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Old 03-08-2012, 10:59 AM   #31
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Originally Posted by csvt2060 View Post
This is rediculous. A/C compressors have always cycled so the evap doesn't freeze over. Even when it's 90 degrees out.
How cold would your AC low pressure cycle tubing (evap) have to be in order to cause an ice buildup with 90 degree air blowing over it? Freezing tends to happen when the fan itself fails. Which is again, why I suggest that it's more of a safety protocol.

In any case, we're not getting anywhere. At this point, even if I'm dead wrong, it doesn't change the fact that turning the AC on and off every 15 seconds while you're driving isn't going to do it any favors. Call and ask a few shop foremen if they think that's a good idea or bad, if you don't want to hear it from me. It's your car, and I don't care enough to argue it beyond this.
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Old 03-08-2012, 12:43 PM   #32
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Holy smokes Altis, you are so wrong about A/C its not even funny.

There are some A/C systems that continuously run, but those are systems that use a variable displacement compressor. Most compressors used are fixed displacement and are in fact cycled on/off through normal use.

If the A/C compressor didn't cycle on/off, then why would Ford have an A/C system called the "CCOT" or "Clutch cycling orifice tube" setup? Since the orifice tube is a fixed restriction, they need to cycle the A/C compressor on and off so the evaporator outlet doesn't freeze up.

The 2012 Focus uses a TXV system. The TXV is what creates the difference between the high and low side and it opens or closes to allow more or less refrigerant flow through the evaporator. (again, to keep the evap outlet from freezing up). When the temps get too low, it will cut the compressor off until things return to an acceptable point to turn the compressor back on.

Yes, evaporators can freeze up even in 90 degree weather. Place your hand on the evaporator outlet next time your A/C is running. it gets damn cold if the system is operating correctly.

And what the hell is an A/C radiator fan? Are you talking about the engine fan in the front of the car that draws air through the condensor.....or are you talking about the blower motor in the dash? There is no other fan.

Here's a tidbit from the service manual for you to ponder....

"The evaporator discharge air temperature sensor is an input to the BCM and is relayed to the PCM over the HS-CAN. The evaporator discharge air temperature sensor prevents icing of the evaporator core by measuring the temperature of the airflow immediately after the evaporator core. An accurate evaporator temperature is critical for compressor engagement. The PCM uses the temperature measurement to regulate the on and off time of the A/C compressor to maintain the evaporator temperature within an acceptable temperature range.

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage."
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Old 03-08-2012, 12:55 PM   #33
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Lots of good discussion here. I want to clear something up; I didn't ever say I was pushing the AC button "every 15 seconds", it's more like half a dozen times during a 15 to 20 minute drive. When the car gets uncomfortably warm, and the fan isn't enough, I press the AC button during deceleration hopefully taking advantage of the DFSO to get some "free" cold air. If things are really bad I will turn it on at cruising speeds as well. What I want to avoid is having it on while accelerating. I always heard that traditional AC systems cycled on and off as part of their normal function so I aSSumed pressing the AC button a few times during a normal trip wouldn't harm anything. I actually thought it might be beneficial because I too have heard of the "cycling to prevent freezing" issue. I figured the AC system is cycling a lot more than I would ever be pressing the button.

Any more opinions on whether or not I am abusing my AC system since we have more participants in this thread?
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Old 03-08-2012, 12:58 PM   #34
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Originally Posted by Visioned View Post
Well, so far I've found that its pretty intermittent on when it happens. For instance, I let off the gas at 30mph and it goes into loop but the next time it doesn't. On one occasion it stayed in loop from 40mph down to 5mph. I should be able to test it above city speeds this weekend but so far its nothing spot on.
Very interesting! I know there are several variables the system takes into account before DFSO activates. Is there any chance you hit the breaks on the one occasion at 30mph where it didn't come on? Does it seem to wait 3 seconds to come on after you let off the gas every time?

I would love to know more details about it going from 40 to 5mph!! That is amazing. I always heard it shut off around 25-30mph.

Thank you so much!
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Old 03-08-2012, 01:29 PM   #35
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Originally Posted by Rat Fink View Post
Holy smokes Altis, you are so wrong about A/C its not even funny.

There are some A/C systems that continuously run, but those are systems that use a variable displacement compressor. Most compressors used are fixed displacement and are in fact cycled on/off through normal use.

If the A/C compressor didn't cycle on/off, then why would Ford have an A/C system called the "CCOT" or "Clutch cycling orifice tube" setup? Since the orifice tube is a fixed restriction, they need to cycle the A/C compressor on and off so the evaporator outlet doesn't freeze up.

The 2012 Focus uses a TXV system. The TXV is what creates the difference between the high and low side and it opens or closes to allow more or less refrigerant flow through the evaporator. (again, to keep the evap outlet from freezing up). When the temps get too low, it will cut the compressor off until things return to an acceptable point to turn the compressor back on.

Yes, evaporators can freeze up even in 90 degree weather. Place your hand on the evaporator outlet next time your A/C is running. it gets damn cold if the system is operating correctly.

And what the hell is an A/C radiator fan? Are you talking about the engine fan in the front of the car that draws air through the condensor.....or are you talking about the blower motor in the dash? There is no other fan.

Here's a tidbit from the service manual for you to ponder....

"The evaporator discharge air temperature sensor is an input to the BCM and is relayed to the PCM over the HS-CAN. The evaporator discharge air temperature sensor prevents icing of the evaporator core by measuring the temperature of the airflow immediately after the evaporator core. An accurate evaporator temperature is critical for compressor engagement. The PCM uses the temperature measurement to regulate the on and off time of the A/C compressor to maintain the evaporator temperature within an acceptable temperature range.

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage."
A TXV control valve means the AC doesn't have to be shut off in order to modulate the coolant.

