From what the helpful mechanic was saying, there was a problem with the PBU. His contention is that the monovalve gets power to open. I don't agree.

From what I'm saying, there wasn't a problem with the PBU. You should see 12V across the monovalve pins with the ACC on MIN because it takes 12V to CLOSE the monovalve.

Here's another test - what voltage do you get with the engine running and ACC set to 0 (off)? This setting sends a full ground signal to the monovalve so it should close.

The diagrams are available in the MB CD manual set. Available from FastLane if you're interested.

Sixto
95 S420
87 300SDL