Hello everyone,

having recently changed the upper radiator hose (just a precaution) and switched to winter tires I thought what else could go wrong? A nice comfy winter ahead of us. Aaaaaaaah sweet dreams. Yesterday, whilst crusing around the city, I heard series of clicking sounds coming from underneath the dashboard accompanied with the center vent going open and blowing cold air. Turning the temp dial up resulted in the center vent closing but the air remained luke warm only. Turning the dial all way up bypassed everything and the car started to heat. So no monovalve at fault. The clicking sounds were heard in long series. Having the car rest with engine off helped to cure this but only for a while after which it started all over. Clicking sounds, no heat worth mentioning.

I took the PBU out and will have it silver resoldered (quite a number of small contacts alas) in a few hours. I hope this will cure the problem. I sincerely do as I am not excited about the thought of having to buy another unit. Lots of $$$ at MB.

I checked my local junkyard. Should resoldering not work they would give me another unit for $7. Yes you heard right. $7. The only problem is they do not currently have it. Having to wait half a year during winter doesn't sound exciting either. I hope resoldering works. Having checked the board, nothing seems cracked or smells burnt. I will keep you informed.

What do PB and PBU stand for?

I wondered the same thing. My money is on "push button" and "push button unit." (Aka CCU.)
Reminds me of a college paper that I once wrote. When I got it back, the professor had marked "UNA" on it. Not knowing what that meant, I asked. "Use no abbreviations" said the professor.

Push button unit indeed. The one to work when cruise control amplifier does not and vice versa