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I think you have a similar problem to mine. At highway speeds is when it happens. And it is not blazing hot? Right?
Your evaporator is freezing up - as this happens the ice surrounding it prevents the flow of air over the evaporator coils and the result is warm air coming out of the registers. If I turn off the AC for about ten minutes and let unit melt the cold air comes back. This does not occur in stop and go driving.
I combat it by either running the fan at a higher than normal setting to increase air flow over the evaporator and then turning off when it gets too cold, OR using the air control to allow warmer outside air to enter into unit at highway speeds to prevent freezing up.
Newer cars have more sophisticated sensors and controls to prevent this but some older vehicles are subject to this anomaly.
Since it works fine under normal conditions I have not messed around with the freon level in system. If you release a tad of refrigerant the low pressure switch will shut off the compressor sooner and help minimize ice buildup. Good luck.
Sounds like the repair place was truthful. About all you can change yourself would be the low pressure switch and then if unsuccessful, the high pressure switch. The A/C Relay could be overheating and kicking out, then resetting when you cycle the A/C controls. The main thing to look at is the compressor clutch is engaged or not. Now if you did not have the system Vacuum purged when flushed, internal moisture could block Freon flow. Since this is a sealed system, it should be exactly like you received it from the Repair place. It is not fixed. Some States require some minimum Warranty on shop repairs. For the problem you describe, you should take it back to the repair shop. Also have the high speed Radiator fan checked if your system includes Electric Radiator cooling. The system works in tandem with the A/C. The repair shop should know at what Freon pressure your unit was pressurized. A change would indicate a problem. Also the A/C system requires an oil. Different makes use different weights. Lack of compressor oil would cause overheating and premature compressor wear. Finally, the pressure switches I mentioned have springs which can get weak. That means the cutoffs would be early.
A mechanic would check the system with a set of pressure gauges. If the compressor is running, the low side would read around 30psi and the high side would be around 250psi. That is how you know if the system is full of freon or not. You may have to get a professional diagnosis to proceed. If you replace the compressor, the accumulator needs to be replaced and a vac pump is used to pull all of the air out before the freon is added.
Hey M, the pressure could be nothing but "hot air" and not freon... (I had this same problem!!) I let off a bunch of pressure..(nothing but clear hot air!!) ..using a small flat punch..on the low pressure side... if freon on comes out right away..stop.. if nothing but hot air comes out.. keep letting it out until freon comes out,,, When mine started letting out freon,, I quit and tried the A/C again... and it has been working fine since.. Just my recent experience!!!!
Not sure how you know if the freon is full without the unit running. There are a number of switches which can keep your A/C from coming on. There is a low pressure switch which is usually located on the larger line and sometimes located on a silver cannister (called an accumulator) which is the low side of a compressor circuit. There is also a high pressure cutout which is usually located on the back of the compressor unit. Both of these switches can be jumpered out to see if they are the reason for the compressor not working. You remove the two wire connector to either switch and place a small jumper wire between the two wires on the connector block. The other reason my be a low idle cutout switch and this will also usually be located near on on the compressor. You can also test it the same way as the other switches. If it is the low pressure switch then of course you will need to add freon which you can purchase from any auto parts store. They usually come with instructions and all the equipment needed to add in freon. If it is high pressure, then some of the freon should be recovered from the system by a technician. If is the low idle cutout then you will have to take it to a mechanic to understand why it is idling so low. Hope this helps.
This is most likely the cycling of the AC unit. As the compressor pressurizes the 134a Freon you get the cold air, once it reaches a certain pressure it cuts out. If you are low on Freon it will cycle out more often. Check your freon level with a gauge to see if your within the proper tolerance. If the compressor runs for a period and doesn't reach the right pressure it will cut out to keep from damaging your compressor so it is important to keep the correct amount of freon in the system.
I can tell you general principles about A/C but have not worked on a Hyundai. If the A/C lines under the hood and directly from the black cylinder tank are frosted or sweating, you have Freon circulating.
There are 2 cutoff switches in all systems. A low pressure and high pressure cutoff. The ideal amount of Freon is in the middle of these 2 pressure points.
In this way the compressor clutch will stay engaged the longest and keep the Freon circulating the longest. This is the way you get COLD air.
There are ways to measure the pressure and to find leaks. You can sometimes loan tools from Autozone or Oreillys for a deposit. Before blaming another electrical problem for the lack of COLD air you should test the pressure of the Freon left in the system. If you are within 90 days or 3000 miles, most minimum warranties cover the parts. With factory warranty it should go farther.
In answer to your electrical concern, yes there is another system controlling the A/C. The ductwork has either vacuum operated or electrically operated doors (flaps) under the dash. These "doors" blend the airflow by mixing cold and hot air to give you the temperature you want. If you want all COLD you keep the heater door shut and so on.
So you look at these areas separately first. Under the hood is the Freon pressure good? Pipes sweating? Compressor clutch on steady? And if everything under the hood is good, then you look inside the car at the ductwork.
You may have created an over charged condition and at this point the a/c compressor will cut off on high head pressures. This can be verified by installing gauges to the systems high and low side service taps and reading the pressures.An overcharged system will start the compressor briefly but soon cuts off once pressure goes to 350 - 400 lbs on the high side. Acceptable pressures for this system should be no greater than 250 high and low suction should drop to 40-50 psi.You might recover the charge and evacuate the system,then try bringing the charge back to proper level.A sight glass in the reciever drier really helps determining proper fill.Once all bubbles have dissipated from the sight glass during charging ,one should stop at that point to avoid overfilling.Too much freon is worse than a low charge especially in hot ambient conditions....
Sounds like the system is working fine, then the condensor or compressor gets too cold and freezes up (or gets WAY too hot). when you turn the A/C off, the system has time to stabilize and return to a more 'normal' condition.
Could be the expansion valve. That's what allows the temperature to actually drop in the system - the contents of the AC system are under pressure, and are at a certain temperature. When the freon goes through the expansion valve, the pressure drops suddenly and dramatically, which "flash chills" the freon, allowing the air to blow cold. If it fails, you may get lukewarm air from the AC despite everything else working. Also though, check to see whether the AC evaporator fan (underhood) is working - on some cars, if it doesn't turn on, the compressor won't be run enough to cool the air.