Does the A/c work on that car? Reason I ask is that when I blew a fuse (10?) once on that year 190D, I lost both my a/c and the tach. I think there is a relay for the a/c that receives input from the tach. Can't remember the reason for that but it made sense when I researched it. Sorry can't help much more. Wish I still had that car...

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Kevin

1978 300D
1979 240D