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Thread: AC Working, But Compressor Won't Cycle

  1. #1
    Senior Member uhhair's Avatar
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    AC Working, But Compressor Won't Cycle

    Last year I gave my AC system a complete refresh consisting of a new compressor, accumulator, lines, and orifice tube. It is also now filled with R134A. I had no issues with the system last year, but this year something is definitely strange. While the air gets cold and works fine, it seems to take a while to get going, and it is also not cycling. The compressor is engaged the entire time the AC is turned on, no matter if I am at idle or the car is revved up. I've never had an issue like this before and am not sure where to start. Any help would be greatly appreciated, thanks!

  2. #2
    Senior Member Bitsyncmaster's Avatar
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    At idle your compressor may not cycle if your blower is on high and the air ambient temps are high. If your blower is on a lower speed it probably will cycle. Or blowing a fan at the front of the car will also help. It could also be a low charge but since your getting cold air I would not worry about it. It probably will cycle when your driving because the air flow over the condenser is better.
    Dave M vin 03572
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  3. #3
    Senior Member 82DMC12's Avatar
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    You will probably have to put a set of gauges on it. What is your vent temp? I had a similar question a few weeks ago but at the time it was hot as s#!T outside and I come to learn that the compressor may not turn off at all when it's working hard in that kind of ambient heat. My recollection is it will cycle on and off periodically in cooler temperatures. I too did a total R134a conversion, replacing everything but the evaporator. This was a few years ago. It's been working great but I think I ended up adding maybe 1.5 - 2.0 oz of R134a to get my pressures right about where they should be.

    Check out this chart and compare your own readings on your gauges (Harbor Freight). The discharge fitting on the back of the compressor should feel pretty cool and probably sweating like crazy in the humidity. Check pressure at about 1,200 RPM.

    https://www.acprocold.com/faq/r-134a...ressure-chart/
    Last edited by 82DMC12; 07-09-2021 at 04:40 PM.
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    Administrator Ron's Avatar
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    Assuming you had the charge correct, since it was OK last year, I'd check that the flaps and the hot water valve are working properly...If it leaked refrigerant, it would probably be cycling too much.

    +1 on hooking up the gauges!
    (You should take/record readings when the clutch cycles out, if possible.)

  5. #5
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    Ron is right. If the system had a leak and it was lower on refrigerant, it should cycle more frequently. If you overcharge the system it will not cycle off, but that is not the case here. Put the gauges on. Perhaps the low pressure cutoff switch is stuck. Once you have the gauges connected observe the suction pressure to see if it is pulled low below the cutout pressure. If it is and the compressor remains on, suspect the pressure switch.

    Ron

  6. #6
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    I have a 30 year old Bronco that is converted to 134. The compressor pretty much runs all the time if it's hot. (And it's still not real cold in the truck.) The newer (1995) Broncos had 134 stock and they work much better. I believe they have bigger condensers. I looked into getting one, but it uses differnt fittings. (Project for another day)

    R12 was such a good refrigerant, that they used it in everything. Now a days, an AC tech must carry all kinds of freon for differnt jobs. 134 is hardly ever used in a freezer because it doesn't work very good. (404 is normal in freezers)

    134 uses higher pressures all around. The low pressure switch is set to cut off around 25 psi. 134 is almost never going to be that low. (Unless your running AC in the winter time) I think it is more likely you will have the high pressure switch cut off the compressor with 134.

  7. #7
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    The "Cycle Rate" for lack of a better term is very dependent on several factors. Change any one of them and it affects the rate. That is why is is so important to charge the system with the exact amount of refrigerant. That way you at least have controlled one variable. If the refrigerant is NOT -12 the low pressure switch should have been adjusted to compensate for the higher suction pressure so you have a good cycle rate when you converted. Typically a loss of refrigerant first shows up as a change in cycle rate but the ambient temperature and engine RPM also changes the cycle rate. It is important to have the compressor cycle so the evaporator core does not build up ice. Before you do anything you should get a set of manifold gauges on the A/C and see what the pressures are for the temperatures you have. My guess is that your system is overcharged or, less likely but possible, the low pressure switch is defective. To use -134 you are supposed to use 10% less than the amount of -12 so that means just a tiny bit less than 2# of -134. if you are unsure of the amount of refrigerant, you should reclaim it and weigh it back in.
    David Teitelbaum

  8. #8
    Senior Member uhhair's Avatar
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    Thanks for the insight everyone! I went ahead and ordered some gauges and will hook them up and check everything out! I do think this was just a case of it being too hot outside, as I drove the car again this weekend and while driving it did start to cycle a little bit. Now if only it would blow a little colder!

  9. #9
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    If you want colder go back to -12. -134 does not have the same cooling capacity as -12. In the meantime make sure you are not losing any air from leaks in the ductwork inside the doors and the rubber boot to the center vents is on properly and not leaking.
    David Teitelbaum

  10. #10
    Senior Member uhhair's Avatar
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    OK so the plot thickens... now my AC is not working at all. The compressor is still always on, and I did hook up some gauges to see what they might mean. The first gauge is the car at idle, compressor engaged (gauge reads close to 40PSI on low side). The second is the car revved up to around 2500 RPM, compressor still engaged (gauge reads PSI close to 20 on low side). The third is the revs higher, around 3500-4000RPM, compressor engaged (gauge reads below 20 PSI on low side). Not sure what is going on here, but the AC worked in the morning after blowing hot for the first 10-15 minutes of driving. I turned it off and back on a few times, thinking that might help, and then it did end up getting cold and stayed cold for the rest of my drive (about a half hour). Going home, no AC, just hot air. Same once I got it in the garage and hooked up to the gauges, it never blew cold but the compressor is engaged and fans are running. Some back story, two years ago I did do an entire AC refresh with new lines, a new compressor, dryer, orifice tube, and r134a.

    Is it possible the system is overcharged? I did add a little bit of freon earlier this year as the ac seemed to be a little warm, but now I'm thinking that may have been a mistake. I admit I don't know a ton about AC or how to diagnose what may be wrong. I'm fairly confident the compressor is still fine, and from what I understand it is unlikely that the evaporator or condenser is bad so I am trying to think of what else it may be. Please let me know your thoughts, and thanks in advance!
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