I've tried checking the wiring harness by the brake booster on two 300E's, and both times I was unable to obtain any voltage signal, even though the CCU was giving a good signal. In both cases, it turned out to be a blower fan related part.
What I have heard is that when the CCU goes, it goes completely. If your A/C compressor is engaging and/or the vacuum elements are actuating the air vent flaps, then the CCU is probably okay.
-anthony
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