How often does your AC compressor cycles?

[luckyforrest]

luckyforrest - I went out of town this weekend in The Blueberry. In both directions I blasted AC. When I got home I let the car idle in the driveway long enough to pop the hood and confirm the following:

At idle, my AC compressor does not cycle. It just keeps on going. That is with it on Recirc, and max fan and me visually watching the compressor clutch. I adjusted it to other fan speeds and it just kept going. I had my son manually push the AC button and I watched the fan clutch and could certainly see it stop. Then start again when he pushed it again to turn it on.

One thing I noted that I didn't expect, is that cruising along at 70 mph, it *sounded like the AC system was turning off then back on somewhat slowly. However, I couldn't feel any difference in the inside cooling, no difference in the running of the car (I can feel a small jolt when the actual compressor starts / stops). So I'm guessing that what I was hearing at 70 mph, was the cooling fan cycling. I.E. with it doing 70 mph the ECU found the coolant temp low enough to turn off the fan. Once turned off, the load on the engine + the load due to the AC running increased the coolant temp (regardless of the air through the radiator at 70 mph) to the point the fan was commanded back on. It was 90°+ and the car was running 70 mph cruise full AC, right at the car's GVWR. So, it was working ... and STILL got over 40 mpg! Helluva machine for $11,400!

Hi 7milesout, thank you for the test. Is the AC in your Blueberry automatic or manual? Mine is manual and as far as I know the compressor cycles is normal in manual AC system to avoid the evaporator from freezing, but I am not sure of the automatic AC. Have you checked the lower side pressure while compressor engaging? I have tested the cycles of my Mirage AC yesterday (ambient temp was around 28 degrees Celsius, humidity: ~46%, not much sunshine ), car was idling and the vent air temp. was ~8 degrees Celsius, not as good as i expected. Interestingly, I also monitored the cycles very carefully: the AC engaged for 7s and paused for 10s, before that the cycle was 9-10 seconds on and 4-5 seconds off in a warmer day. And I have also noticed that the vent air temp was actually lower on a warmer day when driving in city. (warmer days like 32 degrees, the AC can reach 5-6 degrees while driving in city, and it only reached ~8 degrees in a cooler day say 25-27 degrees. Both were recir one and fan on low-medium ) I know my r134a is a bit low (~28 PSI at ambient temp around 21 degrees celsius). Sadly, no R134A is allowed to sell in Canada since 2020 and I am going to the other side of Niagara falls this weekend to grab some cans of pure r134a and top off to ~40-45 PSI (I have the Supertech R134A with sealer and oil from Walmart while in Atlanta in June, but I prefer to use pure r134a at the time being). I like my tiny G4 so far, reliable, cheap on gas and easy to maintain. I had a solo trip to Atlanta and back, that was about 2000 miles, and the average fuel economy was 4.5 L/100KM (AC was on all the time, average speed: 90-100 KM/h) after carefully calculation. Cannot complain.

[7milesout]

Hi 7milesout, thank you for the test. Is the AC in your Blueberry automatic or manual? Mine is manual and as far as I know the compressor cycles is normal in manual AC system to avoid the evaporator from freezing

According to what I learned, a) the compressor is not supposed to cycle, and b) it is CERTAINLY NOT cycling to prevent it from freezing. Yes I have the Auto crap, but my opinion is the only difference is in the control interface. The control of the system (meaning sensors and associated activation / de-activation) and the physics of the system are the same. Now ... when I learned what I learned, I was at Hyundai and was a TFT leader on "Design and Controls" as well as the "AC System Performance." So, what I learned could have been specific to their systems, but I doubt it. My Lexus AC system has some ability to lower the compressor load (improve mpg) in some way that I don't understand. I know it is not cycling or I would be feeling that. Cycling puts a lot of wear and tear on the AC compressor clutch. And, mine doesn't cycle, so I believe that is proof that these systems in the Mirage should not be cycling.

Have you checked the lower side pressure while compressor engaging?

I have not. I resist connecting to it because even losing that tiny amount of refrigerant in doing so, gets my OCD hackles up.

When the system has the appropriate level of refrigerant, the temperature of the refrigerant entering the evaporator is ~29°F to 30°F (from memory). That is just under freezing, so maybe -1°C. In the process of entering the evaporator it immediately begins to warm as it immediately begins pulling heat from the air surrounding and flowing across the evaporator. That is what prevents it from freezing. When the refrigerant level is (at some level of) low, it will actually be colder than those temperatures. Don't ask me why, it a function of how refrigerant works and I'm no chemical engineer. And that (and other factors like high levels of humidity) can lead to the evaporator freezing up. However, when the refrigerant is low, the cycling of the compressor also slows the rate that too cold refrigerant is entering the evaporator, giving the evaporator time to warm things up and prevent freezing.

[luckyforrest]

According to what I learned, a) the compressor is not supposed to cycle, and b) it is CERTAINLY NOT cycling to prevent it from freezing. Yes I have the Auto crap, but my opinion is the only difference is in the control interface. The control of the system (meaning sensors and associated activation / de-activation) and the physics of the system are the same. Now ... when I learned what I learned, I was at Hyundai and was a TFT leader on "Design and Controls" as well as the "AC System Performance." So, what I learned could have been specific to their systems, but I doubt it. My Lexus AC system has some ability to lower the compressor load (improve mpg) in some way that I don't understand. I know it is not cycling or I would be feeling that. Cycling puts a lot of wear and tear on the AC compressor clutch. And, mine doesn't cycle, so I believe that is proof that these systems in the Mirage should not be cycling.

