W900B blower

I dunno how common it is, but the '98 W900L that I run has the same issue. New blower and blower motor and I ended up putting Gorilla tape over the vent on the pass. side to get more air flow. However, I think there may be a problem with the switch that changes settings, because I noticed that even on just regular a/c there iscold air coming from the vents under the dash, as if the setting was bi-level. So I just stuffed rags into those to close them off. Works better than doing nothing.

 

My '99 W900L does it as well, although it's not terrible. I suspect a leak in the box under the dash. I can open the glove box on a hot day and everything in it will be ice cold.

 

Also, make sure you have the box sealed up on the firewall side. I used some foam type window seal tape to seal where the box goes together, then I ran a bead of silicone along the seam for good measure.

To check for leaks, leave the truck shut off and turn blower on high. Listen and feel for air leaks anywhere around the box. If the blower is getting plenty of power and is running as it should, then the air has to be leaking from the box somewhere.

 

I have a '00 W900L. The heater core and a/c evap core are stacked in front of each other. Over time they accumulate dirt, oil, weeds, and anything else you can think of. My truck barely had any air flow. Took it apart and cleaned each core individually and it made a huge difference.

 

Ding,Ding,Ding we have a winner!

 

I am not sure if you can do this on your truck and this is not all my idea but I will blend what I have done with posts from people like Wore Out.
Removing and cleaning those cores is a lot of work. Using high PSI air and or water can fold fins over. Pointed objects can do the same.
Take 2-3' of 1/4" copper tube and solder the end solid. Drill small holes in a line up from the bottom up, at least four. Mine has a short 90 but the other is a better idea. Connect that to an air blow nozzle and blow through cores in both directions. Then take the adapter you made to hook it up to a water hose and wash it out. Some spray foam cleaners work very well but hard to get in there. A garden hose hooked up to a blow nozzle can be a hassle. Maybe put 10' of 1/4 inch plastic tubing in line to make it better to work with. Then you can take the end of the water hose and pore something like Dawn or 409 down it. Air water air water rinse cycle. Hot water helps. One hundred feet of water hose laid out in the sun equals hot water at no cost. Just make sure your drain is open and all your electrical stuff is of of the way.
Yes it sounds like a lot of work but you do not have to mess with the A/C system, worry about damaging at heater core tube getting a hose off of it etc.

 

I used A/C coil cleaner. Comes in a spray bottle at your local Hardware store. Sprayed the air intake side down and used a toothbrush and my shop vac. The a spray bottle with water and shop air. Big improvement on the old Freightliner. Future reference. Keep the fresh air inlet closed at all times. You will get a lot less crap in the evaporator and heater core.