DodgeIntrepid.Net Forums banner
61 - 73 of 73 Posts

·
Registered
Joined
·
4,393 Posts
I ran some more tests. It's only about 62 outside. I just backed the car out of the garage and let it idle. I set the ATC to 84. The compressor ran 99% of the time. It did stop a couple of times for about 5 seconds. The metal tube coming out of the compressor, I'm guessing the high pressure side, was very cold. To the point where water was condensing on it and dripping off. I've seen condensation on it, but never dripping off.

I switched the AC to 70 and the air temp at the vents was extremely cold. At 84, the heat was anemic. I have a feeling that the evaporator sensor is not shutting down the compressor. In the ATC diagnostic mode, there are no codes.
The evap probe only turns the compressor off when it senses the evap core falling into a freezing temp spread. Blend door would have more to do with your heat sucking more than anything.

Compressor can run all the time if A. Pressure never gets to high, B. Evap Core does not freeze.

Comes to your cooling temps, I think you are over analyzing them and expecting new parts to react same as old ones which is not true.

Also here is the 1G fan operation temps for future reference.

Font Material property Number Circle Document
 

·
Premium Member
Joined
·
292 Posts
Discussion Starter · #62 · (Edited)
I don't understand why the compressor would run all the time when a cold evaporator is not needed, as in cool outside temps or turning on heat. It just seems very economically inefficient.

I've seen many car's AC compressors cycle on and off, while the AC is operating, what makes them do this? The pressure hitting an upper threshold?

I was a software engineer/systems analyst, before retiring. I spent years analyzing problems, systems, programs, etc. I guess it's in my nature. It can be a curse at times.
 

·
Premium Member
Joined
·
17,892 Posts
The expansion valve mechanically modulates the cooling effect (no software involved) to control temperature with less need for compressor duty cycling - more like a linear than on-off control. Saves wear and tear on compressor clutch and reduces sudden engine load changes which can annoy people even if they don't realize it's happening.

The a.c. also runs when it's cold out, which is counterintuitive and does hurt efficiency some with compressor load. That's to dehumifiy the air to keep the windshield and windows from fogging up - a safety consideration. So in one sense, the heat and cooling are fighting each other. The "wasted" heat is "free", but of course the additional use of the a.c. is not, but overall, a good trade-off due to lowered humidity and being able to see where you're going so you don't kill yourself and passengers/family. :) (see last paragraph)

You used to hear/read that it was also to run the compressor some even in the winter to prevent the compressor bearings from brinelling due to the compressor main shaft sitting at the exact same position for long periods of time with the normal road vibrations and impacts constantly occurring. Not sure that's an issue with modern compressor construction - you don't see that mentioned much anymore.

Reminds me of one of comedian Gary Mule Deer's jokes: "When I die, I want to go peacefully like my grandfather did, in his sleep - not screaming and crying like all the passengers in his car."
 

·
Premium Member
Joined
·
292 Posts
Discussion Starter · #64 ·
If the expansion valve regulates the amount of liquid refrigerant getting to the evaporator, what controls the expansion valve? The temperature control on the ATC or the slider on a manual system?

How does the expansion valve affect the operation of the compressor? If for example, it restricts the opening to the evaporator, wouldn't that create more pressure between the valve and the compressor. The compressor runs at the same speed, so the pressure would keep building, while the compressor is running. Wouldn't there be a pressure sensitive device to turn off the compressor, while the rest of the refrigerant flows into the evaporator, reducing the pressure? When the pressure becomes low enough, the compressor would turn back on? Conversely, the larger the valve opening, it would allow more refrigerant to flow into the evaporator, thus taking longer for the pressure to build, causing the compressor to run longer?
 

·
Registered
Joined
·
821 Posts
the high pressure varies by ambient temps, by the expansion valve, and how fast/slow freon is flowing.
it would be a problem if the system was overfilled, saturating the condenser with liquid freon.
your compressor also varies according to engine spped.
sensors will cut out the compressor upon too high or low pressure readings.
so, no worries.
the system is set to self adjust and prevent these problems.
 

·
Premium Member
Joined
·
17,892 Posts
The type of expansion valve in our cars is called an H-block expansion valve. The evap's inlet and outlet pipes travel close to each other thru a single block of aluminum. There is a pressurized gas-tube-type temperature sensor at the outlet pipe (built into the aluminum block). The hotter the sensor gas is, the more it forces the inlet orifice (also in the block) to open for higher refrigerant flow which causes the evap outlet to be cooler. The design assures that refrigerant coming out of the evap is gas and not liquid (which would damage the compressor).

The regulation of cabin air temperature is accomplished by measuring the cabin air temperature (sensor built into the ATC head unit) and cycling the compressor clutch to close the loop on the setpoint programmed into the head unit.

The expansion valve does not have a direct effect on compressor cycling. If it is doing its job correctly and the refrigerant charge level is within acceptable bounds, the system will be cooling the cabin air quicker to reach the setpoint for less compressor on time.

The pressure transducer (built into the high pressure hose) is monitored by the PCM. The PCM will disable the compressor clutch if pressure gets too low due to low refrigerant charge. The pressure transducer output is also used by the PCM to control the radiator fan operation.

The radiator fans running greatly lower the high-side pressure. If the fans were to not be working, pressure would go very high, in which case I'm pretty sure the PCM would disable the compressor clutch to prevent damage to the compressor (and of course no cooling of the cabin air).
 

