When you press the buttons on the PBCU, can you tell if the vacuum elements that move the air duct flaps are working? I had a similar problem with my '84 300TD - the ACC was completely dead. No fan, no vacuum actuators, no AC compressor. After a lot of disassembly and testing, it turned out that the PBCU wasn't getting any power at all because the connectors that plug into the sides of the PBCU were corroded and the ignition switch was worn out. I cleaned the connectors and replaced the electrical part of the ignition switch and now the ACC works perfectly.