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Old 03-12-2006, 09:24 AM
mdimbler mdimbler is offline
Registered User
Location: Lehi, UT
Join Date: May 2005
Posts: 66
Mein Auto: 1997 540i
I learned the following from a very knowledgeable fellow in another BMW forum:

Climate control is affected by the outside temp reading. If the sensor is not functioning properly, it can affect (aoming other things) the climate control functions.

There is much confusion about how to set the temp setting, and the use of the blending dial in the centre dash vent.

You should set the "temp setting" to what you "normally" like - keeping in mind that you should probably set it a bit higher in summer than you do in winter - it really is not necessar to try and cool the car down to 70F. It is more comfortable at about 75 as long as you get rid of the humidity.

The blending dial in the center vent has a unique purpose to allow you to add some warmed air to the otherwise incoming air that is either strictly fresh outside air, or chiled by the A/C.

This is most useful in winter to get fresh air to the head area, but slightly warmed for comfort. I keep mine at about 1 red dot in the winter.

BUT - this dial can be a source of problems in the summer. Many people, thinking they want full cooling, will put it to 3 blue dots. This is OK for initial cooling of a hot car , but starts to present problems as the car reaches the desired temp as per your settings.

The system constantly chills all incoming air to the maximum (between 1 & 3C depending on outdoor temp) and then uses air passed over the heater to reheat the output so that it will get the car to the setting you selected. But, the only path for air from the heater portion into the fresh air portion is via the blending flap - which is in the main unit but controlled manually via a cable from that center dial with the dots - it opens a path between the 2 sections.

If you turn the dial to 3 blue dots you have closed that flap and there is no way for the system to reheat the air to the dash vents. In this case when the cabin temp starts to get below your setting the system has only one way to apply heat and that is to briefly open the floor vents and dump heat out there.

There is a whole lot more technical info about this, but to keep your life simple you should just leave the blending dial at 1-2 blue dots in the hot weather.
3 blues will give you a cold forehead and occasional hot feet (just in case that is what turns you on)

Also regarding control of interior temp are 2 additional items.

- keep the small vent area on the left of the climate control free of obstruction. That is where the air is pulled in across the cabin temp sensor. Blocking that vent, or placing a hot or cold drink in the cup holder will affect the reading of the cabin temp. The system will then compensate output in the other direction.

- do not direct the centre dash vents downward as that will pump cool (cold) air directly into the area where it is picked up by the same cabin temp sensor and the system will add more heat trying to get to the setting you have selected. The centre dash vents should be directed more upward as in over your head. The door vent could be more directed toward your lap if required, but even that can affect the temp sensor so adjust the flow at the dial accordingly.

The vent on the rear console is a "fresh air" or "A/C air" vent - same as the ones in the dash.

Later models have the "blending dial" on the right - same as the one in the center of the front dash vent. This allows you to "add" some warm air to the fresh air.

Earlier models did not have the blending dial in the rear, so you had to close that vent (turn the left dial) if it was too cold.

Idea for both these vent systems is to deliver "fresh air" to the upper part of the cabin (head area) to help with alertness.

Heat is delivered via the floor and upper dash vents - including to the rear seat by the large ducts under the front seats.

Hope this helps.
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