malfunctioning auto a/c

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Hi Everybody. The receptionist at work mentioned that her wifes car a/c wasn't functioning properly when her car was at idle, and was slow to cool down when she started driving. Knowing that I do my own repairs on my cars, and light stuff on my work truck, wondered if I'd take a look. They've recently been burned pretty badly by some local shops, so she's wary and hoping to have a rough idea of the issue before going in. A manifold set showed that the low side pressure would drop down into the appropriate range when the compressor was on, but would immediately start to rise when the compressor disengaged, going well above 150psi, with corresponding changes on the high side dropping and rising. I wasn't able to easily locate appropriate pressures for low/high on her car via the internet, and told her I'd ask around to see what the general idea of what's wrong might be before they take it to a shop. If does blow hotter-than-atmosphere air from the vents when idling, as if it's not being cooled before entering the cabin, and seems to get hotter. So, any ideas? I couldn't find if the compressor fan was also the radiator fan, or if it had a separate one, and didn't think to check when I was there. So I'm not positive that the necessary fan(s) are running as they should. I plan to stop back by tomorrow and check that. So, if the refrigerant is at the appropriate charge (a more basic gauge put it at 50psi on the low side running), and there doesn't seem to be a blockage preventing build up of pressure with the compressor on, what are the possible problems, and their potential fixes?
 
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Sometime when the ambient air temperature is 80 Fahrenheit or greater open all the windows in the vehicle and run the air-conditioner at maximum, with the fan at maximum, with the engine at idle. If the system has enough refrigerant the compressor clutch will stay engaged and not cycle on and off. If it cycles on and off it is low on refrigerant.
 
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RichardS

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Being over-pressurized wouldn't possibly cause it to cycle on and off? When I initially checked the low side only, it was registering beyond the realm of readability for that gauge. Also, the low side was far from 20psi when the compressor kicked off and it rose quickly to above the highest pressure marking on the gauge. I had thought about perhaps just evacuating the system and starting anew, but if the system isn't functioning properly, there's no need to waste the money on a pointless evac/refill.
 
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I am not a refrigeration tech, but I have worked with refrigerant systems. I am trying to think what could cause such high pressures on the low side, and the answer is... Some kind of blockage before the gas leaving the evaporator goes into the intake side of the compressor. The location of the low pressure tap, and the combination accumulator/dryer varies. Some systems have the combination accumulator/dryer in-front of the evaporator, and some have it after the evaporator. And, some systems have the low pressure tap in-front of the combination accumulator/dryer, and some have the low pressure tap built into the combination accumulator/dryer that is located between the evaporator and the input to the compressor, and some systems have the low pressure tap right at the compressor input, so depending on where the low pressure tap is and where the combination accumulator/dryer is located in the system a blockage could cause the high pressures you are seeing on the low side. If the combination accumulator/dryer is located between the evaporator and the compressor (which it is on many systems) and the screen that holds the dryer desiccant in place has broken, it is possible that the desiccant is blocking the flow of gas before it can enter the compressor. This would cause high pressure on the low side. If this is not the problem, look for something acting as a block of gas between the evaporator and the input to the compressor, or the compressor itself is not pulling gas into it. I am just trying to think of what could be causing such high pressures on the low side.
 
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Its outside the realm of your knowledge. Say sorry, I dont know enough about AC. I'm afraid you will have to take it in. And yes, check that the electric fans (on the radiator) are working. And check that the door flap valves are working properly. They can be electric, vacuum, or mechanically operated, depending on car. If its open or in the wrong position, it could just suck in hot air from the outside.
 
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Re-reading the original posters post and it says that the low side pressure drops down to the appropriate range when the compressor runs, so it would NOT be a blockage between the evaporator and the compressor input.
 
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RichardS

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spasm3, That was my thought after doing a little online snooping. bubbatime, They're already aware I'm in over my head, and it will likely take a trip to a shop for proper repairs. However, I still don't mind taking a look and seeing if there's an easy/cheap fix. JimPghaPA, I agree that I don't think there's a block internally in all of the bits and pieces. Though that was my initial thought, from what I could see, that doesn't seem to be the case.
 
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The charge isn't getting cooled and condensed in the condenser. Any sounds to be heard inside the car, like escaping gas? You need the fans on low speed to hear that though. You didn't say how high the high pressure side got, but it shouldn't be more than 225 psi.
 

RichardS

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Jetronic, It was well over 250 when the compressor would cycle off, and the pressure would drop back down. It'd get just about equal to the low side, cycle back on, and then back off when the low got into the 40/50psi range. Being that it was 97, I doesn't seem that was too low on the low side.
 
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If the air comes out hotter than ambient, the heater is not turning off fully and is fighting the A/C. One simple check is to feel the low side line. If it is cold, the refrigeration is working OK but the heater is reheating the air before the people in the car feel it.
 
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