During the Spring of 2003, I went to turn on the a/c in my Jetta and found out the hard way that it wasn't working. I started a thread on vortex and quite a few people had ideas on what to check first.
When we opened the hood, it was easy to see that when I turned on or off the A/C, the compressor was never being engaged. It seemed like there should only be a few reasons why this would be so:
I hooked up my Vag-Com and was able to verify that when I turned on my A/C, Vag-Com could see it. This was proof enough for me that the switch was working. In that Vortex thread, a few people suggested checking for wiring / power issues in the battery distribution block, but my wiring appeared to be fine. We checked all the fuses, everything was fine.
By using a small probe, we were able to press in one of the refrigerant valves on the system, and were met with a quick shot of very pressurized R134a. It seemed like there was enough pressure. We attempted (without a wiring diagram) to jump out a low-pressure switch, but ended up blowing a fuse.
At this point we were getting 98 degree days in NJ, and the situation was becoming more and more annoying. From reading what various people had mentioned online, and after conferring with Potterman I decided to take the plunge and order the 'fan control box' at an expense of almost $150.00. The box arrived, and we started to remove my stock one.
To make a long story short, this fixed the problem, and taking a look at my old one made it VERY clear why it had failed. You can't see it well in any of the pics, but in this first shot you can almost see that the top edge of the seal is pulling apart on the box. When we finally got inside there was residue from water, and the failed components were obvious:
Check out the top edge
Complete box with part number
Side view of the guts
The bad stuff!
More toast!
It's easy enough to change, but it is expensive. If you're ever under your car and looking to kill an hour, it might make sense to check this box out and maybe even seal the edges with some black silicone -- it's certainly much cheaper than buying a new one!