AC blowing hot air, after system vacuumed & it worked for 3 days.

Is the clutch engaging?

 

Id start by knocking out the obvious issues. Check to see if your evap drain tube is plugged it could be full/covered in dirt. Also check to see if the lines are frozen up which could indicate a bad evap. Check to see if the fan is turning as well. Could also be the control itself is borked. I know when the one in my 579 died it would still engage the compressor but the controls stopped working for the blend doors. New control unit and it worked fine. Otherwise i dont know enough about freighliners to help beyond that.

Also as an afterthought when you say vaccumed i assume you mean recharged? Id take it in again or get a UV light and look for leaks. Its entirely posible a worn line cracked shortly after a recharge.

 

Temporary help might be shutting off heater core taps .. try checking bunk AC if it’s cold it’s a blend door up front .

 

Is it hot air or is the air temp just matching the outside air.. If it is hot air i would look at whatever module controls the switching between vents.. Since u said it only did that after u switched to defrost mode then back. I am not an expert on a/c systems... The other day my ac was going in and out all day until it finally gave up. I went thru all the diagnostic work from guys on here that was helping me think thru it..i took a flathead screwdriver to purge the schraeder valve (the one u use to add freon) and when i purged it , only air came out. I continued to purge it until freon came out. Then the clutch kicked in and its been blowing cold air ever since.

 

I am unfamiliar with your truck or engine. But you should be able to find some coolant lines coming out of your engine somewhere. Look near the water pump. You should be able to trace them to the firewall and your heater core is usually behind the lower dash on passanger side...

 

I’ve noticed that humidity sometimes prevented my vents from shifting from defrost to cool. Not sure why but it would get stuck when it was humid.