A/C Troubleshooting - Low side pressure= 100 PSI

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So I'm troubleshooting my non-functional A/C problem. I jumpered the A/C relay under the hood, and the compressor kicked in as expected. Next, I hooked a cheapo DIY gauge onto the low pressure port, I got a reading of 100 PSI. That is without the compressor turned on.

Is this normal to have such a high static PSI reading? Even on a cool engine at 80 degrees ambient temp, I'm still getting a 100 PSI static reading.

I have some manifold gauges on order from harbor freight and will have more info shortly. Just wanted to verify that I'm not in danger with a 100 PSI static reading.

FYI - I did a DIY can job six months prior and never pulled a vacuum. Can this be the cause of the problem? If so, how to resolve?
 
One common mistake that do yourself folks make is not purging any air out of the hoses before adding refrigerant.

You can find several pressure/temperature charts on the internet.

The system must be running and stabilized before you will get an accurate reading.
 
The low side pressure should drop to ~30-50 PSI (depending on temp, fan speed, engine RPM, etc.) when the compressor is on. The static pressure with the compressor OFF doesn't really mean much except that there's some refrigerant in the system (or refrigerant mixed with air).
 
Static pressure is a very good way to check your 134a system.
BUT!...
The car has to be dead cold, and off.
Even sitting in the sun will have a dramatic effect.
Overnight in a garage or shade [avoid morning sun] is best.
Then, check the ambient temps [outside air temp].
Here are some numbers:
65 outside - 65 system
75 75
85 82
95 86
105 90

Why don't AC shops use this?
TIME!
Who can wait and ensure a dead cold system when there are cars to be worked on quickly?
But it is a very good and useful test.

It does not test components, of course. Just for a good system pressure test.
 
It sounds like you really shouldn't be working on your ac system.

Before you start to add cans of this and short-circuit that, take the time to learn what you're doing first.

This is the classic scenario where someone damages more than they fix because of inexperience.
 
Originally Posted By: mechtech2

Why don't AC shops use this?
TIME!


What if there is moisture in the system? That will throw your pressures off (or maybe someone didn't vac the system properly during the last service and there is air in there as well).

Say you have a system that is prepped properly, no air, no moisture, just happy R134 and a can of oil. Since you are dealing with vapor pressure in an enclosed system, how can you measure the actual amount of R134 in a static system just by temp/pressure? Whether you have 1 drop of liquid R134 sitting in the bottom of the accumulator, or 12oz of liquid R134 sitting in there, the vapor pressure is the same.

EDIT: Just feeling contrary today.
 
Last edited:
Originally Posted By: mechtech2
Static pressure is a very good way to check your 134a system.


I disagree. Static testing tells you almost nothing about how much total charge is present in the system. If there's enough charge present so that SOME amount of it is in liquid form in the drier, then the pressure will be the same for a given temperature no matter how much liquid is actually there, be it the correct amount, far too little, or even a bit too much.
 
Originally Posted By: 440Magnum
Originally Posted By: mechtech2
Static pressure is a very good way to check your 134a system.


I disagree. Static testing tells you almost nothing about how much total charge is present in the system. If there's enough charge present so that SOME amount of it is in liquid form in the drier, then the pressure will be the same for a given temperature no matter how much liquid is actually there, be it the correct amount, far too little, or even a bit too much.



+1. It is only valid to tell if IF you have some amount of liquid phase in your system or if not. If not, it just tells you that a full evacuation and pulldown is necessary.
 
to OP:

To make things right: you really have to pull a vacuum and clear the system before you can proceed to putting in R134A refrigerant. Otherwise: you are wasting your time dwelling on silly postings.

Better yet: go get a competent A/C shop to do the job for you if you aren't capable of doing it properly.

My 2c's worth. It's your money and your vehicle, afterall.

Q.
 
Originally Posted By: antonmnster

This is the classic scenario where someone damages more than they fix because of inexperience.


Hi guys - thanks for all the feedback. This is on my 93 beater altima, so I don't really care if the AC system gets toasted. It already hasn't worked for 2 years so I consider it toasted anyway. I don't want to drop any money at an A/C shop - even to troubleshoot (not worth it), but if I can troubleshoot myself - then I can make an educated decision weather to repair or not.

I'm waiting for the manifold gauges to come in. Will know more once I can get an accurate reading.
 
Originally Posted By: lairdwd
Originally Posted By: antonmnster

This is the classic scenario where someone damages more than they fix because of inexperience.


Hi guys - thanks for all the feedback. This is on my 93 beater altima, so I don't really care if the AC system gets toasted. It already hasn't worked for 2 years so I consider it toasted anyway. I don't want to drop any money at an A/C shop - even to troubleshoot (not worth it), but if I can troubleshoot myself - then I can make an educated decision weather to repair or not.

I'm waiting for the manifold gauges to come in. Will know more once I can get an accurate reading.



Good thinking. Learn on a vehicle where there's not much chance of making things worse, and ask your questions first. Never be a mental chicken and ASSume that something is "too hard" just because you don't initially understand it.

Now that the pep talk is done, don't forget the safety talk:

DO be aware that the high-side pressures are HIGH, and bypassing things like the high pressure cutoff switches and/or relief valves can make things go boom. Escaping liquid refrigerant can also cause flash frostbite. Never connect a charging can to a high-pressure port. Be careful, and read up about the risks. Wear eye protection so you don't get eyes full of hot refrigerant oil or freezing cold liquid refrigerant if a relief valve does pop off or an old, brittle hose splits.

And by the way, just like there's a forum for oil, there are forums for HVAC out there... make use of them!

http://acsource.net/acforum/
 
Originally Posted By: mechtech2
Static pressure is a very good way to check your 134a system.
BUT!...
The car has to be dead cold, and off.
Even sitting in the sun will have a dramatic effect.
Overnight in a garage or shade [avoid morning sun] is best.
Then, check the ambient temps [outside air temp].
Here are some numbers:
65 outside - 65 system
75 75
85 82
95 86
105 90

Why don't AC shops use this?
TIME!
Who can wait and ensure a dead cold system when there are cars to be worked on quickly?
But it is a very good and useful test.

It does not test components, of course. Just for a good system pressure test.

Now that works for me! Simple, and it works.
 
Last edited:
Nice set of numbers for static pressure. I noticed i don't see mechtech2 posting anymore. He was good poster, guess he just dropped off.
 
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