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Thread: How To: Confirm A/C system is holding vacuum before recharging

  1. #1
    Senior Member 82DMC12's Avatar
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    How To: Confirm A/C system is holding vacuum before recharging

    In this video, I demonstrate how I pulled a vacuum using the Harbor Freight vacuum pump and manifold gauge set. Then, I show you how to confirm that your system is actually holding vacuum since it is common for the manifold gauge set to leak, causing confusion over where or what is leaking. This video is relevant to R134a conversions so you will have to have R134a adapters on your hoses, or they should be replaced with modern barrier hoses that have the correct fittings on them.

    I think this will be the first video in a 2-part series. The second video I am going to show how to pressure test with nitrogen using the same equipment.

    Tools:
    Harbor Freight Vacuum Pump: https://www.harborfreight.com/25-cfm...ump-61245.html
    Harbor Freight Manifold gauge set R134a: https://www.harborfreight.com/r134a-...set-58776.html

    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

  2. #2
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    The video is good, but I have one critique. I see this often, so you are not alone in this issue.

    You don?t leak check with vacuum. The purpose of the vacuum is to evacuate the system. You eluded to this in the video when you said ?if it keeps making vapor, you?re sucking in air somewhere?.

    Vacuum is a very poor way to leak check for a couple reasons.

    1. You can only vacuum to a pressure differential of about 15 lbs. (that?s the difference between atmosphere and a good vacuum) If you use pressure to leak check, (nitrogen or another inert gas) you can have a much larger pressure differential. You could go as much as highest pressure the system will see. In my experience, a 100 psi is enough, but that is still a pressure differential of 85 psi. (100 - 15 atmosphere)

    2. If there?s a leak, as you said, you are sucking stuff in to your system.

    Also, if you were to pull a vacuum at night and came back the next morning and found the vacuum all but gone, it doesn?t necessarily mean there?s a leak. There could be a pocket of non condensable stuff that gradually evaporates. When stuff evaporates, it reduces the vacuum. I work on walk-in coolers and freezers now and than and sometimes I have to leave the pump on over night just to get the system down.

    You will probably meet lots of experienced auto techs that say they leak check with vacuum for many years and they have never had a problem. The fact of the matter is auto systems are very forgiving. I had a vacuum pump that I used for years successfully on cars and couldn?t figure out why a walk-in freezer was having problems. It turned out that my pump was not pulling enough vacuum. (I got a micron gauge and checked it.) A gauge set is a very poor indicator of vacuum.

  3. #3
    Senior Member 82DMC12's Avatar
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    Quote Originally Posted by Helirich View Post
    The video is good, but I have one critique. I see this often, so you are not alone in this issue.

    You don?t leak check with vacuum. The purpose of the vacuum is to evacuate the system. You eluded to this in the video when you said ?if it keeps making vapor, you?re sucking in air somewhere?.

    Vacuum is a very poor way to leak check for a couple reasons.
    Agreed, but I did say in the first post that it's part 1 of a two-video set where the second video is going to show leak detection with nitrogen. I wouldn't even proceed with charging if I was losing vacuum quickly. I think we agree on that point?
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

  4. #4
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

    Posts:    1,654

    My VIN:    11596

    I thought I'd update this thread with some of my own experience with A/C tools over the last few months.

    I really cant recommend the Harbor Freight A/C gauge set. This is my second set that is failing after very light normal use. Namely, the high and low adapters that snap onto the car have bad spots in the valve travel that will leak to atmosphere. The high side was the first one I found leaking. It opens and closes the Schrader valve on the car properly but when the adapter is "closed", it was leaking the gauge side to atmosphere. I replaced the adapter yesterday with a much better quality part from ColdHose.com.

    Then, during further testing, I found the low side adapter has a bad spot somewhere in the middle where it leaks out the gauge side. Not a big problem if you open the valve quickly but who wants to deal with questionable tools?

    Anyway, I ordered a new low side adapter too which should hopefully solve any tool-leakage problems.

    I have also had to reset the low side gauge on the manifold set to zero after I found it is was reading about 15 PSI when it should have read zero.

    So yeah, I was hoping these gauges would be good enough for a single-project DIY but I can't recommend them. Spend the bucks and get something better like this one (thanks for the link, Steve Bano)

    https://www.ebay.com/itm/18511904699...tSecRzpaXMML-U

    If this link dies in the future, it's for Yellow Jacket 42006 Refrigeration Manifold with 60" Hoses, R-22 / 404a / 134a

    Note that this set doesn't come with adapters so you will need to buy them yourself like the ones from Coldhose.com. However it is MADE IN USA!


    Adapters:

    https://coldhose.com/collections/too...35532201066656
    https://coldhose.com/collections/too...35532200280224
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!
    Total frame-off restoration completed 2021-2023

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Kansas City

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