Cabin air filter

The cabin on air filter is on the windshield cowl. You can easily see it if you sit in the passenger seat and lean forward looking out and down.

 

it don't have one

 

I just bought an 01 xl. It's right there on the cowl, just under the right side windshield. If i remember ill grab a picture in the morning.

 

my o5 don't have one there is a aftermarket one that can be slapped on the cowl yes

 

The low switch is designed to shut the compressor off if pressure gets low. Shouldn't have anything to do with fan.

Have you looked to see if there's 2 high pressure switches? 1 would be for fan. Other for compressor if pressure gets high.

Possible relays or wiring issue would be other causes. Along with possible overcharged Freon

 

I have an '02 with the outside filter, if you '01 doesn't have it check here:
Open the hood, look on the passenger side firewall. You should see two silver "clips" remove them and a cover should open to slide out the filter.
FYI: Freightliner in all their infinite wisdom did not have a filter as a standard item. If you have an ex big fleet truck, it may have been ordered without one.