cooling/ac problems

[Ktm200]

Hello everyone, I'm new to BMW's. Bought a used 2003 2.5L Z4. Noticed the AC would stop cooling at stop lights. Left the car running one day with the AC on and the expansion tank exploded. Replaced the expansion tank and bled the system. Started the car with the AC on and the radiator fan spun up and continued to run for several minutes then turned off. It never restarted and the temp rose until i turned the car off. Several days later i started the car and the same thing happened again. This time i removed the fan and cooled the fan motor with compressed air and put it back in the car. Started the car and it spun up and ran for several minutes then stopped and would not start back. Had to shut the car off to keep from overheating. I've searched the forum and can't get enough info to be sure it's the fan and not a sensor. Any help would be great. I don't have access to a dealer, over 80 miles away. Thanks