Last post on Feb 16, 2013 at 10:05 PM
You are in the BMW 5-Series
What is this discussion about?
BMW 5 Series, Sedan
#6441 of 12737 fjk57702
May 12, 2003 (6:52 pm)
I am sure that what you did was to select defrost, which most likely "turned the compressor on".
No, I turned on the A/C. In fact, given that the "Defroster" on both my 1999 328i and my 2002 530i was/is so aggressive, I have only used that mode a few times in the last four years. I find it much more comfortable to simply direct the airflow toward the glass (a mode that BMW thoughtfully includes that is separate and distinct (read mutually exclusive) from the defrost mode) and turn on the A/C.
May 13, 2003 (6:44 am)
Whatever. What you need to understand is that condensation forms on the windows when the dew point in the car is higher than the temperature of the glass. So to get the water off the glass and keep it off you must either warm up the glass or reduce the dew point in the car. (I am assuming that you understand what a dew point is)
If the temperature is below freezing outside and your A/C runs the compressor, your evaporator is going to have ice build up on it. I assume that your engine compartment heat is isolated to a large extent from the outside air inlet to the passenger compartment. This implies that even though the compressor may shut down, the ice will not melt. If the system goes into a recirculate mode, (with the passenger compartment warmed up), the ice could be melted, however, the drain hole will be below freezing and how will the water run out?
#6443 of 12737 fjk57702
May 13, 2003 (7:09 am)
I do in fact understand all of that, however, it seems that the point that you are missing is that the evaporator is only going to be below freezing only until the air circulating through it warms it up enough to bring it above freezing (regardless of whether you are in Recirc mode or not), and what little ice that forms (even at 100% humidity, zero degree air dosen't carry a whole lot of water) will eventually melt off. When it melts off, and most likely the drain hole will freeze shut, but so what, once again, we are not talking about a whole lot of water here, and a night in the 40 degree garage will take care of that.
The point I am trying to make is that it is both feasible and beneficial to allow the compressor to run when the OAT is below freezing, and fortunately, every A/C equipped car I have had for the last 20 years allows the compressor to engage below freezing.
#6444 of 12737 shipo and fjk57702
May 13, 2003 (7:09 am)
I have been reading your guys post with quite interest. I find things like this curious because for some stupid reason I wonder how things like this work. That being said, maybe there is an engineer who could better explain how the defogging of windows works in cars. I can't imagine that BMW has come up with a way to do it any different from any other car. Maybe I might check on the how things work site to see if something is out there.
May 13, 2003 (7:24 am)
What we really need is for someone with the BMW repair manual (if one is available) to check what it says about the A/C system. BMW cannot depend on owners putting their cars in a warm garage everynight.
May 13, 2003 (7:48 am)
I hear you on that one, but since my garage is detached, in the cold michigan winters, I am pretty sure the temperature gets below zero in the garage as well. That being said, I did read something on www.howstuffworks.com. They didn't have anything as specific as how a car defogger works, but they did have the principles of how a dehumidifier works. From that I have derived some theories on how it might work based on my minimal knowledge of cars and HVAC systems in them. I believe that it is possible to place the AC compressor away from the actual condensor coils. That being the case, the explanation stated that a home dehumidifier has a hot and cold coil. As the air is ran across the cold coil the moisture in the are is removed, then it is drawn across the warm coil to heat the aid back up to room temperature. If we can use this same principle in a car then it is possible that once a cars cold coils are cold that the car simply needs to run the in cabin air through these coils and then across the heating coils to bring the air back to in-car temperature if not warmer based on your temperature settings. If the car is not warmed up yet, I would assume that the air would either be cold, just like if you turn the heat on full blast when you first get into a car in the winter. That being said, we can all agree that since these coils will probably be cold and stay cold longer in the winter time without the use of the compressor, it can be possible that the compressor works far less in the winter trying to defog car windows than it does in the summer trying to cool a car down. Does this make sense to you guys? I am just grabbing at air here but I thought I would give it a try.
May 13, 2003 (8:31 am)
In a car's A/C system, the compressor runs off the belt attached to the "front" of the engine in general. The condensor coils are in front of the radiator. The evaporator (cold coils) are in the duct work that brings air into the car from outside, before the heater core. In my car (I think) all of the air comes in fresh unless it is in recirculate mode. So when the compressor is on, the cold coils will remove water from the air coming over the coils. Most people will warm the interior air up to the 60's F for comfort. The windows will be somewhere between the interior temperature and the outside temperature. So they will "fog" up if there is enough humidity inside the car. In general if the air inside the car has come from outside the car and no additional water is added, then no fog should form if the inside of the car is warmer than outside. However, wet people inside the car will add water to the air inside the car, causing the windows to fog.
#6448 of 12737 Comparison with LS
May 13, 2003 (10:56 am)
This may be heresy on this board, but the performance statistics on the Lincoln LS V8 with sport package seem pretty impressive. Does anyone have comments or reasons why the Lincoln is a bad purchase as compared to a 540i? Thanks!
May 13, 2003 (10:59 am)
I went to the Public Library (the University Library wasn't helpful) and found a Mitchel handbook on A/C repair/service for domesitic and imported cars. The bottom line is that there is an ambiant temperature switch that shuts off the compressor when outside air temperture drops to something like 32F (the range is 30-40 F). So, shipo, unless you can find a BMW manual to the contrary (which is possible), I think that below freezing the compressor is off.
#6450 of 12737 defogging a car
May 13, 2003 (11:01 am)
Pardon my ignorance, but what are the condensor coils for? I understand that the evaporator coils are to remove moisture out of the air and and make it colder, but if that occurs before the heater coils, which makes sense, then that will take moisture out of the car. If someone has wet clothes or brings moisture in the car in their clothing then the air must either be heated or recycled so that you are constantly placing dry are in the car to absorb the moisture.