Can I use Hi press to test AC?

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Can I use Hi press to test AC?

Postby 2RON » Wed Jun 01, 2016 6:04 pm

My AC system is low on freon. I plan to add from a can to bring it up. Since I will be adding through the LOW pressure port I can't use the low pressure to determine when the system is full. Can I attach a gauge to the HIGH pressure port and use that pressure to determine when the system is full? I have the high pressure numbers from the service manual.
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Re: Can I use Hi press to test AC?

Postby cornbinder89 » Wed Jun 01, 2016 10:22 pm

Nope, the pressure can not tell you if it is full or not, they can only tell you what the temp of the refrigerant is at the point of measuring. When the system is full, you would expect temps to be in "normal" range, but you can't go backwards and deduce the charge from pressure.
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Re: Can I use Hi press to test AC?

Postby Tom Greenleaf » Thu Jun 02, 2016 1:04 am

First you don't know it needs refrigerant yet if had gauges you can get a yellow hose to the small cans and run it thru real gauges. Gauges still don't tell you it properly filled just the pressure are within perimeters for the conditions you are in all an algorithm to know what to expect.

If someone told you what a pressure should be so easily and know it's wonderful it's wrong - junk info.

Post observation and all temps when compressor is engaged and stays engaged at a raised idle. Then there's something to go on.

Always best to find leak evidence if suspected now while there's pressure in it you may find it by oil evidence if anything.

Again - nothing so far says to add refrigerant so get pressures. If not able A/C is a real lousy DIY risk to wrecking things so send it out to be done,

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Re: Can I use Hi press to test AC?

Postby Cusser » Thu Jun 02, 2016 2:25 pm

If your system is low on refrigerant, then you have a leak that must be fixed first.

Agree, refrigerant needs to be added by weight, everything else is just a guess. And usually not a good guess.


Do NOT add anything containing a sealer either.
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Re: Can I use Hi press to test AC?

Postby 2RON » Thu Jun 02, 2016 7:52 pm

I really don't understand all these replies saying not to use 'PRESSURE' to determine if the system is fully charged.
The "GM Service Manual" and ROBINAIR both refer to 'PRESSURE' to determine the state of the system.
I certainly would believe them before anyone else.
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Re: Can I use Hi press to test AC?

Postby cornbinder89 » Thu Jun 02, 2016 9:10 pm

pressure can tell you when things are wrong, but can not be a determining indicator of when things are right.
The reason is simple, the refrigerant passes thru a phase change (from gas to liquid or liquid to gas) The temperature determines at what pressure this happens.
If the temp of the system is below the condensation point, the liquid will have no pressure, so if it is cold enough, you could have a fully charged system and zero pressure. SO if you follow this, you can see that pressure can not be used as a guide to amount, only to temperature
Too many people think of A/C like air in a tire, it doesn't work like that, or we could just use air as the refrigerant.
When you start to charge an empty system, both pressures are below 0, or a vacuum, if you continue to add refrigerant the pressure rises. the refrigerant whether added as liquid or gas, turns to gas very quickly and expands, as the temp is above its condensing temp (unless you are trying to charge at temps well below zero). At some point enough gas is in the system to reach a point where some of it stays liquid as the pressure is high enough to be above the boiling point at that temp. Now how do you know how much more is needed? you can't tell by pressure because that remains the same. you can only tell that you have enough in the system to reach a point where some remains liquid, but no how much liquid is in the system.
If you heat with propane, you know on a summer day your tank can be nearly empty and the pressure can be near or over 100 psi, you can have a full tank in the depth of winter and the pressure can be 15 psi or less, the pressure only tells you the temperature of the propane in the tank, not how much. This is why propane tanks have level gauges and not just pressure gauges.
An A/C system is the same, pressure can tell you temp, but not amount.
Systems need the correct amount to be able to transfer heat, too little and the liquid in the evaporator rapidly turns to all gas and absorbs very little heat, too much and it can't boil off because the low side pressure remains above the desired boiling point.
The system designer has determined what amount is as close to ideal for all conditions.
Like the aforementioned propane tank, what the pressure in the system is varys by the temp where it is being measured. Lets imagine you have a really efficient condenser, and it can cool the high side to 80 deg, the high side pressure than will be 87 psi (for 134a), is that too low, or is the condenser doing a very good job? they answer is it depends on the refrigerants temp and boiling point, not how much of it there is.
In a system that is working correctly, and you measure the air temp, refridgant temp, you would expect to see pressures in a certain range, that would tell you that heat is being moved from one point to another. In a way, it is telling you how efficient the system is preforming, but it can not tell you why it is under preforming. The only true way is by weight of refrigerant, the weight doesn't change with temp or pressure, it is constant by amount.
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Re: Can I use Hi press to test AC?

