tenp the sensor circuit closes, but I would imagine it to be around 195 deg F ????

I have to repeat that I "BELIEVE" this is how the 940 works. I personally own a 740 which is slightly different - a combination of both a belt-driven fan and an efan.