I just did my first automotive AC repair, after the compressor in my 2004 Lancer 2.0 ate itself, cracked its casing, and blew all the refrigerant out.

I think I did the repair & recharge correctly-- and the AC is blowing very cold-- but I'm concerned that the operating pressures are not anywhere close to what they are spec'd to be.

(During recharge, I added ~18oz of r134a, per the yellow sticker under the hood.)

Before recharging, I googled 'r134a pressure chart', and found the following: r134a pressure chart

At the time, it was 82F in my garage, so I was expecting to end up in the middle of the range (both low- and high-pressure) for the 80F row.... 45psi for low, and about 190psi for the high side. (When referring to this chart, I assumed that the target low readings were for when the clutch was disengaged, and the target high readings were for when the clutch was active.) However, after my recharge the highest low reading I see (right before clutch engages) is 40psi, and the highest reading I ever see for the high side is 150 (right before the clutch disengages.)

Should I be concerned about these low operating pressures, given my starting ambient temperature? This car is 11 years old, and so the computer & pressure switch have about 160K miles on them.

As I said, AC appears to be working very well. The compressor clutch cycles about 12 seconds on, and 12 seconds off. (I'm not sure what's an exceptable duty cycle there. Also I was parked in a garage, not running down the road, air flow through the condenser wasn't exactly real-world. Of course the fans kicked on whenever the clutch engaged.)

Static pressures (with engine off): 80-85 for both high and low sides.

In case anyone has questions about my methodology, this is what I did:

  1. Remove old compressor, condenser, dryer, and expansion valve
  2. Flush lines and evaporator with recommended flush solvent
  3. Installed (all new) compressor, condenser, dryer, and expansion valve, coating all rubber seals with PAG oil
  4. Added UV dye and 1.75 oz of PAG-46 oil to condenser (compressor came pre-filled with 3 oz, and total required by manufacturer was 4.75) (Mitsubishi originally called for 'SUN PAG-56', but after-market compressor called for PAG-46, so I used the latter)
  5. Connected hi/low gauge set
  6. Pulled vacuum to almost -30 psi inches Hg, observed that level held for 1 hour
  7. Resumed pulling vacuum for 1 additional hour
  8. Closed off high & low sides, detached vacuum pump, attached 12oz can of r134a (no additives)
  9. Pierced first 12oz can, allowed yellow line to pressurize.
  10. Burped yellow line @ the manifold to purge air
  11. 12oz can+valve+hose weighed 20.7oz at this point
  12. Opened up low side, allowed vacuum in system to suck in some refrigerant
  13. Started car, resumed slowly-turning can... compressor clutch engaged for the first time pretty early into this step.
  14. After first can felt empty, new weight reading was 8.2 (so, 12.5oz discharged from 1st "12oz" can)
  15. Closed off low side
  16. Disconnected yellow hose from can's adapter, then connected to different adapter already installed on 2nd can (at this point, I realized I had made a minor mistake, should have moved the first adapter over to the second can without disconnecting the hose. But I compensated for this by burping the yellow line again.)
  17. Initial weight of second (partial) can+adapter+hose: 15.1oz
  18. Final weight of second: 9.6 (so, 5.5oz discharged from 2nd "partial" can)
  19. Total estimated charge: 12.5+5.5=18oz (actually a little less because of burping and final contents of hoses. I think this is OK because the service manual's specifications page gives an acceptable range of "16.93 - 18.34")
  • How low does the low side pressure go when the compressor is running? If my memory is correct, the low pressure sensor usually stops the compressor if it drops below about 30psi. – HandyHowie Oct 8 '15 at 7:20
  • @HandyHowie I didn't make a note of that-- I think because I was mostly worried about not overcharging-- but the service manual says that the low-pressure switch will transition ON --> OFF when dropping below 28.4psi, and will transition OFF --> ON once above 32.1 psi. – Ryan V. Bissell Oct 8 '15 at 7:49
  • It sounds like you may be undercharged, that would explain the 12 second cycling of the compressor. If the low pressure is causing the low pressure sensor to kick in, you probably need more gas. – HandyHowie Oct 8 '15 at 8:00
  • @HandyHowie I'm above the ON --> OFF transition point (40 > 28.4) when it goes off. I'm actually starting to think that it is shutting off because of temperature (cold), not pressure. I say that because when I first turn the AC on, the cycle times seem longer (haven't measured exactly... tired at the moment.) And, at the time of the 24-second[1] cycling it's blowing 42 degrees at the vent (80 ambient) [1] 12-seconds on, 12-seconds off – Ryan V. Bissell Oct 8 '15 at 9:05
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    If you are above the low pressure limit all the time, then you should be fine. If you drop below that when the compressor is running, the compressor will stop. The pressure when the compressor is not running is irrelevant. – HandyHowie Oct 8 '15 at 9:21

That chart is not universal. You should be fine.