Postby cornbinder89 » Thu Jun 02, 2016 9:15 pm

One other thing, If you read the GM manual or Robinair, no where will it say charge to XX psi. If it were that simple, then they would tell you exactly what psi a full system was. If they did, you wouldn't be here asking these questions.
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Re: Can I use Hi press to test AC?

Postby Tom Greenleaf » Thu Jun 02, 2016 10:11 pm

NO - it's WRONG - nobody can say by pressures it's properly and exactly charged - no way. They shouldn't tell you that you can and you can - I do "tweek" systems from part charge when the exact charge weight isn't known - talking about now antique vehicles where close was also OK for some - that game is over.

Look at the dial of Robinair gauges and see how much info is on them you can barely make out. They show a temp/pressure relationship as help.

Now it's almost easier to use an infrared thermo and if you know the temp you know the pressure hence know where it's liquid and where it's still a gas. It's a must so 99% of all suggestions should be first find the leak if not behaving by gauges - that you can see -- the pressure are wrong for the performance you have noting all temps and where plus vehicle's RPM. You can't just print that and say what they should be so easily! OMG - it's a two year technical course to fully understand the sciences of A/C not a kit and blast stuff in deal.

OK - This long and concise as can be link show what you would do with a known proper working system to charge it from a well held full vacuum. Please read it.........

http://aircondition.com/tech/questions/ ... Procedures

That without paying strict attention is as close as you'll get for how when you don't have a way to measure in the precise weight.

As Cornbinder said it's very much like propane. You weight the tanks if refillable ones if you have as it's liquid then.

Step one: Find any leaks. Three ways are common are evidence of oil stains on A/C parts anywhere and include the condensate drain for oil there should be none. Next and my personal favorite is an electronic sniffer. No renting those they cost big bucks and have learn how to use them for false alarms.

Plain soapy water can show a larger leak or leaks at service valves well. You check all areas as said, lines, hoses, compressor, evaporator and condenser and items along the way.

When found you evacuate it by recovery and reuse by machine the will measure how much came out or at least recover it. You can buy a scale to charge with your gauges for the source as well using "tare" weight along the way and know. High end machine measure it in and out exactly not practical for anyone not in biz for service routinely.

YOU don't know you have a leak yet. What are the pressures and temps now? Does the compressor stay engaged or cycle when checking? Yes, you could know it's low by all observations. Then you know you have a leak to fix is the point.

Look up top for info already printed at this site as "Knowledge Base" and the chart locked up top of this page shows clearly temp/pressure relationships.

It's doable just not by pressures only so easily. Perhaps I can but I've already written this much and haven't scratched the surface of what I know to do that and wish it was that easy and it's totally NOT,

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Re: Can I use Hi press to test AC?

Postby 2RON » Fri Jun 03, 2016 7:08 pm

From what I am reading is that if my AC system is 12 oz low on freon (of course I wouldn't know the exact amount, but lets say it is) and I am not getting proper cooling I should evacuate the system, throw away 24 oz of freon, and recharge. Sure the leak should be fixed but for the time being I would rather ADD. I understand you people are PRO'S and I certainly will not argue against you. But there are many manufacturers of AC equipment (can dispensers with gauges, etc) who use pressure. So I guess I'll use the Temp/Pressure chart in the GM service manual and add freon with a sealant in it and see what happens. Thanks
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Re: Can I use Hi press to test AC?

Postby GM Tech » Fri Jun 03, 2016 7:45 pm

If you put sealant in it- say goodbye to the system, or anyone to work on it. Sealant is a death trap.

I answer the "amount in there" question by recovering and weighing the reclaimed refrigerant- then after fixing the leak- put the reclaimed refrigerant back in at the specified factory amount.
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Re: Can I use Hi press to test AC?

Postby Tom Greenleaf » Fri Jun 03, 2016 7:54 pm

If you must - just waste the refrigerant and put back the 36oz by your guesses that are invalid it holds 36 oz which is three little cans. Whoop-de-do out ~$30 but you wont know diddle about where any leak went.

