Charcoal Canister (Tank Vent) solenoid - when and why?

[zipwax]

I posted this on BimmerForums.com, but nobody there pays attention or knows. You guys seem much more technical. (I use Pelican Parts for all my M3 needs)

Car: 1999 M3, ODBII.

On the newer style charcoal canister valves, the kind in the rear spare tire well on the E36 - what I want to know is: when does the DME open it, and when does it close it? Be specific if you know the answer, it will help out lots of people, I'm sure.

Background: My car is throwing EVAP codes: purge valve stuck closed, charcoal canister valve stuck open. Neither is true, and they're both operating fine. Gas cap is fine. All the lines from the purge valve on back to the tank hold a vacuum. Expansion tank is fine too. But I'm getting these mysterious codes. Not once in a while, but every time I start and run the car.

Some interesting data: When I start the car, after a few minutes, the purge valve starts opening and closing 10 times a second, with a square wave being applied to it via the DME. But the charcoal canister valve is open (no voltage applied), which means the intake manifold is drawing in outside air through the charcoal canister via the vent tube. Is this normal?

Thanks