Should Auxiliary fan always turn on when AC is on?
 

My AC does blow cold, but I noticed today that sometimes it's not quite as cold at a stop. I also notice that the aux fan is not on when I turn a/c on. It's really not a big enough deal to spend $550 to replace it, but I'm wondering how this thing is supposed to operate.

My aux fan runs when the AC is on and the car is idling in the garage. I've never stopped at an intersection and checked if it's still running.

Edit: Cooler when driving rather than at a stop may be a result of more airflow through the intake vents on the hood.