I changed the ignition on my 81 300sd now the vents only work on defrost and my heater will turn on randomly when the wheel is set on cold... my switch used to take a few tries to get it to switch from defrost to center but always worked... now i get nothing... it does blow on the feet too... is this a vacuum issue or is this the main hvac switch going out...

I'd look into vacuum issues first.

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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
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13