The first shows the wiring and the second shows the location. I can't tell from the picture where this is located.

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84 300SD
85 380SE
83 528e
95 318ic

What is it for; relief for what?

Confusing FSM terminology!!

Overflow relief valve = switchover valve = ALDA overboost switch.

Usually located on the DS bulkhead , follow the boost sense line from the intake manifold.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

I by passed that crap years ago.No alda,boost @15lbs

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Here is a picture of the switchover valve on my `1981 300SD. It is on the firewall beside the brake booster. This is the new style which I installed about ten years ago. It is actually installed upside down from the old style switchover valve.

Were you looking for this?

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Thanks. I thought that was it but I do not think that's what the photo shows.

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84 300SD
85 380SE
83 528e
95 318ic

The boost sensor wire goes through the same loom that houses the glow box to glow plug wires. On my car it goes on connector 153 mounted on the front side of the diagnostic plug mount. From there it goes into the loom which travels through the bottom of the fuse box beneath the fuse bus.

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84 300SD
85 380SE
83 528e
95 318ic

The boost sense line is the tube than runs from the intake manifold to the ALDA.
It is interrupted by the switchover valve that vents the pressure to the atmosphere when it receives an overboost electrical signal from wire coming from the intake manifold pressure switch (the pressure sensor on the intake manifold).

I've taken a higher resolution picture of what the FSM figure was trying to show. Gotta love those grainy FSM closeups.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Tangentially related, but when people talk about removing the switchover valve and installing an overboost warning light, are they tapping into the signal at the valve? Anybody know which wire?

Also, isn't boost limited by the switchover valve acting on the ALDA to cut fuel when it sees too high of a boost (15psi or so I believe)? That valve seems small to do the job of venting a lot of boost, so that's what I always assumed. Perhaps I've been understanding it wrong all along.

-Rog

I'm trying to track the wire off the sensor which was depicted in attachment 1 of my thread starter. I appreciate the photo and had indeed saved a picture of that unit. To point out my error, the wiring appears to travel back through the firewall so I'll add it to my list of which wire to rob off this body and install on the SE.

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84 300SD
85 380SE
83 528e
95 318ic

For an overboost warning light, use the same ground trigger the valve uses. It's the inch diameter switch with a single wire on the intake manifold. Hmm... the other end of the wire at the valve is closer to the gauge cluster.

It doesn't have to vent boost, it just has to block the boost signal to the ALDA. The turbo can't blow anything close to 15psi or even 10psi with no boost signal to the ALDA.

Sixto
83 300SD

Thanks. So I think my understanding of overboost protection was correct. Otherwise you could remove the ALDA and still have the protection with just the switchover valve, which isn't the case, correct?

-Rog

No ALDA, no overboost protection.

Sixto
83 300SD

Yes, the overboost protection works by limiting fuel. That's also why the car gets so slow when the boost "vacuum" line gunks shut... no boost to the ALDA = no extra fuel = not enough exhaust energy to spool the turbo = no boost.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

That BR/BK wire runs to connector 153 then into the bottom of the fuse box. It goes into the large rubber boot which connects the fuse box to the cabin area then makes a loop inside that boot and exits the box on through the rubber boot on the right side of the box. It exits that large mass of wires in that loom and heads toward the engine bay beneath the huge AC line.

It is joined on the last part of that journey by a BK/YL/RD wire when originates at fuse 10. The BK/YL/RD wire is attached to the F10 with another
BK/YL/RD wire so if you're going tie into or move those wire be careful which you cut. This wire also makes a loop toward the cabin and gets tangled in that mass of wire before it comes out and makes it's way to the ALDA side by side with the BR/BK.

How do I know? It took an hour and a half to dig through that mass of wiring and get this sorted out.

__________________
84 300SD
85 380SE
83 528e
95 318ic