a/c problem (evaporator freeze?)

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  #1  
Old 11-20-04, 07:39 PM
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a/c problem (evaporator freeze?)

AC (R134a) works a while and then slowly loses cooling. Sometimes, fog comes out with the loss of cooling. I have not visually checked evaporator but I tend to believe it freezes after a while.

Gauge readings are 175-210 psi HIGH and 12-15 psi LOW. I know 12-15 is low. My question is if 175-210 is also on the low side. The condenser fan cycles between fast and slow. When it runs fast, pressure drops to 175. During low cycle, it increases to 210.

If 175-210 is too low, I will add freon. High high can this go?

Test condition: idling (about 700 rpm), panel mode, max fan speed, lowest temperature control know setting, ourside air in mode, all windows rolled down, all vents open.

My main concern is if the freezing is caused by blocked evaporator. I had the compressor replaced. After about a month, I had to get the evaporator replaced for blockage. That was about a year ago.

Many thanks for the advice.
 
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Old 11-21-04, 02:33 AM
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You never mention what kind of car or system you have. They all work different. They all have built in freeze protection. If it's an oriface tube system, it should be cycling the compressor off around 24 lb and back on at 48. If it's an expansion valve system, it should have a "ranco" type thermostat in the evaporator to shut the compressor down when the temp gets too cold. It does sound like it may be a little low on refrigerant but you'll have to evacuate and recharge with a measured charge to know for sure. Someone may have jumped out a thermostat or switch sometime in the past.
 
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Old 11-22-04, 03:16 PM
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My apology. The car is 98 Plymouth Breeze 2.0L 4 cyl. After reading your reply, I studied the shop manual. the car has an Evaporator Probe that senses freezing and cycles compressor clutches off and on. Also, it is an Expansion Valve unit.

I suppose I was wrong to assume icing of evaporator. I will remove the duct and visually check if the evaporator is indeed freezing. If it does, then I will focus on your suggestions (disconnected switch or thermostat). If not, I will add 1-2 oz freon and watch the high side pressure. Many thanks for the advice.
 
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Old 11-22-04, 05:04 PM
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I would get an accurate digital thermometer in the center vent on low blower speed and drive around for a while. It really shouldn't drop into the thirties without cycling the compressor off. If it does that, check the thermostat.
 
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Old 11-22-04, 06:55 PM
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Thank you. I will do that first.

Let me ask a related question. Somehow the low pressure port started leaking a little, as I can tell from the green dye bubbling at the shroeder valve slowly but noticeably. Is there a simple way to seal it without replacing the valve core? Like an extension valve on a tire? Thanks.
 
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Old 11-23-04, 06:26 PM
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Usually a good cap with a good rubber seal will hold that back but the correct way to resolve it is to replace the core.
 
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