I posted this in the Expedition forum but thought I would post here also to hopefully snag an expert outside of the Expy/Nav world:

My 03 Expedition A/C quit working, the compressor was cycling on and off so I took it in to be recharged thinking it was low. They said was completely out and it took 3.5lbs to recharge although they couldn't find any signs of leaking. When they recharged it they put more dye in to help find the leak, the technician said he suspected a compressor since it was completely empty and had been working in the spring. I was hoping it was a compressor as I have changed one before on my 97 Expedition and am pretty comfortable with it, however I can't see any thing leaking using a UV light. I have checked the compressor and every line I can see all the way back to the rear (it has rear air). When they recharged it worked very good for about 8 days on the 9th day it was working and went from blowing cold to hot instantly. Any ideas on where or what to look for??

 

Using a UV light and yellow-lensed glasses, look for the dye. Also look at the evaporator drain in case it leaked inside the dash.