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Discussion Starter #1
I have just bought a 1986 W124 300D . I also own a W123 300D of which I am conversant with the engine workings.
When I turn off the the W124 it continues running , although at a reduced idle . The Fuel, temp, oil gauges go back to zero. No gnition lights come on. I then found if I turned the heater/A/C fan on first click, halfway between 0 and 1 the engine stops. At 1 it will still continue to run.
I am confused, as the switch I presume is electrical and the cutoff is vacuum .
Keeping the heater switch in this intermediate position does not worry me . Engine turns off with the key.
I am just curious as to why.
 

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W124 Moderator
86 190E 2.3L 16V, 2 95 320TE's, 02 S500
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Sit tight, we have some very well versed diesel owners on the forum and they will chime in.

If you would, fill out some info in your profile while you wait.

Jayare

Sent from my XT1710-02 using Tapatalk
 

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Discussion Starter #4
I realize that the shut off valve is vacuum. It has me puzzled as to why the car shuts off instantly with the key when fan switch is in the intermediate position between Zero and One. Any other position and the engine idles at reduced revs .
 

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'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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Sorry, I'm not familiar with the W124's systems. But, since it seems like the W124 has vacuum actuators for its ventilation system like my W202, then perhaps a leak on the ventilation system overcomes the air suction rate of the vacuum pump. Maybe this diagram will help? It appears that the diesel shut-off channel to the ignition barrel must be connected to the "unused" (in this diagram) end of the check valve 36. On my W123, I have a similar situation with the central locking, which also gets its vacuum from the engine (the ventilation on that is all-mechanical).
 

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Discussion Starter #6
Sorry, I'm not familiar with the W124's systems. But, since it seems like the W124 has vacuum actuators for its ventilation system like my W202, then perhaps a leak on the ventilation system overcomes the air suction rate of the vacuum pump. Maybe this diagram will help? It appears that the diesel shut-off channel to the ignition barrel must be connected to the "unused" (in this diagram) end of the check valve 36. On my W123, I have a similar situation with the central locking, which also gets its vacuum from the engine (the ventilation on that is all-mechanical).
Thank You for your input. Having only owned the car two days, I have yet to delve into its innards. If I find the reason, I will post here. 😀
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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Dollars to donuts the vacuum diaphragm inside the injection pump has gone bad.

Also check the vacuum tubing going to the shutoff valve, particularly at the T where it joins the master vacuum line. The T tends to get very brittle with age.

Lastly, don't discount the possiblility of a worn engine vacuum pump although this would also be reflected by a different brake pedal feel.
 

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'86 W123 200, OM617 non-turbo, bastard 5-speed; '95 W202 C250 Diesel, OM605 non-turbo, 5-spd man
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P.S. I couldn’t open the link.
Just search for OM603 shut-off valve on Google

Dollars to donuts the vacuum diaphragm inside the injection pump has gone bad.
Not saying "no", but wouldn't that make the engine simply never shut down?

Lastly, don't discount the possiblility of a worn engine vacuum pump although this would also be reflected by a different brake pedal feel.
This is partly the problem on my W123 as well, but is aggravated by the central locking leaking badly.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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One more thing, on diesel cars the ignition barrel mechanism also has 2 vacuum connections. With age, the rubber connections tends to get weak and let air in. When the system can no longer hold vacuum properly, it will result in odd behavior such as the engine not shutting down properly among other things.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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Sorry, I'm not familiar with the W124's systems. But, since it seems like the W124 has vacuum actuators for its ventilation system like my W202, then perhaps a leak on the ventilation system overcomes the air suction rate of the vacuum pump. Maybe this diagram will help? It appears that the diesel shut-off channel to the ignition barrel must be connected to the "unused" (in this diagram) end of the check valve 36. On my W123, I have a similar situation with the central locking, which also gets its vacuum from the engine (the ventilation on that is all-mechanical).
This is where I would place my bet. The rubber diaphragms in the vacuum actuator pods on the W124 HVAC systems fail and cause a vacuum leak when that function is activated. The "halfway"between 0 and 1 switch position may be putting the system in limbo and causing activation of the leaking pod(s).

Try adjusting the airflow direction (vents, floor, mix, re-circ, etc.) and see if the behavior changes.
 
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