Adding Freon to Car AC – Gauge Readings Explained

7:29 am Air Conditioning, DIY, How To Auto Repair

Continued from Page 1

Both sides are lower than normal. Both low and high side readings are lower than normal, this indicates a car’s AC system is low on refrigerant and is under-charged.

Both readings are higher than normal. If both low and high side readings are too high, this indicates an overcharged system – too much refrigerant. This also can indicate that the condenser fan is not working, is too slow or the car is overheating and heat is transferring from the radiator to the condenser.

Low side goes into a vacuum and high side is too high. When the low side goes so low that it’s reading shows it is in a vacuum, the most likely cause is a bad expansion valve or blocked orifice tube. Another possibility is a restricted condenser. Blocked condensers are not as common as they used to be but if a compressor fails and comes apart inside the remnants can end up in the condenser causing it to restrict the flow of refrigerant.

Low side is too high and high side is too low. When the compressor clutch is definitely engaged and the low side is high and the high side is low, the most likely cause is that the compressor is failing – it is not pumping sufficiently. Rarely an AC clutch could be slipping but usually this will be accompanied with a squeal or chirp. Also be sure not to overlook the obvious, like a loose belt!

Question: I have to add a can of freon every two weeks, where could it be going?

Answer: Obviously, the system has a leak. Some cars come with UV dye in their system already. Usually you can see some of the dye in the service port (a little might show after disconnecting the gauge set hoses). Look for dye or clear refrigerant oil on the AC components that would indicate a leak. In addition to a visual inspection, an electronic leak detector can be used to pinpoint the source of a leak. See our article with more information about Finding A/C leaks.

Question: I see the small line going into the evaporator on my Ford Explorer, it’s really sweating and very cold at one point. Is there a restriction in the line somehow?

Answer: Most Ford Explorers have an orifice tube (expansion tube) in the inlet of the evaporator. It’s normal for it to be cold – if it is frozen and the vent air is not cold then it may be plugged up. If upon removal there is metal debris caked onto the orifice tube, the compressor will need to be replaced.

Question: Why is there such a big temperature change in the AC lines? Some are very hot and some are cold is this normal?

Answer: The line going from the compressor to the condenser is the discharge line – it is normal for it to be very hot to the touch. The other line going from the drier or accumulator to the compressor is normally colder. The liquid line can be hot up to the point an orifice tube is in place. Just remember that LOW Pressure = COLD and HIGH Pressure = HOT.

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Be sure to check out the related links below for A/C tools and more AC repair articles.

Related Links
A/C Repair Articles on this Blog

Car A/C Tools for Sale

Car AC Troubleshooting Tips

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415 Responses
  1. brandon m :

    Date: July 18, 2016 @ 1:35 pm

    i have an 2000 dodge ram 1500 5.2 liter
    the high side valve has a slight leak. When i fill the system and put the cap back on it builds pressure and blows the cap off? the air is actually cool while this is happening?
    question 2: can you check your pressures one at a time all i have is a single line hose

  2. dennisb - Auto Tool Sales :

    Date: July 18, 2016 @ 1:43 pm

    It will cool until enough refrigerant leaks out that it cannot. I’d try tapping on it slightly to see if the valve stops leaking. If not it will need to have the refrigerant recovered, the valve replaced and the vacuumed down and recharged. To check each side requires different fittings and gauges. High and Low. Good Luck.

  3. Real Mechanic :

    Date: July 27, 2016 @ 4:39 pm

    Vehicles manufactured starting in model year 1994 use R134a, not Freon. Freon is Dupont’s trade name for R12. Freon is NOT R134a!

    I’m an ASE and EPA certified. Plus, you need an EPA license to even buy Freon.

    To avoid any confusion, please remove “Freon” and add, “R134a” to your article.

