Oh, and obviously (maybe), keep an eye on that boost gauge to make sure it doesn't go above normal when the ALDA is off. Though if it does that would be another productive result that can be explored.

-Rog

Here is a pic of a switchover valve on a 1981 300SD. It is on the firewall near the brake booster. As I remember this new style is basically installed upside down from the original and looks just a bit different. On the new one the nipple on top is open to the air and the tube hooked to the other nipple on top goes to the alda. The tube below fits a nipple on the mechanism just in front of the VAV - cannot remember what it is called. Maybe shutoff valve.

So three nipples: two hoses on two nipples and one nipple always open to the air. Actually you cannot see the hose on the bottom; sight is blocked by the wire plugged into the switchover valve.

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I see... although I know that's not my issue since I dont have one!