To be honest, I don't know what this is for on my 1991 300D. It was disconnected for I don’t know how long.

Could have been since august and a few thousand miles.

Now the reason why I ask “should I be concerned” is because the car runs and drives beautifully more or less all the time. It idles at the right speed, starts well, has boost (I believe), etc.

What is that connection for? Should some operation be affected?

Car runs so nicely.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)