Here's a question for y'all that is confusing me. Why does the ac compressor run when the defroster is selected with just heat? Seems like since the temperature selector is all the way to the hot side there would be no need for the ac compressor to run... what are yalls thoughts on this.
Chris

 

So you get dehumidified air to let the defroster work quicker/better.

Steve

 

In addition to the correct info mentioned, that is also a nifty way for the A/C to keep in shape all year. When it's not used in wineter, would you run it full blast to keep the seals in good condition? Probably not.

I just got the clutch fixed on my Ranger A/C....no more being fogged out on rainy, muggy December nights in the boonies. That's nice for driving visions....

 

Whats a rainy muggy december night?
Only part I understood was the boonies...

 

Ahh, ok... So the air is ran through the ac to dehumidify it and then it is ran through the heater to achieve the temperature wanted. The dehumidified air can then absorb the humidity off the glass or out of the cab. I should have known that...just finished taking thermodynamics 2 and one of the questions on the final involved a similar setup. Thanks for making me realize how little knowledge i am retaining from college, lol.
Chris

 

Does the a/c compressor run at all times when set to defrost or does it have some sort of cycling control?

 

Bigted -

On my Explorer it seems to run all the time in defrost; the winter temps are pretty mild where I live, so I don't know if the system has a low-temp cut-off switch for defrost.

Steve

 

Mine seems to be intermittent. So I was wondering if the controls had some way of turning it on and turning it off from time to time

 

Some AC systems use line pressures to cycle the compressor. Some use a bypass. Not sure what is in Ford trucks as I just got mine and haven't studied it yet. I suspect the cycling type as it is a bit better on MPG that way.

 

Mine has a low pressure switch that drops the compressor out but I know that is for safety [for the compressor] reasons only. I can see why it is good to have it run during defrost but at all times!!?? Hardly cost effective.

 

If your compressor is running all the time it could be low on refridgerant.

Most of the systems I've seen have a capillary tube going into the evaporator core to sense temperature and when it gets cold enough it cuts out the compressor. If the compressor ran all the time the evaporator will turn into a solid block of ice and no air will flow through it. Since the block of ice has much less surface area than the evaporator, cooling/dehumidifying efficiency goes to near zero.

 

Okay. Lets rephrase the question with this in mind. In real life I am an HVAC tech. Not automotive . I work on commercial and residential stuff. Assume that I am hip to all of the nuances of low charge, overcharge, dirty evap, dirty cond, etc. With that said the question still stands. When in defrost is the compressor supposed to run at all times or is there something that limits or varies it's run time while set on defrost? This question has vexed me for years and I have never gotten an answer that made any sense. Also remember that I am talking about a 1986 F-150 and not a 2005 Caddy which probably has controls to cycle the compressor as needed to dehumidify based on an algorithym taken from the physcometric charts. This is just an old truck and the answer is either-yes, on defrost the compressor is "called" at all times or no,the compressor is told to run at some times or the compressor is told to run at all times and the suspected low ambient temperature that one would expect at times when defrost is needed would cause the compressor to cycle on and off due to low pressure conditions induced by the cold weather that demanded defrost use in the first place. Any automotive A/C pros out there who would like to weigh in on this? And let me also add that if anyone has any questions about commercial or residential A/C or heating issues please feel free to ask me.

 

Not a pro and, as stated, not familiar with my '93 F-250 AC yet, but on lower end vehicles, and older, the AC compressor is called to be on full time with the defrost and it cycles based on the normal factors that cycle auto AC compressors. This is based on my reviews of wiring diagrams. The switches in the feed to the clutch coil are the AC switch and the def switch in the control head and those switches, usually pressure I believe, on the gas system.

The "all the time on" AC I was refering to earlier is 70's vintage GM where the compressor is run all the time and some kind of gas/liquid valving (POC, POR?) regulates the AC evaporator temperature. This system then did not run with the defrost on the cars I had then (one I still have, '75 Vette, one owner, me).

 

If you are willing to discuss res/comm HVAC put a post on the garage and workshop forum. I'll bet they can keep you busy.

 

So if I understand you right you are saying that in my case[1986 F-150] on a call for defrost the compressor will run until such time that the low pressure switch [which is a safety] drops it out? Wow. I am surely not saying that you are wrong. You are certainly well read. But if that was Ford's intention- Who the hell was watching the store the day that idea got approved? LOL