The fan comes on when the high side of the a/c system reaches a set pressure of about 250psi( benzmac knows the exact spec). You need special tools to test this pressure. The fan also comes on when the coolant is close to boiling. If your car isn't over heating and the a/c is working, I wouldn't worry about the fan as the other fan is handling the load.
'87 300E 325,000 km
[This message has been edited by CMuc (edited 09-08-1999).]