I recently bought an 83' 300 SD. I am currently trying to get the engine to shut off like its intended too. Over the past week or so it has gotten worse and worse. I use to shut off about 20 seconds after the key was turned off and now can go for about 3 minutes and die painfully.

Here's what I have done so far.
1. Tested vacuum on the shut off valve. Works great after about 5 inches of vacuum.
2. Watched the vacuum pressure coming from the hard line hose to the shut off valve. Instantly pops up to 16 inches of vacuum after the key is turned to the off position.
3. Used a 'T', 3 way, connector and watched the system to see what happens. Vacuum slowly builds up from 0 to 5 inches of vacuum slowly. Been getting worse.
4. Replaced the rubber hose to the shut off valve and I have ordered extra 3 way and 4 way vacuum junctions to replace them all but do not have them yet.

What could be the cause of this? The system appears to work well when tested separately but when put together the vacuum slowly builds.

The other side effect of this as far as I can tell is that the vents in the car slowly change from the defroster to the main vents. When I disconnect the shut off valve they work perfect. Door locks work great too. Every system I have tested (doorlocks, transmission, EGR) works well.

Is there a good place to find out how to test the vacuum system, i.e. - disconnect this house with engine on and watch vacuum and see what it does?

My last question is how the heck do i remove the switch panel to get behind the radio and to the ACC system? Mine has no round turn knobs, only switches. I will try and include the picture of it at the bottom of this post.

Sounds like the shut off switch on the ignition my be causing problems.

If you used a mity vac on the shut off valve, the engine should shut down after only 1 or 2 pump. If you have to pump more than that your shut off valve it bad.

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If there is proper vacuum going to the shutoff valve, it is entirely possible that your shutoff valve is bad. I was a total newbie with vacuum related issues with my 300sd and Im glad to say its really more simple than you may think

here is a good pictorial to help you with replacement.

shutoff valve replacement

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1985 300SD- 212K and running

Open ashtray and remove insert. Take two phillips screws out. By wiggling remove ashtray frame. With the ash tray assembly gone you can now see two plastic tabs under each side of the Becker. Push and hold down these tabs and wiggle the radio out. Slide CCU pannel down to expose.

Here is a pictorial also

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15 VW Passat TDI
00 E420
98 E300 DT
97 E420 Donor Car - NEED PARTS? PM ME!
97 S500
97 E300D
86 Holden Jackaroo Turbo D
86 300SDL
(o\|/o)

The shut off valve goes off when the mightyvac has two pumps for sure and sometimes one. That leads me to believe the shut off valve is good. At the same time the vacuum coming from the ignition switch seems fine when tested separately too. If I do have a vacuum leak with the ignition switch where would I look? None of the vacuum diagrams I have found show one for the ignition switch. Is it connected directly to the key switch or does it go over to the ACC area?

Forgot to add, thanks for the radio removal. If I need to remove the cover plate above the radio where the fan controls and other switches are how do I do that? I tried prying it off from the bottom but the switches seemed to stick and the top felt like it was screwed in somewhere. Is there a special way to remove the switches and get to some screws behind it?

There are two lines that go through the firewall to the ignition switch. "t" into those lines and see if it holds a vacuum. Although somewhat rare the switch valves do go bad.

Slide the switch pannel down - it has tabs on the top.

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BENZ THERE DONE THAThttp://www.peachparts.com/shopforum/...c/progress.gif
15 VW Passat TDI
00 E420
98 E300 DT
97 E420 Donor Car - NEED PARTS? PM ME!
97 S500
97 E300D
86 Holden Jackaroo Turbo D
86 300SDL
(o\|/o)

Ok quick update. I did quite a bit of testing on the vacuum system. I found that there is a leak in the ACC system. I guess when I did my original testing I did not think to change the controls on the ACC with the engine running. Anyway here is what I found.

Numbered according to the picture.

I tested by pulling the rubber adapter off the electrical switches and checking to see if each one would hold vacuum.

1. Bad - also missing from the top of the sticker on the picture, color= red/white
2. Bad - Red/Yellow hose.
3. Good ??? takes a few pumps and then holds.
4. Good - same issue as 3 - Green hose
5. Bad - didn't hold vacuum without dropping above 15 inch Hg
6. Good - Green and Blue

Does anyone have a clue as to where these hoses go and what would be wrong exactly. Should I be worried about the slow buildup lines or is that a side effect of the another part of the system with a leak.

I figure with my luck that the dash will need to be removed. My car slowly changes vents from the defroster to the cabin vents. I found an 83 300D diagram for an ACC2? diagram, but the colors dont match what I believe I saw. Anyone else have a diagram for an '83 300SD?

Thanks.

Attached Thumbnails

 

bump.

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My first Mercedes
1983 300SD - 136K

1982 Toyota Supra - 160K currently for sale

figure out where your vacuum reservoir is and see if it will hold a vacuum, don't pull it's line and check that way, use the existing line without messing with it and see what that gives you. no vacuum reservoir, slug performance on most if not all of the vacuum using parts. the slow building vacuum leads me to think that the tank is taking a longer than normal time to build vacuum but the system ideally would already have residual vacuum form the last time your vacuum pump ran.. that's ideal for a mercedes vacuum system, almost too ideal