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AC blows warm on long trips


Matthew Coe

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My AC will begin to blow warm air on trips of about 3 hours or longer. It starts out fine, perfectly cold air. After a few hours the driver side will feel warmer than passenger side. Eventually it will become warm, and the passenger side will also start to become warm to the point where there is no AC at all. The AC compressor is still on and cycling when this happens. Once the car sits for a few hours the AC works perfectly again. My thought is AC Evaporator freeze over. I checked the cabin air filter and cleaned it with no better results. I also just checked the evaporator drain and it is clear. Would low refrigerant cause this problem, or something else? I guess I need to check refrigerant pressure next. Any thoughts?

 

-Matt

2012 Edge SE

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Taken from my earlier post:



could be lots of things, but my 2011 Limited started acting odd on just the drivers side. I would have to put the temperature all the way to high and then bring it back down for it to react. Took it to the dealer 2 weeks ago. Their diagnosis on my invoice says " HVAC Control Module reprogrammed with latest calibration per TSB 11-8-7"



Seems to have fixed mine. Not sure if will apply to 2012



pdf.gif TSB 11-8-7_Unable to maintain Temp old.pdf


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Thanks, probably not my issue unfortuately. The SE has manual controls so I don't think it has an HVAC control module, although I could be wrong.

I'm also starting to rule out evaporator freeze over since the volume of air coming through the vents is unchanged only the temperature. The fact that the AC compressor is still cycling but I'm not getting cold air is strange to me. Maybe blend door actuators are failing after a few hours of use??

-Matt

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Do you see water dripping out onto the driveway after you come to a stop, having used your A/C? If not, then there is either a clog or the evaporator is freezing over from some other restriction (like a cabin air filter/screen). If it's bad enough, it will eventually throw water into the interior of the vehicle, and/or generate sloshing sounds. Potential for blower motor destruction in the process.

 

Do both fans run when you run MAX A/C on max airflow?

 

Has the system been checked for charge and leaks? Have you cleaned out the condenser that sits in front of the radiator (also contains the trans cooler)?

 

Yes, there IS a control module, it is called the EMTC instead of the EATC.

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Plenty of water drips, and I even checked the evaporator drain for debris and it is clear. I cleaned the cabin air filter (which is the screen type) and it didn't help at all. Both fans do run when MAX A/C is on...now whether or not they are running when the AC system has been running for over 3 hours I will need to check. That's something I haven't looked at when it is having the problem. It could be that they are cutting out for some reason after a long period of use.

 

Condenser is clean and actually only 2 years old as it was replaced after I hit a coyote on the interstate. I need to check the system refrigerant but I have a feeling it will be OK since the system works perfectly fine except when used longer than about 3 hours continuously.

Thanks for the input though!

-Matt

Edited by Matthew Coe
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Hmmm.

 

Possible that this is an "extreme" situation/scenario. Maybe it is by design, kinda like paper shredders that have thermal overload protection. They will time out for a while, maybe 15 minutes, before they can be used gain. The A/C can cut out by design during WOT acceleration, to give full power for acceleration.

 

Ok, the FSM states (note the statement in bold):

 

The Refrigerant Cycle

During stabilized conditions (A/C system shutdown), the refrigerant pressures are equal throughout the system. When the A/C compressor is in operation, it increases pressure on the refrigerant vapor, raising its temperature. The high-pressure and high-temperature vapor is then released into the top of the A/C condenser core.

The A/C condenser, being close to ambient temperature, causes the refrigerant vapor to condense into a liquid when heat is removed from the refrigerant by ambient air passing over the fins and tubing. The now liquid refrigerant, still at high pressure, exits from the bottom of the A/C condenser and enters the inlet side of the A/C receiver/drier. The receiver/drier is designed to remove moisture from the refrigerant.

The outlet of the receiver/drier is connected to the Thermostatic Expansion Valve (TXV). The TXV provides the orifice which is the restriction in the refrigerant system and separates the high and low pressure sides of the A/C system. As the liquid refrigerant passes across this restriction, its pressure and boiling point are reduced.

The liquid refrigerant is now at its lowest pressure and temperature. As it passes through the A/C evaporator, it absorbs heat from the airflow passing over the plate/fin sections of the A/C evaporator. This addition of heat causes the refrigerant to boil (convert to gas). The now cooler air can no longer support the same humidity level of the warmer air and this excess moisture condenses on the exterior of the evaporator coils and fins and drains outside the vehicle.

The refrigerant cycle is now repeated with the A/C compressor again increasing the pressure and temperature of the refrigerant.

A thermistor, which monitors the temperature of the air that has passed through the evaporator core, controls A/C clutch cycling. If the temperature of the evaporator core discharge air is low enough to cause the condensed water vapor to freeze, the A/C clutch is disengaged by the vehicle PCM.

The high-side line pressure is also monitored so that A/C compressor operation will be interrupted if the system pressure becomes too high or is determined to be too low (low charge condition).

The A/C compressor thermal protection switch will interrupt compressor operation if the compressor housing exceeds temperature limits.

The A/C compressor relief valve will open and vent refrigerant to relieve unusually high system pressure.

So possibly 1 or more conditions creating the scenario you have described.

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Yea, I'm leaning towards the Evaporator thermistor and re-evaluating possible evaporator freeze happening. It looks like that thermistor is a problem across a lot of different Ford models. It also seems like a complete pain in the ass to replace it. If done at Ford probably $$$$.

-Matt

Edited by Matthew Coe
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It's freezing up, plain and simple. When you travel at high speeds for long periods of time, the A/C is able to get a lot colder than it does sitting in a traffic jam. If Ford set the A/C system to where it's 100% impossible to freeze up, people would complain it's not cold enough. So under certain "ideal" conditions, it can get cold enough to freeze up the core when there is lots of humidity involved.

 

I've seen it happen on our 08 Taurus (once), and my parent's 05 Cadillac CTS (multiple times). It happens during long drives at high speeds on really humid days.

 

It took the Taurus about 3 hours to fully melt/drain the core after it was parked. The driveway had an area about 10'x10' square completely soaked from the melting ice.

 

You can trick the computer to go colder by adding resistance to that thermistor circuit (F150foum.com has details on how to do it), but I don't think you can go "the other way" so to speak and make it run warmer/shut off the compressor at a higher temp.

Edited by IWRBB
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  • 1 month later...

Low pressure is in normal range of about 25-30 or so psi, high pressure though is way high at about 320-350 psi (should be more towards the 220-250 range). Therefore it's either a stuck expansion valve or bad condenser. It works fine for about 2 hours or so but then the high pressure switch cuts off the compressor until the condenser cools. Therefore letting the car sit a few hours and cool down brings down the pressure enough for it to start working again. It is not evaporator freeze up. I should know more by tomorrow, its at the shop and they are replacing the expansion valve.

In 2014 they replaced the condenser after I hit a coyote on the interstate doing 70 mph. Of course the condenser took the brunt of the hit. They should have replaced the expansion valve when replacing the condenser but apparently they didn't. I'm betting that is the entire cause of all this.

 

-Matt

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