1994 300d won't shut off. I checked all elements which all appear good. (I have a hand vacuum pump that works properly.) All other parts of the vacuum system, like door locks, work OK. Here is what I have done:

1. verified that vacuum pump on engine is working OK. Good vacuum is present, when engine running, at vacuum line offtakes from main line to brake booster to vacuum system plastic tubes.

2. shutoff gizmo at rear of injection pump. With engine running or not running I cannot draw a vacuum at the shutoff diaphragm with hand pump. But: with engine running I removed the existing line and rand a bypass line from the shutodd gizmo to one of the oftakes at the main line to the booster, mentioned above, where I knew vacuum was present. It shut off the engine immediately. That led me to suspect thaty the shutoff gizmo was working OK.

3. valve at ignition switch with brown and brown/blue plastic lines. I replaced this in light of apparent successful test of gizmo, above. New valve failed to solve problem. The brown/ brown/blue tubes were verified to be correct installed. I even tried reversing them as experiment. No success either way. I removed valve, still connected to tubes, from ignition and started engine. The valve is operated by a push rod in it which is driven by a cam in the ignition switch. With engine running I pushed the rod with my finger, but engine did not turn off. I concluded problem was not this valve and that I had wasted my money on this new valve. Therefore, I had to conclude the problem was also not a cam failure in the ignition switch.

4. vacuum tubes/lines and connectors: With hand vacuum pump I verified that all plastic tubes and rubber connectors were intact and holding vacuum. I concluded I did not have a leak in this part of the system.

5. Conclusion: I am driven to conclude that, despite my success in activating the shutoff diaphragm by putting a tube directly between the shutoff diaphragm and the main vacuum line, the problem must nevertheless be the shutoff diaphragm at the injection pump. This is for two reasons: (a) everything else works OK, and (b) I could not get a vacuum using the hand pump with engint running and not running even though I could stop the engine as described above. What am I missing? Where else should I look?

Advice and insights are solicited. Thanks.

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Douglas
1959 M-B 220S cabriolet