Should Auxiliary fan always turn on when AC is on?

[rph74]

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.

[haigha]

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.