I have just looked up the ACC on my new CD manual, and found a great 35 pg. discussion of the function of the ACC, as well as some diagnostic tests to run with a multimeter. I'm just diving in now, but I have reason for hope that my PBU is not dead. Here's what I found:
1. The PBU will protect itself from short circuits in its outputs by shutting the ACC system off completely. The PBU manufactured by Kammerer will immediately turn the ACC on again once the short circuit stops. The PBU manufactured by Bosch will wait 30 seconds.
2. The Blower Regulator (BR) will not allow the blower to power on when the engine is cold, the outside air is cold, and the inside temp is lower than the temp selected on the rheostat of the PBU
3. The BR will shut off the blower motor if there is a short circuit or excessive temperature.
Here's what my system has done:
1. When I have gotten into the car on a cold morning and turned it on, the blower motor has at times gone on and off rapidly right after I start the car. This would indicate a faulty blower regulator or temp sensor, because the blower should not be allowed to come on until the car warms up.
2. Other times, the blower/ACC system has not turned on and off rapidly until a few minutes into my ride, long enough for the engine to reach the required temp to allow the BR to turn on the blower. So sometimes the blower regulator waits until the car warms up, functioning properly in that respect.
Here's the theory:
1. The blower regulator is faulty, creating a short-circuit condition. The PBU's response to the short-circuit is to shut the system down. The blower regulator's response to the short-circuit is to cut power to the blower motor, thereby eliminating the short circuit. This then allows the PBU to turn the ACC back on, and the whole thing starts again from the beginning.
2. Why does it only happen sometimes? Maybe the BR has something loose inside that causes an intermittent short? Maybe the wires to the BR are shorting intermittently? I will study the manual and report back...

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87 300D 162,000 miles: It's not just a car, it's an adventure!