With the newer type three vent boost pressure switchover valve, which side should vent to atmosphere when activated by the manifold pressure switch? The turbo boost, or the Alda? I ask because when it is activated one of the other sides closes.

The side from the intake should vent.
https://www.startekinfo.com/StarTek/outside/12253/program/Engine/617/09-450.pdf

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Sam

84 300SD 350K+ miles ( Blue Belle )

The ALDA should be vented to atmosphere when the switch is engaged. The idea of the overboost protection is to kill boost to the ALDA and restrict fuel to spool the turbo down. It is a safety system to protect against a failed wastegate actuator. The description in the attached document in Reply 2 says the same.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thank you for the pdf document, thats very helpfull. I agree, as the document said that it should be vented to atmosphere on the ALDA side of the switchover valve.
However, as a related note. The switchover valve is useless if it doesn't receive an electrical ground from the manifold pressure switch. So, out of curiosity I took out the manifold pressure switch an tested it with air pressure, an air gauge tee in line and a electrical continuity tester connected to the plug end of the switch and the ground end threaded part. It closed at 20 psi. So it would have activated the switchover valve, over boost protection at 20 psi of boost. Not what it should have as the FSM document says at 1.1 bar + - .15 (which is 15.95 psi) or, if adding the +.15 bar is 16.1 bar. I checked the pressure switch rating, and it was stamped on the side 1.1 bar +-.15 so it must be defective.

My overboost switch was completely full of carbon and as a result, doesn't work at all (never trips). I've routed my boost pipe straight into the ALDA, and I'm not losing sleep over it. That said, I think the 603's also run higher boost than the 617s did. I'm running 12.5PSI and the FSM states that 12.5-13.5PSI is normal.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

I'm glad you read the pdf attached! Sorry about the misinformation. At first, I didn't understand why it was done this way. Then I realized that the ALDA might hold pressure if that side didn't vent.

Diseasel300
I also had the boost pipe straight to the ALDA for a day while I got a switchover valve off a car at the wrecking yard. The car was like a rocket ship with the direct connection. That is probably in my opinion the most night and day performance improvement on this car one can encounter. That of a blocked switchover valve, ( I've been driving it like that for a long time) and then to directly route the boost to the ALDA.

Hi SD Blue
Yes thanks again for theFSM pdf, I have a other few bits of it printed out. Yes that exactly what I thought too, that the high "trip" pressure would be contained in the line and ALDA thus still still introducing more fuel. Although I tested my ALDA today and it doesn't hold a vacuum. So I guess the trip pressure would leak down and thus reduce fuel too.

I'm not sure if vacuum is a legitimate test for the ALDA, as it works on pressure. It has an aneroid that reacts to pressure for both altitude compensation and boost pressure.