·
Premium Member
Joined
·
292 Posts
Discussion Starter · #67 ·
Thanks peva. That's very informative. Once you described the expansion valve, I knew exactly what it looked like on the car. The shop replaced that along with the evaporator, about 10 years ago.

It was 67 in my garage, according to the ambient air sensor. I had the car halfway out of the garage, idling, while warming up, in the shade. I set the ATC to full auto at 78 degrees.

As I watched the compressor run, I could feel the condenser get hot, which makes sense. At that point, the compressor clutch would disengage and in a couple of seconds the high speed fan would turn on. As soon as the condenser no longer felt hot, in about 30 seconds, the compressor clutch would engage and the high speed fan would turn off. It repeated this cycle over and over. The air coming out of the vents was warm as expected. The clutch remained engaged about 95% of the time, only turning off when the condenser felt hot.

It seems that the necessary events are happening at the right time to keep the evaporator cold. I'm wondering if you could explain what the system is actually doing? I know the basics that it is absorbing heat from the interior and expelling it outside through the condenser. It's just a hard concept for me to understand why the compressor would run 95% of the time, when I want heat. I could understand it running occasionally, but why does it need to run about 95% of the time?

I switched the ATC to 72 and the process was the same as it was when it was set to 78 degrees. The air at the vents was immediately ice cold. The compressor clutch was engaged about 95% of the time, same as when the ATC was set at 78 degrees.

I had the car out this evening. The ambient temp was in the low 60's. It felt cold to me outside. Inside the car was a comfortable 74 degrees. That's what I had the ATC set on. While I was waiting for someone and with the car idling, I opened the hood. The compressor was on 95% of the time. It was repeating the same process as it did this morning. As soon as I switched the ATC to 70, the air at the vents was immediately ice cold.

I'm sorry to keep asking about this. I learn more about how the system works with each response. It just seems odd to me that the compressor would always run the same amount of time, regardless of the ATC setting and keep the evaporator very cold. It seems like, if I know I only want heat, to just manually turn off the AC with the switch on the ATC module. I use AC the majority of the year. I just hate to wear out the compressor, by it's super cooling the evaporator all the time and not need it.

Thanks.
 

·
Premium Member
Joined
·
17,892 Posts
You're welcome.

If your a.c. ever goes out during moderately cold weather and your windows and windshield suddenly fog up to where you can't see because of the moisture in your breath, you will appreciate the fact that the PCM is programmed to run the a.c. even when outside is cold to dehumidify the cabin air. It happened to me one time when the a.c. was on the fritz. It took me a while to get home because I had to keep pulling off the road to wipe the fog off the glass and then rapidly drive as far as I could before they fogged up again. I do believe I also had a pinhole in the heater core at the time that was adding to the cabin moisture load.
 

·
Premium Member
Joined
·
17,892 Posts
Just to confuse things more, technically the a/c system does not cool the air. It just removes the heat from it and displace it outside via the condenser.
Removing heat from something = cooling it

C'mon man!! 🙂
 

·
Registered
Joined
·
4,393 Posts
Removing heat from something = cooling it

C'mon man!! 🙂
Yes, but most people think it of as chilling the air directly, vs removing the heat from it. . Kinda like, which way is the thermodynamics is moving vs overall action.

Same output, different weird methods of doing it.


I just wanted to be difficult.
 

·
Premium Member
Joined
·
17,892 Posts
OK. I'll play. 😁

It's cooling the air using a heat exchanger interface (evaporator with internal latent heat of phase change thing going on to suck heat out of - i.e., cooling - the air). When you say "chillng the air directly", what practical method did you have in mind for that - other than by using a heat exchanger interface? Swamp cooler?

You're maybe thinking that people envision a piston device cooling the air by pulling a vacuum on the air without refrigerant - hence "direct chilling"?

Continuing the "being difficult" theme, I just take issue with "...technically the a/c system does not cool the air". I'm of the opinion that the a.c. system does a decent job of doing exactly that. 🤷‍♂️
 

·
Registered
Joined
·
4,393 Posts
OK. I'll play. 😁

It's cooling the air using a heat exchanger interface (evaporator with internal latent heat of phase change thing going on to suck heat out of - i.e., cooling - the air). When you say "chillng the air directly", what practical method did you have in mind for that - other than by using a heat exchanger interface? Swamp cooler?

You're maybe thinking that people envision a piston device cooling the air by pulling a vacuum on the air without refrigerant - hence "direct chilling"?

Continuing the "being difficult" theme, I just take issue with "...technically the a/c system does not cool the air". I'm of the opinion that the a.c. system does a decent job of doing exactly that. 🤷‍♂️

More of splitting hairs on the terminology used when describing the actual thermo deals going on. By means it does not cool the air, it removes the heat. And absence of heat is cold air. So on a technical note it does not chill the air. Just produces it (cold air) via said heat removal.

Kinda like standing in front of a fan outside, on a 100 degree day. Fan is still blowing 100 degree air at you. But because it is helping remove heat from your skin (skin acts like the evap-core) it feels like cooler air. I think that's the right analogy. I think.

I thought it was interesting when reading on how Heat pump systems work in both directions, and how when heating they draw heat out of the air and displace it in the house. Do not create heat, just displaces it somewhere.

This was learned when I was reading up on how to properly clean the coils outside and the benefits from it.







At least that is my understanding. Of course in cars the heat transfer is strictly one way deal.
 
61 - 73 of 73 Posts
Top