Hi and thanks for helping.
I have a 1984 300SD with an oil leak coming from the oil pressure switch.
Being new to working on MB, I am unfamiliar with how the oil pressure switch works.
I took the wire off the switch and oil comes out of the pin on the switch.
Is this normal????

The wire has a connector that when it is on the switch it seems loose and does not seal tightly.
That is where the leak is from.

Do I need a new switch or a new wire harness?

Please help, thanks.

needs a new switch.

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83 300SD Dark Silver Dark brown inside

Can please explain the connection?
is the oil pressure switch out put only an electric signal and NOT oil pressure?

Thanks.

Yes. On the 126 SD's the sender puts out an electric current (or I think more accurately, provides a variably resistant ground) not oil.

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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

THanks, that is how other oil sensors I've seen worked.
Now why this diesel one has a hole in the pin is an interesting question.
There must be a diaphragm that gives way and MB thought this would be a good way to identify that the sensor was bad.
(How about looking at the gauge?)

Time to order up a new switch.