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  1. #14
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    Check for restricted cap tube by checking condenser temp. If condenser is mostly ambient and drier is warm, cap tube restricted.

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  2. #15
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    Oct 2009
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    Raleigh, NC
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    Drier ambient, rather.

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  3. #16
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    Apr 2010
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    93
    Quote Originally Posted by ammoniadog View Post
    R134a and R404a both use the same crappy oil.
    This is good to know. Thanks.

    Quote Originally Posted by MicahWes View Post
    Check for restricted cap tube by checking condenser temp. If condenser is mostly ambient and drier is warm, cap tube restricted.
    When I added a couple of ounces, the condenser did warm up slightly, but then back down. Tubing was warm entering the condenser, not hot like you would expect. Drier wasn't even warm.

    I have ordered a new compressor and cap tube. I should have it done by the end of the week, I'll report back on my findings. Thanks for all the help.

  4. #17
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    Jun 2009
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    DFW, TX
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    Quote Originally Posted by MicahWes View Post
    Check for restricted cap tube by checking condenser temp. If condenser is mostly ambient and drier is warm, cap tube restricted.
    Micah, not trying to pick a fight, because I know a LOT of people who test units this way. Just sharing my own personal opinion.

    I've talked to a lot of different techs who have told a lot of different "tricks" for determining whether a small critically charged unit is either low on charge or restricted. Various tricks, like blocking condenser air flow, adding a few oz's of refrigerant, feeling condenser u-bends, checking sub-cooling, etc.

    But in my experience, these "tricks" can often mislead technicians, because every unit is a little different. Case in point: I recently worked on a very old Randell that had what I consider an oversized condensing unit by today's standards. This caused a cooler drier than normal - and a lower suction pressure than I typically see (about 12# r134a). Next unit I worked on was a Beverage Air that had (what I would consider) an undersized refrigeration, and the drier was hot, with a suction pressure of about 20# (r134a). This was after the repair, new cap tubes, nameplate charges. Very recently was with another tech that I was training, we found a unit running in a vacuum. He tried a "shot" of gas, and it was still in a vacuum. He said "must be the cap tube." I said, let's get the scale and weigh in 3-4 more ounces. We did, and unit began cooling perfectly: it was just low on charge. We ended up finding and repairing the leak. If we had assumed "must be the cap tube" we would have gone through the hassle of backordering a cap tube, coming back out, installing new cap tube - then unit would work fine (since we charged it to nameplate charge) for about a month until it ran low on refrigerant again. Then we look stupid to the customer. This type of situation is (unfortunately) somewhat frequent.

    Bottom line is this: If the evaporator is starving, it's usually low on charge or has a restriction. If you pull the charge and weigh in nameplate charge, and it starts working, then it was obviously low on charge. If it still doesn't work then its probably restricted. I personally have misdiagnosed a few in the past, but since I started doing it this way, it is almost foolproof. And occasionally, you find a combination of low on charge AND restricted: recently repaired a True T-49 cooler; suction was running down into a vacuum - weighed in the nameplate charge, and started cooling some, but suction stayed at about 5# (r134a) - turned out to have a leak in evaporator, plus restriction. If I had just been adding gas, I might have thought "Oh I just need to add more gas". But by weighing the charge, there is no question, and I am 100% confident of the diagnosis.

  5. #18
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    Oct 2009
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    Raleigh, NC
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    790
    I don't really know why you would think you are picking a fight?! You are correct that every situation is a little different. Thing is, I work in a busy shop in the real world. Many of the small, self-contained units we service don't even have service fittings. I must make a diagnosis quickly and sometimes with no pressures, or more commonly just suction pressure available to me. I can make a diagnosis with a suction pressure and my hands and ears with few occurrences of misdiagnosis. Most of the time if you have a warm drier and low suction, you have a low charge. As long as there is a little bit of the charge remaining the drier will likely be quite warm. The cap tube COULD also be restricted, but you would know this as you proceed with your service and solve the first problem by adding charge. If you have low suction and the drier (and usually part of the condenser) is at ambient temperature, then you almost always have a restriction, and it is usually the cap tube. In either case, if your compressor won't pull down the suction quite low within a reasonable amount of time, you could have an inefficient compressor as well.

    So far as there being other problems after a cap tube change that you might miss, namely leaks, you would figure this out because you should pressure checking with nitrogen and then pulling a deep vacuum using a micron gauge. This will show you any leaks.

    None of these basic troubleshooting processes are tricks. While I certainly misdiagnose problems sometimes, it is relatively rare. I cannot afford to spend an extra 30-60 minutes on a single call just to prevent that 1 time in maybe 30-40 machines that I might misdiagnose.