I have not. I resist connecting to it because even losing that tiny amount of refrigerant in doing so, gets my OCD hackles up.

When the system has the appropriate level of refrigerant, the temperature of the refrigerant entering the evaporator is ~29°F to 30°F (from memory). That is just under freezing, so maybe -1°C. In the process of entering the evaporator it immediately begins to warm as it immediately begins pulling heat from the air surrounding and flowing across the evaporator. That is what prevents it from freezing. When the refrigerant level is (at some level of) low, it will actually be colder than those temperatures. Don't ask me why, it a function of how refrigerant works and I'm no chemical engineer. And that (and other factors like high levels of humidity) can lead to the evaporator freezing up. However, when the refrigerant is low, the cycling of the compressor also slows the rate that too cold refrigerant is entering the evaporator, giving the evaporator time to warm things up and prevent freezing.

So the cycle of my AC compressor is caused by the low refrigerant level I guess. I recall that in the first day/year I bought it, the compressor cycled, but I never checked the pressure of low side before. I totally agree that connecting manifold gauge to AC will lose a bit of refrigerant probably 1-2 OZ, but I used the gauge on Supertech can which is much shorter, I am not worried about this. I noticed that the low pressure pipe under the hood is very cold even in a hot day, especially the area entering the cabin, I can see dews on it everytime, and the H pressure pipe to the cabin is warm (maybe 30-40 degrees celsius) and very hot (probably around 80-90 degrees celsius) entering the condenser, which is normal. Anyway, I will grab some pure R134A and top it off and check how the AC performs.

[Fummins]

So the cycle of my AC compressor is caused by the low refrigerant level I guess. I recall that in the first day/year I bought it, the compressor cycled, but I never checked the pressure of low side before. I totally agree that connecting manifold gauge to AC will lose a bit of refrigerant probably 1-2 OZ, but I used the gauge on Supertech can which is much shorter, I am not worried about this. I noticed that the low pressure pipe under the hood is very cold even in a hot day, especially the area entering the cabin, I can see dews on it everytime, and the H pressure pipe to the cabin is warm (maybe 30-40 degrees celsius) and very hot (probably around 80-90 degrees celsius) entering the condenser, which is normal. Anyway, I will grab some pure R134A and top it off and check how the AC performs.

https://carsupercare.com/how-often-s...le-on-and-off/

I object to the fix it until it's broken theory. I haven't seen an ac compressor run constantly before. But we don't get a ton of +30c weather here either. In my car and every other vehicle I've ever owned or looked paid attention to(with working ac), the compressor clutch kick on and off as needed. It's normal(for this climate). If it cycles on and off every second then you're likely low on refrigerant.

I forgot to preface that I didn't read any of this thread other than a couple post's.

[luckyforrest]

https://carsupercare.com/how-often-s...le-on-and-off/

I object to the fix it until it's broken theory. I haven't seen an ac compressor run constantly before. But we don't get a ton of +30c weather here either. In my car and every other vehicle I've ever owned or looked paid attention to(with working ac), the compressor clutch kick on and off as needed. It's normal(for this climate). If it cycles on and off every second then you're likely low on refrigerant.

I forgot to preface that I didn't read any of this thread other than a couple post's.

Hi Fummins, yes, I agree. It makes sense to me that compressor cycles as needed, but short cycles is very likely caused by low refrigerant. I will top off the r134a and check how frequent it cycles.

[7milesout]

The only way to top it off is to evacuate it, then recharge it by weight. Pressures just give you an idea, but are not a good way to gauge if your system has the specified amount, too much or too little.

However, I don't think you'll hurt it much by adding some amount that lessens the cycling.

[luckyforrest]

The only way to top it off is to evacuate it, then recharge it by weight. Pressures just give you an idea, but are not a good way to gauge if your system has the specified amount, too much or too little.

However, I don't think you'll hurt it much by adding some amount that lessens the cycling.

I agree. But my refrigerant is just a bit low, may not worth the effort to evacuate and full recharge. I will check the static presure before topping off.

[7milesout]

I agree. But my refrigerant is just a bit low, may not worth the effort to evacuate and full recharge.

That's what I always thought too. Regarding my Dodge Ram. But when I finally got off my butt and did it, man what a difference. But I have all the equipment and tools to do it right.

[klroger]

If It's low, you have a leak...Find it & Fix it & Carry on... That's What I would do instead of always putting cans of stuff into it with no idea of how much refrigerant is already in the system... When it doesn't work, do you have too much, or not enough??? You put more in thinking it needs more & Hydro lock the compressor??? It may be better to Evacuate & measure what you put back in... But Just My Thoughts, But I'm not very smart about stuff,,,...

[7milesout]

That's what I always thought too. Regarding my Dodge Ram. But when I finally got off my butt and did it, man what a difference. But I have all the equipment and tools to do it right.

I drove the ole Ram out last night just to clean it up and let it stretch its legs. The A/C system was reset last spring to the exact amount of refrigerant for which the system is designed. Yesterday it cooled extremely well. The truck is 23 years old and the AC system has only been touched by me. When I pulled up to where I was going, I let it idle, popped the hood, and the compressor never stopped.

LF - Are you certain you're not considering the radiator fan operation for the AC compressor? Or are you actually watching the clutch on the compressor when you time the on/off intervals. I don't know if the compressor works simultaneously with the AC clutch or not.