I found a guy selling his 240d ac compressor. I want to put it on my 300d. Is it compatible??

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'90 300SE --- 173k miles --- (Odometer just stopped working!)

'85 Cali 300D --- 193500 miles --- (Second one) Sold Nov '10

'82 300TDT --- 236,xxx miles --- (My first one) Sold July '09

what year 240d compressor and what year 300d?... and what shapes are they?

As long as the donor compressor is an R4 from a 616/617 it will work.

There are two types used on the W123. One is the York piston compressor used up till 1979. The other is the Delco R4 used from 1980 on. The York looks like a big aluminum brick and the R4 is a round cylinder. If the other guy's looks like yours, then it will fit.