  6. #19
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    Sep 2002
    Location
    Virginia
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    3,080
    I assume your boss doesn't care for line taps ?

  7. #20
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    Jul 2011
    Location
    Lehigh Valley, PA
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    427
    Quote Originally Posted by trippintl0 View Post
    Bottom line is this: If the evaporator is starving, it's usually low on charge or has a restriction. If you pull the charge and weigh in nameplate charge, and it starts working, then it was obviously low on charge. If it still doesn't work then its probably restricted. I personally have misdiagnosed a few in the past, but since I started doing it this way, it is almost foolproof. And occasionally, you find a combination of low on charge AND restricted: recently repaired a True T-49 cooler; suction was running down into a vacuum - weighed in the nameplate charge, and started cooling some, but suction stayed at about 5# (r134a) - turned out to have a leak in evaporator, plus restriction. If I had just been adding gas, I might have thought "Oh I just need to add more gas". But by weighing the charge, there is no question, and I am 100% confident of the diagnosis.
    What I have found on R134 units, adding a few ozs will quickly rise the discharge psi while the suction climbs slowly near 5#'s before settling back down again to where it was when I started after 10 mins or so. I've been fooled once too many times evacuating, vacuuming & recharging R134 systems to nameplate although I am not saying this is not the correct way of doing things.

    Those darn partially restricted cap tubes....PITA

  8. #21
    Join Date
    Apr 2010
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    93
    Update to my thread: Well, to be honest, I will never know exactly WHAT the problem was. However, when I arrived, the unit had been unplugged and sitting, waiting for me to work on. I decided to plug it in and double check things - on start up, the compressor sounded pretty bad - hopefully this wasn't from the extra few ounces I put in - and my suction took too long IMO to come down.

    Either way, I had the ok to change the compressor and cap tube - did both and by the end of the day, the unit was below zero, pressures were textbook.

  9. #22
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    Nov 2006
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    Southeastern Pa
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    Quote Originally Posted by Kevin_1963 View Post
    Update to my thread: Well, to be honest, I will never know exactly WHAT the problem was. However, when I arrived, the unit had been unplugged and sitting, waiting for me to work on. I decided to plug it in and double check things - on start up, the compressor sounded pretty bad - hopefully this wasn't from the extra few ounces I put in - and my suction took too long IMO to come down.

    Either way, I had the ok to change the compressor and cap tube - did both and by the end of the day, the unit was below zero, pressures were textbook.
    I have seen sitautions where the compressor got hot and stopped running, but with the noise in the kitchen and fans running, the fact that the compressor has stopped running went unnoticed. To avoid this, I look for FLA with my clamp meter.

    In your case, the thing had been unplugged so it could cool down, and then you got to hear what it really sounded like when it WAS running. Another notch in the belt!
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  10. #23
    Join Date
    Jun 2009
    Location
    DFW, TX
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    645
    Good work! I will never condemn a tech for replacing a small pot compressor on a unit like that, because, even if it just had a restricted cap tube, you're still better off with a brand new compressor. As we all know, once the oil breaks down, you have a much higher chance for repeated cap tube failures. And to add to it, you heard the compressor getting noisy, so IMO you definitely did the right thing.

  11. #24
    Join Date
    Oct 2004
    Location
    Central WA
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    Quote Originally Posted by trippintl0 View Post
    Good work! I will never condemn a tech for replacing a small pot compressor on a unit like that, because, even if it just had a restricted cap tube, you're still better off with a brand new compressor. As we all know, once the oil breaks down, you have a much higher chance for repeated cap tube failures. And to add to it, you heard the compressor getting noisy, so IMO you definitely did the right thing.
    The boss laid down the law a couple of years ago, and we no longer replace cap tubes. Convert to txv is the only option.

  12. #25
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    Oct 2009
    Location
    Raleigh, NC
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    Quote Originally Posted by cjpwalker View Post
    The boss laid down the law a couple of years ago, and we no longer replace cap tubes. Convert to txv is the only option.
    Wow, that is extreme! I don't think cap tubes are THAT big of a problem. If you keep the condensers clean, they are very reliable. I know, I know....In the real world this is asking a lot.

    Just remember...For every box in which we are replacing a cap tube, there are 20 more that run for years and years with no service needed.

  13. #26
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    Oct 2004
    Location
    Central WA
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    The problem is once there is crap in the system (wax, desiccant, etc) changing the cap tube doesn't eliminate it. The txv will hold up better to this, and has a removable screen as well.

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