if your a/c works, the fans come on with the a/c and stay on the whole time. till the a/c is turned off.
if the a/c is not working, aka no cold air, the fans will not come on at all when the a/c is turned on.
fans wont come on till the temp reaches 212 in a 3800 not sure if thats the same for a 3100 or not.