2003 freightliner Columbia AC issues

Did you check the blend door to see if it operates?

 

Ok
If you don't have good air flow coming out of the dash you could very well have a problem with the fins in the evaporator being pluged with dust and debris so the air can't blow thru the fins and be cooled.
My 07 has a filter behind the lower dash panel in front of the passenger seat.
Mine was pluged solid when I bought my truck with 800,000 miles on it.
Also my truck has a cabin air filter on the fire wall on the right side of the truck.
And lastly my truck has a filter on the bunk ac much like the one up front.
Did you check all three of these filters?

 

Change or remove your cabin filter. Its only about ten bucks and can make a huge difference. Also start pulling the passenger side dash apart so you can get to the evaporator. Find an ac supply store and but a can of condenser cleaner. Use it to clean the fins out really good. Oven cleaner also works but can actually eat away aluminum so i don't recommend it.

 

You have 3 filters. 1 at the bunk blower. 1 at the dash blower. And 1 just behind the truck air filter under the hood.

 

With the hood open look at the truck air filter housing. 2 inches or so to the left of it is a rectangular clipped on door. About an inch and a half wide 8 inches tall. The cabin filter goes there.
in front of the passenger seat is a kick panel. Pop out the trim that is where a glove box would be on a real truck to access the top 2 screws. 2 more down lower to the floor. Lift out out of the way and you should see a filter around the blower cage. If you don't you should order one. It keeps dust from being sucked back into the evaporator.