cooling system/electrical

ok, im haven some major cooling problems with my 91 305 rs. the damn cooling fan wont turn on! but it does turn on when the AC is on. a
friend of mine told me that so i can keep the car cool. the problem is i dont want to have to leave the air on in order to cool down the car. The AC doesnt work so it just blows hot air n its already too hot in there as it is. I tried changen the fan motor, checked the relays and at a loss. My next step is checking the coolant temp. sensor. But does anyone know how to tell if the sensor is bad before i just go out and buy another one? I've been told there is a way to test the volts/amps or sumthin to tell if the sensor is bad. any one know? thanks in advance
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