The A/C system in the 123 is a pretty easy system to fix and diagnoisis. You do need some specialized tools but they are not that expensive.

http://dieselgiant.com/repairyourac.htm

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1981 300D 147k
1998 VW Jetta Tdi 320k
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1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair