I am curious about anyones thoughts on the AC compressor coming on and off with heat selected.
I unplugged the compressor last winter and hooked it back up this season for AC.
Today I run heat because it has been chilly here today and the compressor comes on.
Is there a reason I need the compressor to kick on and off while using heat, doesnt make sense to me.
Also, anyone have an idea on a minor circuit modification that would delete this circuit funtion,,ie- been there done that kind of thing?
my hvac has climate control as well., 87 5.0 w/full digi cluster / climate control
It comes on so the inside of your car doesn't fog up-A lot of cars do that. My dad used to unplug his a/c compressor on his Yukon in the wintertime.
I thought it came on with the "heat" so that the compressor will get lubed throughout the winter so come summertime it still works, just a guess...
Usually the compressor will run in all positions except "Off", "Vent" and "Heat" to prevent window fogging like kendoo130 said. I believe there is a thermal switch to prevent the compresor from running below freezng temperatures. Running in "Heat" would be a malfunction on a manual control, but I'm not that familiar with the ATC.
I believe it'll run in a climate control system in any setting except for vent or off...
It runs in defrost and a/c modes.
Defrost to dry the air hitting the windshield and remove the moisture(fog).
It should not come on in the vent(non a/c position)or floor position.
On the atc systems it will come on depending on temp in most positions,just hit the a/c button and shut it off.
I have to do this in the tc most of the time.
The thing that stops it running when it's cold isn't a thermal switch, it's the pressure switch. It's the nature of refrigerant. It "boils" at slightly below freezing, and A/C relies on this "boiling" to work.
To vastly simplify how A/C works (don't ask me why I'm typing all this out - part is because I know Scott likes technical stuff, and part is to reassure myself that I still remember this stuff after being out of the trade for a decade): The compressor compresses the refrigerant into liquid form. This compressing creates heat (just like a turbo or supercharger does to air). It's not so much that the compressor creates heat, it's that the refrigerant contains a certain amount of heat energy by volume, and by compressing it the volume is greatly reduced but the heat energy stays the same (more energy per unit of refrigerant, which makes it hotter). The hot liquid refrigerant goes into the condenser, where that heat is removed by the engine fan. The cooled refrigerant then passes from the compressor through a restriction, which is actually the dividing line between the high and low pressure sides. The moment the refrigerant passes through the restriction it starts to "boil" (which is simply a term for "rapidly evapourate"). Now the opposite of heating occurs: The volume of refrigerant is suddenly increased back to what it was before the compressor, but the heat energy is reduced because heat was removed by the condenser (less heat energy per unit of refrigerant, which makes it cooler). It's next step is, as you can guess, the evapourator in the dash. The blower fan forces hot air through the evapourator, which transfers heat out of the air and back into the refrigerant (IE air gets cold, evapourator and the refrigerant it contains gets warmer). The refrigerant then passes into the receiver/dryer (that can on the firewall) so that the rest of the liquified refrigerant can boil off (convert to gas). This last "boiling off" is necessary because if the compresser sucks liquid refrigerant in it will hydraulic lock. The gaseous refrigerant then passes back into the compressor, where the proccess starts all over again.
Remember, this is vastly simplified. There are lots of scientific principles at play here, most importantly phase change dynamics. It takes a lot of energy to make a substance change phase (from liquid to gas), and a similar amount of energy is released to change back (from gas to liquid). In the case of A/C the energy to change from liquid to gas is provided by the air that passes over the evapourator (that cool air you feel is actually warm air that has had its heat energy removed). The energy released when the process is reversed is the heat that comes off the condenser and is disspated by the engine fan. When you feel heat, energy is being released from the hot object into you. When you feel cold, energy is being transferred from you into the cold object you're touching.
A "heat pump" for a house is basically a large, reversible air conditioner. In summer it transfers heat energy from your house into the air outside, and in winter it transfers heat energy from outside air into your house (no matter how cold it is outside, there is always heat energy in the air).
Anyway, when it's too cold outside, there is no "boiling". The refrigerant simply stays in liquid form. This leads to low system pressure, which keeps the system off.
Both reasons for running the compressor when not in A/C mode have been addressed here already: Keeping the system's seals lubed (and the oil evenly distributed), and drying the air. Engineers figure that when you select "defrost" you want to clear the windshield, and the quickest way to do that is with dry air. Passing warm air over a cold evapourator dries it because the cold evapourator attracts moisture (in much the same way a cold beer on a hot day does). The water you see running out from under the car on a hot day is that moisture.
if my head hurts now, id hate to see the long version of what has been said. Thats wayyyyyy over my head and I like chemistry to.
Yep I have the Electronic Climate Control in mine and it does the same thing with the heat on......the few times I actually use the heat in early spring. I couldn't say what it does when it's really cold as the car is put away in the winter. ;)
I guess my point was that it steals power if it actually doesnt "need" to cut the compressor on or off.
the white coug has been missing AC for many years, heat would eventually remove the fog. I never really paid attention to how quickly the HVAC removes the fog inside the car.
To answer one of your original questions, yes, it could be easily defeated. Simply find the A/C enable wire on the back of your HVAC panel (I don't have my EVTM handy, but you'd find the wire easily enough with yours) and put a switch on it. I would agree it's not really needed and drawing off unneccesary power (and burning unneccesary fuel). Cars without A/C clear their windshields just fine...
Read page 227 in your EVTM under manual overrides.
I know what you mean about the A/C compressor running when it's not needed. The basic idea behind having it running is that with ECC you set the temp and then the car uses a combination of heat and A/C to keep the interior at the temp you want. Then you never have to roll down the windows. I don't know why every car with ECC had power windows if you never have to use them ;)
gonna go piddle some more in the garage. I think i see a simple solution without a swtch,,,,,,i think.
Too easy,,
Nice one softtouch:D
I have solve the problem on my t-bird by replacing relay in ventilator control module. The old one have welded contacts- the AC system were switch on all time during run. The current in the circuit is quite large for this small relay . Something like this http://www.brownbear.ru/goods/8711.html
It is necessary to find strong relay, rated 20+A. I have take from old security system board.