  4. dennisb - Auto Tool Sales :

    Date: July 28, 2016 @ 9:56 am

    Hi, thanks for the comment. I’m also an ASE Master Tech with many years of experience in the automotive repair industry including AC repair starting back in the 1980’s. You are correct, since 1994 134A has been used in cars in the USA. Many people refer to refrigerant as “Freon” although you are correct in saying that 134A is not technically called Freon. This article is not a text book. Instead it is practical info relating to gauge readings from my personal experience over the years. Dupont used to own the name Freon, they spun off the division that owns the Freon name to The Chemours Company in 2015. Luckily, R134A connections are completely different from R12. And R12 is not readily available to the public. At this time anyone can purchase R134A at their local parts store without any type of license. Although it may not be 100% correct to call R-134A Freon, if you go into any parts store in the USA and ask where the Freon is, they’ll point you to 134A. Don’t believe it? Try it.

  5. Rick :

    Date: September 26, 2016 @ 3:24 pm

    Just coming home today and my a/c stopped blowing cold. Working good then not. It has a static pressure of 90psi high and low, even with the truck running and the a/c on max. I think it is a faulty compressor. Your comments please. Compressor is not cycling.

  6. dennisb - Auto Tool Sales :

    Date: September 26, 2016 @ 3:31 pm

    Rick if the compressor is actually turning, then yes the compressor is not pumping. I’d make sure the clutch disc is turning to verify though. Good Luck.

  7. Ron :

    Date: May 25, 2017 @ 8:41 am

    I.m working on the AC in my 1993 Corvette. No freon ( R 12 ) in system. Tried to attach my Manifold Gauge set to diagnose problem. My high side (red) hose would not thread on to the high side service port. No adapter in my set would fit. High side port is smaller in diameter than normal R12 service hose schrader valve. Has no visible valve core in it. Has what appears to be a small black rubber ball about 1/2 inch down in the port. Is this something specific to the 1993 Vette? My 1990 Corvette has normal R12 schrader valves. Will appreciate any help. Thanks, Ron

  8. dennisb - Auto Tool Sales :

    Date: May 25, 2017 @ 10:12 am

    Ron, it’s been a long time since I worked on R12. Thought most were converted to 134A by now. You may want to check in a Corvette forum for help on the service port connector. Good Luck.

  9. Rick :

    Date: June 2, 2017 @ 4:28 pm

    Hello Dennis,
    I’m pulling my hair out. Replaced compressor last fall. Worked very well until winter came then didn’t use it. First hot day it did not work at all. Clutch will not engage. Tried jumping the low side plug. Will not engage. Ran an external 12v and ground from the battery and it worked well, it engaged and cooled. I’m getting 12v to the low pressure switch on the drier. Replaced dashboard controls, no difference. Static pressure is 95 on both sides. Both high and low switches are new as well as the drier last year. I also replaced the oil pan and starter last winter. I went through the wires, don’t see anything broken. Any thoughts?

  10. Rick :

    Date: June 2, 2017 @ 8:35 pm

    I may have found it. The high pressure switch is broken. I will order a new one and see what happens.

  11. dennisb - Auto Tool Sales :

    Date: June 3, 2017 @ 9:24 am

    That’s great!

  12. Rick :

    Date: June 9, 2017 @ 9:34 pm

    That was the problem. The high pressure switch plug was broken but still plugged in. I ordered and installed a new one and the compressor engaged. Thank you so much for your help. You have helped me a lot in the past with complicated A/C problems.

  13. dennisb - Auto Tool Sales :

    Date: June 10, 2017 @ 9:11 am

    Perfect. Glad I could help.

  14. Dodgeman2 :

    Date: August 26, 2017 @ 8:28 pm

    I find your articles on repairing the air conditioning systems very interesting and informative. It didn’t answer my questions. I have a 2002 Dodge van B1500 5.2 with front and rear ac units on the same system. my system was low on Freon so I added Freon, as I continue to investigate found out that My ac takes 46 Oz of r134a and 10 oz of pag oil to completely fill the two units. my system was not open, Didn’t have a leak, what would the pressure reading on the hi side and low side be when the system is filled to manufacture specification. Or how do I determine the correct pressure
    Thank you Dodgeman2

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