You're right that this is a fixed displacement compressor, which means its either on or off, but that has nothing to do with the fact that it can, and does, run for extended periods without cycling on or off.

The conditions that generally cause the evap to freeze are a fault in the system, most notably the fan that pumps air into the cabin, and thus over the evap... Or certain rare conditions like having the AC on cold, leaving the car off for a short time, then starting it back up... This can cause it, but the system detects it and acts accordingly.

The compressor doesn't need to be constantly cycled to prevent ice buildup. It only deals with it when it needs to. The things you mention are protective elements that are used as needed, but it does not prove that the compressor needs to be continuously cycled between on and off. Usually minor adjustments in the pressure valve is all the adjustment it requires.

The AC has its own radiator, with a fan. Look into how AC units work and you'll see that the high pressure freon must be cooled in order for it to be colder than ambient temperature in the low pressure (evap) circuit. When you're moving, the fan is usually off, but when you stop, you can often hear the fan kick in. The engine fan operates in a similar manner.

OP, it wasn't clear how often you were doing it when I first cautioned... My point was simply that overdoing it isn't good for the compressor. Doing it here and there is unlikely to have any effect while you own the car. I just don't recommend doing it every time you slow down.

I apologize as I did not mean for this thread to get hijacked for a small word of advice that one can consider for themselves.
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Old 03-08-2012, 01:38 PM   #36
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Quote:
Originally Posted by GoShockers View Post
Very interesting! I know there are several variables the system takes into account before DFSO activates. Is there any chance you hit the breaks on the one occasion at 30mph where it didn't come on? Does it seem to wait 3 seconds to come on after you let off the gas every time?

I would love to know more details about it going from 40 to 5mph!! That is amazing. I always heard it shut off around 25-30mph.

Thank you so much!
This is strictly coasting, I've never seen it when I was applying brakes. You are right about the 3-5 seconds after letting off. It also doesn't seem to care if you are in neutral or not. If I knew which variables activate it, I could give better results.
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Old 03-08-2012, 01:42 PM   #37
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Originally Posted by Altis View Post
I suppose you believe that the engineers write it, perhaps? There are technical writing specialists, who work in conjunction with lawyers, to ensure the whole manual is completely fool-proof and lawsuit-proof.

The engineers tell the team how the things work, the design limits, etc... and the technical writers and lawyers go to town with it.

Can you imagine the lawsuits if something described in the manual wasn't clear to the bone and someone got hurt? We're talking about people who sue because there isn't a sign on a steaming cup of coffee indicating it's hot...
As an Engineer, I disagree. Technical writers and lawyers may make edits for clarity or legal purposes, but do not change the technical content.

Besides, you can tell a lawyer had nothing to do with writing that excerpt from the service manual because it is written in plain english and it is much too brief of an explanation.
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Old 03-08-2012, 01:56 PM   #38
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As an Engineer, I disagree. Technical writers and lawyers may make edits for clarity or legal purposes, but do not change the technical content.

Besides, you can tell a lawyer had nothing to do with writing that excerpt from the service manual because it is written in plain english and it is much too brief of an explanation.
In my engineering experience, anything written for the general population and consumer (ie. not other engineers), lawyers and technical writers are always consulted.

The fact that you say it's so brief only supports this, because an engineer has no problem explaining things... But lawyers don't want to say too much ;)

The manual would be 100x more useful if the engineers were in charge of the content. In any case, the lawyers have the end say of what the final content contains.
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Old 03-08-2012, 01:58 PM   #39
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Quote:
Originally Posted by GoShockers View Post
Lots of good discussion here. I want to clear something up; I didn't ever say I was pushing the AC button "every 15 seconds", it's more like half a dozen times during a 15 to 20 minute drive. When the car gets uncomfortably warm, and the fan isn't enough, I press the AC button during deceleration hopefully taking advantage of the DFSO to get some "free" cold air. If things are really bad I will turn it on at cruising speeds as well. What I want to avoid is having it on while accelerating. I always heard that traditional AC systems cycled on and off as part of their normal function so I aSSumed pressing the AC button a few times during a normal trip wouldn't harm anything. I actually thought it might be beneficial because I too have heard of the "cycling to prevent freezing" issue. I figured the AC system is cycling a lot more than I would ever be pressing the button.

Any more opinions on whether or not I am abusing my AC system since we have more participants in this thread?
Yes, as the technicians have pointed out, some level of A/C compressor cycling is normal, and has been in many systems for decades. The level of use you are describing shouldn't have an appreciable effect on the life of the clutch. This is just my opinion, you should probably consult a lawyer though.

I would like to give you props for thinking outside the box on this one. What you are doing is very similar to what some hybrids do to assist in recharging batteries, using the vehicle momentum that would otherwise be wasted during braking to accomplish a task. In your case of course to run your A/C compressor.

Hybrid powertrains are optimized to accomplish this task. I am not sure how efficient the DCT is with engine braking, but the main objective would be to stay in gear while slowing down as long as possible. Any neutral condition (i.e. between downshifts or during coasting while slowing to a stop) will result in the engine itself compensating for the extra compressor load, which begins to negate the advantages. This might be better accomplished by downshifting with the toggle switch. Honestly I haven't had my car long enough to really play with the DCT.
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Old 03-08-2012, 02:13 PM   #40
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Quote:
Originally Posted by Altis View Post
In my engineering experience, anything written for the general population and consumer (ie. not other engineers), lawyers and technical writers are always consulted.
I agree, yes.

Quote:
In any case, the lawyers have the end say of what the final content contains.
Not for technical content. To do so would be practicing outside of their profession, much the same as if I were to provide legal content or advice. Illegal and in itself would invite lawsuits.

/done. I have engineering work to do.
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