Mike,

Are you certain the Black/Yellow is not carrying 12V? That wire runs to several other places including the cold start valve, diag. connector, Motronic, etc. The main engine connector is attached to the back of the fuse box. Start by separating the connector and see if you've got 12V on the black/yellow at this point. Also check the diagnostic connector to see if it is making it that far. A 12V test light makes this job easier as you can see it from the driver seat while turning the ignition.

A bad solenoid will act the way you describe. The extra voltage generated by going directly from the battery is often enough to jolt the solenoid into operation eventhough the normal connection won't.