My 300SD is not shutting off with the key. Intermittently this would happen during the summer. A key cycle on-off usually caused the engine to stop when that happened. Now I can hear rpm change but the engine does not shut off. A quickie check at the shut off device indicated vacuum but how much is unknown. Aftermarket key and tumbler was installed before I purchased the car but I do not know if the entire switch or just the tumbler was replaced. I will be inspecting / diagnosing tonight, outside in the cold.

I have tried to use the shop's service software to find these answers but it is distinctly lacking in vintage MB information.

1) Is there a minimum or typical value I should be testing against at the shutoff device? With the key "off" what reading should I see?

2) Is there an acceptable or typical leakage rate for the shut off diaphragm, should I find it's leaking?

Thanks.

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When life gives you lemmings... make lemmingade.

Test the shut off valve itself with a mighty vac, or just by connecting a length of vacuum hose and sucking. Testing the line tells you whether or not the valve behind the ignition switch is good. A good shut off valve needs less than 5" of vacuum to shut down the engine. I'd leave it in there as long as it works. When the diaphragm is really gone the vac line will start to suck oil out of the injection pump.

If you turn the ignition key back to the run position after the engine shuts down, it will zero the vacuum out on that circuit-- assuming that all of the check valves are functioning properly. That may help to extend the life of a weak diaphragm for a while.

__________________
'79 240D

Put a piece of rubber vacuum hose on the shut off. Start the engine. Three strong sucks should be enough to shut it off. A mouthful of oil proves the diaphragm has failed.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

#1. Attach a Mityvac Pump direct to the shutoff vacuum box.
#2. Pump it as low as it will go.
#3. It should hold with no loss for 24 hours.
#4. FYI: I pass most units that hold with no loss for 15 minutes.

Odds are 90% that your actual issue is:
#A. Central locking vacuum element diaphragm failure.
http://www.peachparts.com/shopforum/tech-help/158363-door-lock-issues-link-thread.html#post1214456

#B. Climate control vacuum element diaphragm failure.
http://www.peachparts.com/shopforum/diy-links-parts-category/142408-climate-control.html

Vacuum Diagrams
http://www.peachparts.com/shopforum/diesel-discussion/105297-vacuum-diagrams.html


http://www.peachparts.com/shopforum/diesel-discussion/229331-vacuum-pump-rebuild-thread.html#post1927867

http://www.peachparts.com/shopforum/diesel-discussion/163455-hens-tooth-1985-300d-vacuum-filter-dark-black.html#post1265296

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__________________
ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

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Thanks for the replies.

I left work nearly an hour late and forgot several items I intended to bring home including my vacuum pump / test kit. So I proceeded to do what I could at home without it.

First clue that something was funny was during the visual inspection. The engine was running with the key in the "off" position while I looked for problems. I happened to bump a vacuum line and the engine shut off. Restarting the engine and carefully moving lines back and forth slightly for several minutes resulted in no direct correlation between line position and correct function, but the engine did shut off one more time. Removing the vacuum line to the diaphragm revealed no oil and looking at the vacuum switch under the dash showed the same.

The next test was a direct connection from the middle vacuum Tee on the main line to the diaphragm. The engine did not shut off. With no gauge I was unable to measure vacuum at the tee. With no pump I was unable to apply a known vacuum signal to the diaphragm. At the Tee, however, it felt like the signal was very weak. But the power brakes seemed to work fine with speedy recovery from fully depleted and plenty of reserve in the pedal. Ultimately the vacuum signal at the center tee was insufficient to move the diaphragm and hose position seemed to change the signal. There may have been contamination in the Tee because at some points there was no vacuum available there while the vacuum signal near the brake booster was consistently strong.

I ran a small pick into the Tee carefully to check for restrictions and found "something" blocking the path. Check valve?? The fast answer was to break out the cordless drill and a 1/16" drill bit and remove the restriction. Now the car shuts off very quickly.

So ultimately, I need to learn about what I've done. Is that Tee part of a check valve, or is there a simple restriction in the fitting? Am I going to regret this move down the road? So far the vacuum diagrams I've found require some study to determine if the Tee connected to the vacuum switch on the ign cylinder is the correct Tee to us. I believe I have a vacuum line from the '82 126 that I junked and I'll probably install that at a later date.

As an aside, I noticed the linkage for the vacuum vent valve to the transmission was disconnected due to a faulty / missing clip. I reconnected it using a metal carb linkage clip I had in my toolbox. I was worried that altering the vacuum Tee would cause shifting issues but a brief road test didn't indicate any major issues. Now it's off to locate more service data...

__________________
When life gives you lemmings... make lemmingade.