You don't even know you have a leak yet!!!!

Pressure could be way off because of fans not right, blend door may be adding heat from heater core which would wreck performance and throw the pressures way off. That has nothing to do with refrigerant - many issues don't and you really don't have a clue yet. The mere thought you could know it's low 12 oz. by pressures is just plain and totally impossible.

OK - You really don't want to take advice so at least follow charging procedures of the link I posted above. This means you need a real vacuum pump to remove all refrigerant and any air from the system at sea level your gauges would read 29.92 or close and hold there for over 1/2 hour wait time. Then under vacuum you can add one little can thru high side as a liquid, engine off after purging the line of air if touched while vacuuming that line has air in it.

Air means moisture which will turn PAG oils into an acid then in some time whole system is junk - 100% unfixable all new part and thousands of bucks - your call.

If you mess up and add too much somehow you can choke a compressor and wreck it as you don't know what the trouble is now so it might suck in liquid. Game over again and mega bucks to fix.

You haven't yet posted any pressures and temps when taken nor said what the vehicle is!

Oh - by the way those kits with gauges on them have junk that claims to seal leaks and add oil unknown so system is screwed if you use those at all - CALLED IN THE TRADE "DEATH KITS." Sold everywhere as a couple times someone gets lucky then blows the system up and can include getting really hurt as it gives you no clue what you've done but add stuff unknown and a gauge that isn't accurate most of the time and at best only useful at 80F and engine at 1,500 RPM and compressor staying engaged.

Not far from where that junk is sold everywhere look for the can the says "Restore" engine restorer. See you don't need to fix a thing just add that to a blown engine and it's all fixed. Who needs to know anything? Read one - the lies and corruption of auto crap is out of control A/C the most costly if wrong the other just don't work. If it sells they make it.

I'm really trying to save your azz some real potential problems from lack of real understanding. Don't be fooled by the web and every bit of bull that claims everything is so easy.

For a laugh I hope watch this dude show you how to wash the inside of a catalytic converter with laundry soap - hell no need to buy new one just wash it! It's one of the web's worst bullsh*t shows and people fall for it. Link.........

https://www.youtube.com/watch?v=5icTmYItwiE

I hope you laugh - that's total crap.

Hey - trying to make the point that there's lots to know and how to use and HAVE equipment to do much of anything. If you'd tell us more we could help you determine if you need refrigerant or have a leak at all - still an unknown,

Tom
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Re: Can I use Hi press to test AC?

Postby cornbinder89 » Fri Jun 03, 2016 8:22 pm

2RON wrote: But there are many manufacturers of AC equipment (can dispensers with gauges, etc) who use pressure. So I guess I'll use the Temp/Pressure chart in the GM service manual and add freon with a sealant in it and see what happens. Thanks

Then why don't they just tell you fill it to xx.x psi and be done? the fact is you can't tell amount by pressure, if you could, both the people here and the tech manuals would say so.
I understand you want to just top it up. Unfortunately there is no good way to top it up by pressures. This is especially true with ccot systems. These systems (and almost all cars made in the last 25 years or so use it) are especially sensitive to refrigerant amount. A little too much or too little and the performance drops rapidly. Very hard to tell if you are a little low or high. If you are way too high or low, the pressures will likely show it, assumeing everything else is right on the money.
As everyone here has said, sealers are a bad idea, and can cause expensive problems.
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Re: Can I use Hi press to test AC?

Postby 2RON » Sat Jun 04, 2016 2:14 pm

OK - I'M CONVINCED. THIS IS NOT A JOB FOR AN AMATEUR LIKE ME. I'M GOING TO TAKE THE CAR IN TO A A/C REPAIR SHOP AND HAVE THEM FIX IT. I BELIEVE YOU PRO'S SAVED ME FROM A VERY HIGH REPAIR BILL FOR MY BLOWING UP THE ENTIRE SYSTEM.
MANY THANKS
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Re: Can I use Hi press to test AC?

Postby Tom Greenleaf » Sat Jun 04, 2016 3:03 pm

Good move Ron: You are NOT at fault for misinformation out there. YOU absolutely could do this but at risks mentioned. We (regulars here) have nothing to gain or lose just state what we know. A/C is probably #1 of DIY things you can screw up and cost THOUSANDS to save a couple hundred bucks sometimes or less?

I mean it. Best of luck in an easy, reasonable cost pro fix. It's the right thing for most to do, Tom
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