I had a new mechanic "fix" my a/c system. Before when it was 90s weather, everything was fine. But now that I need heat, I have a problem.

In a nutshell, now, whenever I press any of the buttons on the heater/air con controls, the A/C ALWAYS comes on. If I rotate the dial to heat (red numbers), then the heat AND A/C comes on simulataneously - Heat out the bottom and A/C out the front. I know, pretty crazy. If I rotate the dial to A/C (blue numbers), the heat turns off.

The only time the A/C does not come on is when I press the "E/C." button, which is supposed to be just outside air.

How could be wrong here?

Vacuum problem? Wiring problem?


Anything that happens when I got the car back , is the auxillary fans never spin at high speed any more , just low speed.

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1990 Mercedes 300 SEL (320,000 miles)
1994 Mercedes E420 (140,000 miles)

Sounds like the vacuum actuators are not directing the air through the proper pathways. There are a number of components that are involved with the vacuum circuits that can cause the problem.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Any guesses were I can start.

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1990 Mercedes 300 SEL (320,000 miles)
1994 Mercedes E420 (140,000 miles)