Are you saying it works on defrost but nothing else? Or that it only initially works on defrost and afterwards will not work at all, including defrost?

I believe I have read on this board that a failure in the auxilliary pump of some kind will cause the pump to draw too much current and burn out the board in the control unit. Some people put a fuse in the auxilliary pump circuit to stop this problem I believe. Perhaps this is your issue.

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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