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

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

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, deutschland deutschland uber alles uber alles in der welt

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

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 is some great info, thanks for posting it.

So I'm guessing the single wire from the sensor grounds the circuit. Although I've found some strange wiring methods on these cars, so I've learned to never assume. But if that's the case you would just wire up a light and have the ground go to the sensor wire. Wish it was easy to test is all.

-Rog

My issue with a system which put a light in the ground circuit would be the bulb failing which would cause the circuit to fail.

If I wanted a light to tell me when the ALDA is limiting boost I'd run a hot off of one of the fuses and splice into the BR/BK going to the intake sensor.

Since the BR/BK goes through the main ele loom and into the lower cavity of the fuse box that is where I'd make the connection. It would also give access to the bottom of the fuse bus which would accommodate the attachment of the hot for that light to a fuse.

That entire fuse bus comes up by removing two screws from the front of the platform and lifting the platform while pulling it toward the front of the car. with a little tug the frontmost part of the bus will lift straight up allowing for work in the lower cavity of the fuse box.

The ground connection could also be done in a connector, C153 which is mounted vertical on the diagnostic plug housing, connector pin #3. Fishing the wire through the fuse box would be the tough part of that exercise. I wouldn't trust another path due to the possibility that a faulty ground would trigger the ALDA and limit boost.

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

At least the warning light also informs you of a ground fault Do you really need such a thing? Don't you know your car well enough to wonder, "Dang, she's particularly peppy today... " In a previous life I added warning lights to a turbo gasser for fuel cut (its version of overboost protection) and knock sensor frequency.

Sixto
83 300SD