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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