I have just started using my heat in my 1985 300TD and noticed that my auxiliary fan was on when my heat was on. I checked to see if the compressor clutch was also engaged and, sure enough, it was (this is when the temp. dial is set to its hottest setting and the defrost is not activated). The second to last owner, a mercedes mechanic, mentioned the other day that he had rewired the AC system so that the auxiliary fan is always on (I thought it would have been anyway). I'm wondering if he inadvertantly wired things such that the compressor and auxiliary fan are activated whenever any climate control option is activated.

In the meantime I have disconnected the compressor switch at the drier but this is obviously not a long term solution. Anyone have a wiring diagram for the full climate control system?

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1985 300TD "Ted", 220,000 miles

ok. ok. Should have looked at shop manual first. It appears that the compressor comes on at 2 degrees C if I have the center button of the control unit on. If I just want heat I hit the "EC" button, apparently.

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1985 300TD "Ted", 220,000 miles

Hopefully, "always on" means only when the A/C is selected.

That's it.

Keep in mind that in auto mode (center button) the compressor coming on will dry the air entering the cabin. Dry air means better windshield defogging. The way this system should be set 95% of the time is "center center center" meaning center button on horizontal buttons, center button on fan and the temp wheel rotated to center of its travel. The system will raise or lower the interior temp depending on how far it is off from these center settings. The fan will blow as hard as it needs too to get the interior comfortable then lower its speed automatically. When I owned my 81 CD I never varied more than 10 degrees on the temp wheel all year long. I used ECON in the warm evenings if the sunroof was open or a window was down. My feeling is that if you use the system as if it were a manual system, you risk the chance of wearing out the vacuum valves and switches early on.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

I'll give that a try. Thanks for the advice.

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1985 300TD "Ted", 220,000 miles