Hey Geniuses of the 5Series forum. I need help with the AC on my 2005 525i

I bought it with some front end damage so I've replaced several body panels and some parts under the hood to get it looking straight. Now I'm trying to chase down the cause of my AC only working sometimes.

I've evacuated and recharged the refrigerant. It seems that when I'm rolling along at steady speeds, the AC works great. When I slow down, it stops.

I replaced my coolant fan behind the radiator with one from a salvage yard. I know it works to keep the engine cool but it doesn't run constantly when I turn on the AC. If I sit and let the car idle, the fan will turn on and off and vary at different speeds regardless of the AC settings.

So...I'm wondering if there is some sort of sensor or something that tells that fan to run when the AC is turned on? Any ideas on how to test it or what may be causing my problem would be greatly appreciated!