A/C engine fan issues

So my truck, a 2007 W900 with a C15 ACERT constantly kicks the engine fan on at slow speeds (under 15 mph) whenever the compressor is on. It doesn't matter if it's cold or hot outside, it's like it is programmed to do it, not that the high side is overheating or anything. I completely replaced the heater box and both coils thinking that it would help that but it didn't make any difference. It blows harder and colder but the fan still comes on. I need to know if this can be fixed. I spend a lot of time in dirt lots where that fan just kicks up a crazy amount of dust, to the point where I can't see where I'm going. Thanks for any insight you can give me.