About 3 weeks ago the radiator fan bellied up in my 1999 Nissan Quest. After replacing the radiator fan asembly we discovered that the radiator fan motor did not function. Further inspection found the 15 amp fuse had blown opening the circuit. After replacing the fuse the fan rotated and appeared to function normaly. Now here is the problem, as long as we are running at a normal speeds without the air conditioner on everything functions properly. If the air conditioner is turned on the car begins to over heat nothing can be done to stop it unless you pull over and allow it to cool down normally. I have reset the computer, thinking a line spike, when the fuse blew, may have scrambled the programming. What appears to happen is the fan runs at one speed but when it gets hotter than normal the fan shuts off. I believe this is a multi speed fan but unsure of how it works. Any suggestions or help in correcting this problem would be appreciated.