High-side Pressure is Low But Air Vent Temp is Normal

[thartman]

Hi,

I have a 2009 Hyundai Elantra GLS and I've been having some issues with my A/C that runs on R-134a.

I noticed at the end of last summer that it wasn't blowing as cold as it usually does. I started as a full-time Automotive student at a community college last summer and one of my instructors (ASE Master Tech) did a full evac/recharge to 1.16 lbs (factory load) on 8/22/22 but it still didn't feel like it was cooling as much as it should (I didn't test with a thermometer because I was a newb and it didn't occur to me). Yesterday (after putting on another 8,000 miles since 8/22), I performed standard tests at school to try and determine what might be wrong.

-Compression test (after reaching operating temperature) showed low-side at 29 psi and high-side at 110 with ambient temperature of 60 degrees Fahrenheit
-We did a full evac/recharge (it recovered 1.12 lbs, so a loss of 0.04 since 8/22), held it under vacuum and injected UV dye to perform a leak test. I checked with a UV light afterwards and there are no leaks.
-Compression test afterwards (after reaching operating temperature) showed low-side at 28 and high-side at 74 with ambient temperature of 53 degrees
-We placed a thermometer in the air vent and it was dropping to 40 degrees Fahrenheit, so that was normal
-I checked for leaks again several hours later when I got home and still no leaks.

So, my low-side is normal, but my high-side is much lower than it should be. It took me a while to find a R-134a pressure chart for temperatures below 65 degrees, but that also showed my high-side is low.

Does anyone have an idea as to why my high-side is so low?

Any assistance would be greatly appreciated.

Thanks!

[JohnHere]

The low high-side pressure could indicate a compressor problem, but it's very difficult if not impossible to pressure-test and performance-test a system at such low ambient temperatures. I suggest waiting until the ambient temperatures in your area rise to 80°F or above, and then proceed with diagnosing the system. I normally don't pay much attention to P/T charts.

Did you recharge the system before injecting the UV dye, or was it only the dye that was in the system when you leak-checked it? Also, it's "pressure-testing the system," not a "compression test," which is something entirely different and not related to MVAC.

[thartman]

The low high-side pressure could indicate a compressor problem, but it's very difficult if not impossible to pressure-test and performance-test a system at such low ambient temperatures. I suggest waiting until the ambient temperatures in your area rise to 80°F or above, and then proceed with diagnosing the system. I normally don't pay much attention to P/T charts.

Did you recharge the system before injecting the UV dye, or was it only the dye that was in the system when you leak-checked it? Also, it's "pressure-testing the system," not a "compression test," which is something entirely different and not related to MVAC.

Yes, the system was recharged, my instructor was watching me.

And, thanks for correcting my vocabulary. I am constantly using the wrong words since I'm learning all of this a little at a time and I've asked my instructors to correct me whenever I say the wrong thing. The instructor that was helping me yesterday was messing with me and asked me to get him a can of compression. I thought he meant a can of compressed air and started walking of to get it before he let me in on the joke.

I took Engine Service 1, Electrical, Powertrains and Brakes last semester. I wanted to take Heating and A/C this semester, but I couldn't make it work with my schedule, so I'm having to learn through research afterhours. I really appreciate your help.

[JohnHere]

I noticed at the end of last summer that it wasn't blowing as cold as it usually does. I started as a full-time Automotive student at a community college last summer and one of my instructors (ASE Master Tech) did a full evac/recharge to 1.16 lbs (factory load) on 8/22/22 but it still didn't feel like it was cooling as much as it should (I didn't test with a thermometer because I was a newb and it didn't occur to me).

The specs that I have also call for a maximum of 1.16 pounds of R-134a. The minimum is 1.05 pounds. So the charge amount is within the manufacturer's tolerances, even taking into account the 0.04-pound (0.6-ounce) perceived loss on recovery. The specs also call for 5.07+/-0.33 fluid ounces of PAG-46.

So, my low-side is normal, but my high-side is much lower than it should be. It took me a while to find a R-134a pressure chart for temperatures below 65 degrees, but that also showed my high-side is low. Does anyone have an idea as to why my high-side is so low?

Probably due to the below-80°F ambient temperature as already discussed.

[tbirdtbird]

"Probably due to the below-80°F ambient temperature as already discussed."

I will second this. We seem to have a rash of questions on here about "abnormal" pressures and temperatures and performance when in fact the posters are below 80F. Surely your instructor is aware that no meaningful testing can be done at anything below 80F. The reason for this is because at such low temps, no meaningful work is being done by the system.

And def get your vocab straightened out....the sooner the better. Start reading case studies at night. You can also read case studies on this forum and go back more than 20 years in the archives and learn a hell of a lot

[thartman]

Surely your instructor is aware that no meaningful testing can be done at anything below 80F. The reason for this is because at such low temps, no meaningful work is being done by the system.

Hi, tbird.

Thanks for the response. I did check with my instructor after JohnHere's response, and he is aware of the futility of further testing, but wanted me to figure that out for myself.

One of the reasons I like him so much is that he doesn't just answer every question for me. He helps me get started by showing me (and explaining the purpose of) various testing procedures, allows me to bang my head against a wall for a bit before stepping in and suggests that I find other resources in the outside community in order to expand my network and knowledge resources since I won't be a student forever. That was what led me to find this forum and why I really appreciate you and John offering your knowledge and advice.

I took my first automotive class because I got tired of being ripped off by dealerships whenever I needed to take my car in for service, and wanted to learn how to do some things myself. I continued taking classes because I had never felt so welcomed by a community that I had no previous experience with, and I could see how much all of the instructors cared about the success of their students.

All of them are ASE Master Techs and could be making a lot more money doing nearly anything else in the automotive industry, but it is their desire to help the generations following theirs that has kept enrollment maxed out every semester. True Story: One of the kids I frequently partner up with wears a required school uniform shirt that is a slightly different design than the rest of ours. I asked him about it and it turns out that his father was a graduate of the program over 20 years ago and he wears his dad's old shirt to school. And, our lead instructor was one of his dad's instructors back in the day.

So, again, thanks for your advice both to me and everyone else that has visited this forum. I'll make sure to recommend this forum to any of my classmates that gets stuck with an A/C problem, and I'll check back in 3 or 4 months once summertime weather allows for further testing.

[JohnHere]

So, again, thanks for your advice both to me and everyone else that has visited this forum. I'll make sure to recommend this forum to any of my classmates that gets stuck with an A/C problem, and I'll check back in 3 or 4 months once summertime weather allows for further testing.

Great backstory. Thanks for that.

And appreciate your recommending this Forum. Come back anytime.

[tbirdtbird]

Also, Tim, the site owner, has great prices on great parts at ACKits.com. You may want to spread the word on that , also