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  #1  
Old July 28th, 2008, 08:05
MattS MattS is offline
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A/C blowing hot only when it is hot and humid

Ok, my A/C works great when it isn't blazing hot outside. When the outside temp gets over 85 degrees, my A/C will "freeze up" when driving around town. Usually, I have to turn it off for a few minutes, and all will be fine for a little while, then do it again. I've replaced the low pressure cycling switch, I think that's what it's called, had the system evacuated and refilled by the dealership and leak tested. I've had several other guys and mechanics hook up gauges to the system. I have great pressure/levels on the low side, but my high side was "a little" off. I think it was showing low. I'm kind of at a loss.
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Old July 28th, 2008, 11:19
yellowta yellowta is offline
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Re: A/C blowing hot only when it is hot and humid

condensor fan? if it's not moving air then the freon doesn't get warmed up and it will freeze the evaperator coil.
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Old July 28th, 2008, 14:07
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Ecomike Ecomike is offline
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Re: A/C blowing hot only when it is hot and humid

Quote:
Originally Posted by MattS
Ok, my A/C works great when it isn't blazing hot outside. When the outside temp gets over 85 degrees, my A/C will "freeze up" when driving around town. Usually, I have to turn it off for a few minutes, and all will be fine for a little while, then do it again. I've replaced the low pressure cycling switch, I think that's what it's called, had the system evacuated and refilled by the dealership and leak tested. I've had several other guys and mechanics hook up gauges to the system. I have great pressure/levels on the low side, but my high side was "a little" off. I think it was showing low. I'm kind of at a loss.
What do you mean exactly by freeze up? Do you mean the evaporator coil ices over on the outside of the coil? or something else?
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  #4  
Old July 28th, 2008, 15:59
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tomcat tomcat is offline
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Re: A/C blowing hot only when it is hot and humid

my evaporator core is currently clogged up due to a mouse making its home in the heater box. i found him dead in the squirrel cage in the heater box. if you pop out your blower motor resistor, you can just make out the bottom of the evaporator core. if it is clogged up, it can't shed the cold as fast as the system wants it to, and will freeze up.

take a peek, and see what you can see.
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  #5  
Old July 29th, 2008, 07:28
MattS MattS is offline
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Re: A/C blowing hot only when it is hot and humid

I guess I shouldn't call it "freezing up" I haven't seen icing, unless it is somewhere I can't see. What I do experience is that it will blow cold for a little while, then start blowing warm. Sometimes it will cycle back to cool on it's own, most of the time I have to turn it off. The aux fan comes on and seems to work the way it is supposed to. The attention getter, at least to me, is that it only behaves this way when it is hot and/or humid. For instance, this morning, it was about 71 on my way to work, and the AC worked great, I can guarantee that on my way home, it will start blowing hot air after 10 minutes.
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Old July 29th, 2008, 07:40
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xj88superjeep xj88superjeep is offline
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Re: A/C blowing hot only when it is hot and humid

What is your coolant temp gauge telling you? Are you running hot? You gotta keep engine temps cool for the air to run cold. If you're not overheating, it could be a bad heater valve, but I only say that because a radiator guru told me once that the heater valve in my XJ was important for the air to function properly.
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  #7  
Old July 29th, 2008, 08:17
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Ivan Ivan is offline
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Re: A/C blowing hot only when it is hot and humid

It still sound like it's your low pressure switch, that is located on top of the accumulator. Maybe you got a defective one.

When this switch fails (which is common in XJ's), it will create the symptoms you are describing.... icing up the evaporator, which blocks air flow, giving you that luke warm air you are feeling. Once the ice melts, the AC works fine until it ices up again.

You can buy the switch at the dealer and replace it yourself. Just unscrew it and install the new one. You will not loose refrigerant, as there is a schrader valve on top of the accumulator, preventing the loss of refrigerant while you replace the switch....
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Old July 29th, 2008, 08:17
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BIGSLVRXJ BIGSLVRXJ is offline
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Re: A/C blowing hot only when it is hot and humid

Quote:
Originally Posted by xj88superjeep
What is your coolant temp gauge telling you? Are you running hot? You gotta keep engine temps cool for the air to run cold. If you're not overheating, it could be a bad heater valve, but I only say that because a radiator guru told me once that the heater valve in my XJ was important for the air to function properly.
His profile says he has a 99, if that's the case he doesn't have a heater valve. I really wonder if you have a leak that's causing most of the refrigerant to leak out quite fast. I know you say it's been leak tested, but often times evaporator leaks won't show up since you can't get a black light behind the dash. Do you know the time it was evacced how much they pulled out?
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  #9  
Old July 29th, 2008, 09:13
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Re: A/C blowing hot only when it is hot and humid

I didn't get a chance to look at his profile... But you are right, no heater valve on that year. So, I'm out of the loop on this one... Good luck Matt!
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  #10  
Old July 29th, 2008, 10:45
Newblue Newblue is offline
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Re: A/C blowing hot only when it is hot and humid

My 2000 does the exact same thing. This moring going to work it was in the low 70s and it worked for 30 minutes, all the way to work. When the temp goes over 80 outside it works for less time as the temp rises. I have replaced the low pressure cutoff switch with a mopar one and it didn't help. The low and high side pressures were tested and only the high side was off - it was a little high. I hope a solution can be found.
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  #11  
Old July 29th, 2008, 11:37
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Ecomike Ecomike is offline
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Re: A/C blowing hot only when it is hot and humid

Several possible causes.

Low refrigerant charge is one.

Bad vacuum actuator valve on the outside air / recirculation door that is triggered to recirculate air inside (No outside air) when the setting is on Max AC. If the vacuum is lost to the valve due to a vacuum hose problem or leak, or if the diaphram in the valve goes bad, the door does not close (spring loaded to open) and 100% outside air (hot and humid) increases the AC load beyond its design limits. In hot humid weather it must be set on Max AC and the door must close for the AC to work properly.

Both of the above!

I doubt it is the low pressure switch in your cases.

Engine coolant operating temperature (too high....) has nothing to do with the AC working properly or not.

One low probablity possibilty is a busted wire to the AC/Heat blend door that runs from the Cold - Hot blend control to the blend door, but it would not be very sensitive to the outside temperatures.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091

Last edited by Ecomike; July 29th, 2008 at 11:44.
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  #12  
Old July 29th, 2008, 12:19
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Re: A/C blowing hot only when it is hot and humid

CHECK YOUR EVAPORATOR CORE FOR BLOCKAGES
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  #13  
Old July 29th, 2008, 12:52
menos menos is offline
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Re: A/C blowing hot only when it is hot and humid

Loud noises!!!!!!
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