This vacuum switch goes from intake to the fuel pump
I don't have power to it. What is it and do I need it working?

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If that is the one between the intake and the ALDA, you can bypass it if you'd like. It's the overload prevention mechanism. The boost pushes the diaphragm in the ALDA to give more fuel, if that valve leaks or blocks the boost you'll have a slow car. Most of the 617 turbos I've had that valve goes in the trash and I run a straight hose between the intake and ALDA. I'm not sure how it actually worked, but if there was too much load I think it would stop allowing more fuel somehow. They were a bit cautious about overboost etc on these originally.

Agree with identification but disagree with the solution. If this is on the firewall, I believe that is the switchover valve which often becomes clogged and needs to be replaced. Be careful to note which rubber hose goes on which nipple. On my SD I reversed them and the vehicle would hardly go over 30 MPH. Looked at the manual and corrected the hoses and immediately got original power.

If I remember correctly the new switchover valve (original was 1981) was upside down from the original which is why I got the hoses misplaced.

It is there for a reason; trust the MB engineers.

Have been without this for over 20 yrs. Four different 300sd cars.

Yes, there is purpose to the valve.


If you are going for a original build, and want to have redundant protection as designed, it would be a good idea to restore all of these systems to 100% functionality. If a overboost condition occurs this will cut fuel, and maybe some other condition. Redundancy and safety systems like these are why these cars run forever.


However, like most things on these cars, this is over the top. I don't recall the value of the overboost switch, but all of these systems are in place for maximum longevity, and have a lot of headroom. The first 617 car I removed this from was because I wanted to run more fuel and boost. That car lasted several hundred thousand miles with no issue well over the stock 13psi, though I kept an eye on EGTs with a gauge in the cabin and kept it under 800F.


It depends on how you use the car and your ultimate goals for it. The valve is a safety mechanism and is redundant to the working system, if disabling it just keep that in the back of your mind and pay attention to changes in running condition. There are quite a few of these running around out there without safety systems functioning. If you are trying to restore the car and/or plan to sell it, then yeah sure put it back in place and pressure check the trigger switch etc. If this is a beater that you are running into the ground I would not worry about it.

It's my daily driver and I baby it. It is never going to activate the way I drive...

Update. Saw a video and the guy opened the Alda and Removed a copper lookin disk and said it really improved turbo power by allowing more fuel. I'm afraid I'll break it rem9ving it with my luck.

I do have an electric valve in line that does not have power leads to it. I'll remove this valve and see what happens. Let me know if I'm flirting with disaster...

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Whats a good way to test this thing?

No one knows how to test this electric valve???

I took valve off and when I drove one morning it smoked big time out tail pipe. I bet you don't want the added fuel when engine is cold. The switch may open after its at operating temps.