Every model will have its own characteristic low-side and high-side pressures.

This is why some manufacturers recommend charging refrigerant by mass rather than by high-side, low-side and ambient temperature values.

As long as you have charged the system with the correct amount of refrigerant (which you have according to the tolerance defined in your manual), you should be fine.

Another sanity check is that the low-pressure switch activates at 32 psi, which is lower than the 40 psi you are seeing on the low-side.

I'm not sure if your Lancer is equipped with a high-side pressure switch but the same principle applies.

Enjoy your ice-cold AC!


Some observations about your procedure:

  • Pulled vacuum to almost -30psi, observed that level held for 1 hour

    This tells me that there are no leaks.

  • Added UV dye and 1.75 oz of PAG-46 oil to condenser

    The UV dye is added if you want to detect leaks with UV light. Else it shouldn't be necessary. PAG-46 is the right oil for your car and is absolutely necessary to since it lubricates the internals of the compressor.

  • This car is 11 years old, and so the computer & pressure switch have about 160K miles on them.

    Not sure about the Lancer, but on some vehicles the pressure switch is a capacitor where the refrigerant pressure controls the distance between the two plates, thereby altering capacitance and voltage.

Once again, the level of detail provided in this question is stellar. I'd consider this to be the golden standard for all AC-related questions.

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    Thanks! It's actually my step-daughter's car. I'm hoping I've now established my position as Best Step-Dad Evar. I'll give this question a few days to collect more answers before accepting one, but I certainly like the sound of yours. – Ryan V. Bissell Oct 8 '15 at 9:14
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    @RyanV.Bissell : I've added some more of my thoughts. Given the amazing amount of detail you provided it was too good an opportunity to let go of :) – Zaid Oct 8 '15 at 9:23
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    @RyanV.Bissell, you can never have enough Dad points! – Bob Cross Oct 8 '15 at 12:56
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    I added the UV dye to be kind to my future self, should a leak occur later. – Ryan V. Bissell Oct 8 '15 at 16:14
  • @RyanV.Bissell : That is a fantastic idea! – Zaid Oct 8 '15 at 19:40

@RyanV.Bissell I am impressed with how much attention you gave to this task. Last year, I took my VW to authorized dealer to replace the compressor and they overfilled it. After that, they did not know what was wrong simply because the computer did not give any errors and actually did not believe me that something was wrong at first :D The problem appeared only at very hot weather so it was difficult to prove at first :) As fix they have re-done the flush/refill from scratch, hoping it would fix the issue. But meanwhile I have researched how to diagnose AC problems.

Unfortunately, they used some robinair machine and it showed the correct amount put into the system but it was still wrong...

You should be able to check the correct operation even though you do not have the vehicle specific values. This can be accomplished by checking super-heat and sub-cooling.

While the chart you have put is not universal, the temperatures that the refrigerant turns into liquid and vaporize only depend on the pressure and it has a chart also. Same as water boiling at sea level at 100C(212F) but at lower temp at high altitudes because pressure is less, same rule applies to refrigerant.

So, using laws of physics you can tell a lot about the operation of the system. For example you can tell if the refrigerant is in liquid form when leaving evaporator and entering the compressor (which would be bad for the compressor).

If you do a web search with keywords 'ac troubleshooting superheating subcooling' you would easily find a lot of good articles. Here is one: http://www.achrnews.com/articles/93445-troubleshooting-with-superheat-subcooling

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    "So, using laws of physics ... you can tell if the refrigerant is in liquid form when leaving evaporator and entering the compressor (which would be bad for the compressor)." That is a good point. Asking the compressor to compress an incompressible liquid is bad news. – Ryan V. Bissell Feb 27 '17 at 18:54
  • That was just a simple example. Knowing the sub/super heat values can tell you if your system is overcharged, undercharged or you have problems in txv etc. So, for example you wouldn't need the system specific pressure/temperature values to tell the status of your AC. Make a web search for some articles about sub cooling and super heating. I think you will find it interesting :) – Evren Yurtesen Feb 27 '17 at 19:07

From my personal shade tree experience this sounds like a properly done job. Good vacuum, proper oil and refrigerant amounts, and blowing really cold air. I would not worry about exact values on the gauges.

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