I have a 2006 Ram 1500 QC Hemi with ~70k miles. Recently the fan has started slowing down when running the air conditioning. When the truck is started cold, it seems to always blow fine for a while (minimum 10 minutes, or more) on the normal or recirc settings. The fan will eventually start to slow down to the point where the only air moving through the system is if I'm driving on the highway. The good thing is that the compressor is still making cold air, so on the highway it's not too bad, but in the city it's miserable. Eventually the fan will increase to it's normal speed again, but this process repeats.

I've been through many Google searches regarding this problem. I've ruled out the recirc door issue. One interesting thing that I noticed is when "Defrost" is selected, air comes through the main AND defrost vents, and when "Floor" is selected, a lot of air comes through the defrost vents and little through the floor. I assume this is due to the mode door 2 failing and maybe this is unrelated to my A/C fan problem. I'm not sure how long this problem has been going on but I noticed it while trying to troubleshoot the problem with the slowing fan.

Does anyone have any ideas?

Have your alternator checked sounds like a problem we ran into at work with a Medic recently. Works fine when batt at full charge then slows down tio what att and alt. can run together

I would pull the fan out and connect it directly to 12v and see what happens. That way you would know if it is the fan itself or maybe also the resistor block