Climate control works in foggy weather...
 

A 1987 300TD:

When it's not working: From a cold start, with the middle button selected, the fan speed on auto, and the temperature wheel anywhere but the extreme settings, the fan blows very softly through the dash and foot vents until the coolant temp hits about 50C, then the fan speed slowly ramps up to maximum air pressure and maximum heat.

The interior quickly gets too warm for comfort, so i have to cycle the system on and off.

The low fan button turns down the fan speed but the temperature is still at max, the high fan button makes no change from the auto setting, since the fan is already at max speed. If I set the temp control to MIN, it blows unheated air at a medium speed. The defrost button turns on max hot air through the windshield vents, even with the engine cold, as I believe it's supposed to.

I tried another CC control unit (which I presumed to be good) which produced similar results.

The part that drives me nuts is that sometimes, when it's wet and dreary out for a few days, the climate control will seem to work perfectly, heating up the interior and then automatically reducing the heat and fan speed to maintain a comfortable temperature. I'm pretty sure the car is doing this out of spite, so i know what I'm missing the other 99.5% of the time.

Because of the connection with wet weather, I'm guessing it's some connector under the hood that is not quite connecting, but I don't know where to start looking.

Any tips?

Also, the A/C system is dry, but I'm not concerned with that at the moment, because it's getting cold out. Not sure if that makes a difference.

There's a bunch of temp sensors that could be the culprit:

- cabin temp sensor = beside the front dome light
- outside temp sensor = on the passenger side of the blower case under the hood
- evaporator temp sensor = beside the accelerator pedal
- heater core temp sensor = behind the ACC controls

My guess is the outside temp sensor is most sensitive to ambient humidity.

Sixto
87 300D