I have a 500 SEC, 1984, European model. When the car is at idle the heat and a/c, including it seems fan speeds, work. As soon as I get up to highway speeds, I get neither heat or a/c. The fans blow but no moderation of air temperature. That years dash has a fuel efficiency (a vacuum ?) gauge on it, and it seems to me that when the needle goes up, the heat/a/c stops.

Good chance you have vacuum leaks in the servos that move mode doors. This shouldn't directly relate to temp control but the euro modle didn't mix the temp control well in the manual package.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician