BMW Repair: e36 auxiliary fan running high, climate control unit, m3 coupe

Question
QUESTION: Hi, my 99 M3 coupe aux fan runs high and then stop every 15 to 20 seconds whenever I turn the A/C on.  I have changed the temperature sensor located by the radiator and also tried a different climate control unit, but same result.  Do you think it is the high and low speed relays? Or the aux fan motor?  Or someone even suggested there is low in feon (or leak)?  Thanks in advance,

ANSWER: Without having the car in front of me its tough to say, but when you have the A/C on, the auxiliary fan should cycle off and on especially if it's cool outside.Ive seen lots of blower motor resistors cause lots of weird problems some similar to this one. Freon leak possible but not likely especially if A/C blows cold, do you know the high and low pressures of the A/C system ?  that would be good to know.  Hope this helps   Thanks Nathan

---------- FOLLOW-UP ----------

QUESTION: Hi Nathan,
I will replace the blower resistor as you suggested.  Do you know what the high and low pressure of the A\C system should be?  Also there is the safety AC pressure switch for the receiver dryer, do you also think if a faulty one will cause the intermittent problem?  The auxiliary fan only runs high for 15 sec and stop for about the same amount of time then stays on this cycle through out when the A\C button is pressed.  I can hear the fan noise from inside the car when this happens.  Thanks again,

Answer
Depending on the outside temperature where you are at the pressures should be Low 40-50  high 150-180psi.  I think you're pressures may be off due to a leak, find out what the pressures are. And yes the a/c systems does have high/low switches they are on the Dryer receiver..Thanks Nathan