I'm trying to diagnose the HVAC system in W116 chassis 1980 300SD. No heat in cabin, A/C compressor only comes on when DEF button is pressed. I have CD manual but most of the climate control stuff is greek to me...any sources for HVAC testing procedures? I have basic tools, like a hand vac pump and a multimeter.

Thanks,

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1978 300SD 149K miles silver-blue "Maxine" (mine!)
1979 300SD 318K miles red "Helga" (SOLD)
1980 300SD 321K miles yellow "Linda" (younger daughter's)
1980 300SD 148K miles blue (SOLD)
1980 300SD 153K miles green(parts car, SOLD)
Yes, I have a W116 fetish :-)