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A/C Recharge System Pressure Question

I have a '97 M3, and today I purchased an R134a recharge kit. I plugged the gauge to the low valve, and I got a 45 PSI reading, which is at the maximum level. Nevertheless, I dispensed nearly one whole bottle into the system. Before I pulled the gauge off the valve, I got a 36 PSI reading. It seems that the A/C blows a little colder, but I'm confused because I thought that adding more freon into the system increases the pressure. Does anyone have an explanation?
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Old 05-16-2008, 09:33 PM
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Old 06-09-2008, 09:26 PM
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Sorry I don't have an answer. But I am about to do the same thing. Where is the Low pressure fill port? My front fan doesn't go on and my tech told me that if there is not enough pressure it will not. So I am going to try to recharge it. My car is also a 97 M3. Kurt.
Old 06-10-2008, 04:50 AM
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This calculator http://www.csgnetwork.com/r134apresstempconv.html
will tell you the pressure in your can at ambient temperature. Should have been somewhere around 80 psig at the beginning. That would also have been the pressure in your system before you started the car and turned on the AC.

The can pressure drops as you empty it because of the cooling effect (which you felt if you held the can in your hand). So long as the pressure in the can is higher than the pressure on the low side of the AC system, refrigerant moves from the can to the AC. I usually put the can in a pan of warm water to keep it warm and speed up the process.

The AC pulls a 'vacuum' on the low side of the compressor, between the compressor and the expansion valve. The longer the compressor runs, the lower the pressure on the low side will be. If you put too much refrigerant in the system, the compressor will see lots of high side pressure and will turn off sooner, the low side pressure won't get low enough, and you won't get as much pressure differential across the expansion valve. That means the air won't get as cold. The other extreme is not enough freon, when the high side pressure does not get high enough, the compressor runs a lot but still can't build enough high side pressure to cool the air.

The best way to fill an AC system is to empty it and put in a measured amount. The second best way is to monitor both the low and high side pressures, and compare them to a temperature chart, as you fill the system. The way that most of us fill our systems is to drive until it fails to cool well, then add a can. If it cools OK, we drive a few more years.
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Kurt - The low side pressure valve is located near the firewall on the left side of the engine if you are facing toward the front of the car. There is a black cap that you must unscrew, and after doing so, you will see the valve. Don't worry about making the mistake of connecting into the high side because that is located near the front of the vehicle just to the left of the radiator. Besides, most R134a kits have a fitting that only lets you connect to the low side anyway.

Manolito- Thank you very much. Your explanation makes sense. I fear that I may have over filled my system.
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Old 06-10-2008, 08:17 AM
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Thank you, The valve behind the headlight was the first I saw and assumed it was the one to use. Thanks again. K.
Old 06-10-2008, 10:36 AM
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Mine took a whole can as well. The strange thing steming from a past post was my fan was not working??? I stuck a screw driver though the grill and gave it some help and it started??? AC seems to be working the same but according to the guage I need more R134. Is my fan going bad or is their not enough pressure to start it? I am going to drive the car and notice the fan more often. K.
Old 06-10-2008, 07:39 PM
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My explanation was a bit simplified. To completely understand what is going on inside there, you have to understand the liquid/gas phase change that takes place in the condenser and the evaporator, but for our purposes, we can gloss over that.

The pressure sensor (the one you can see) has to see pressure or it will open a switch and not allow the compressor to start. This keeps the compressor from burning itself up if all the freon and oil leaks out. That is why you have to short out the plug on the pressure sensor to force the compressor to start when you need to charge a system that is very low on freon. If the pressure were so low that the electric fan would not start, I would expect that the compressor would not start either.

When an A/C system is very low on freon, you have to worry about whether air has leaked in. A/C will NOT work well if it has any air or water vapor inside. It must have only pure refrigerant and the proper oil and nothing else. If you open up your AC to repair a leak, you MUST get it pumped down to a hard vacuum to evacuate all the air and water before refilling it with refrigerant and oil. If you don't, you will never get very cold air out of the vents.
Old 06-10-2008, 08:19 PM
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I'll try to explain the first question you asked.

Temperature and pressure are relative, when the temperature of freon or any thing else for that matter decreases so does the pressure.

The system wasn't operating efficiently until you added refrigerant (Freon is just a trade Dupont trade mark) to the system, so the evaporator was not working at capacity.

The temperature of the can also was warm until the liquid refrigerant in it started to boil off into a vapor, due to you opening the valve releasing it to a lower pressure (the suction pressure of the car A/C).
Thus the temperature in the can dropped, so the pressure did also.

You were adding freon to the low side. So your gauge was actualy reading the pressure in the evaporator.
As you added refrigerant, the A/c system started to go to work.
The temperature in the evaporator started to drop, causing the evaporator pressure to drop.

If you had been using a regular commercial gauge set, the small numbers on the inside of the scale actualy tell you the evaporating temperature for any given suction pressure without even having a thermometer.

All of this gobltygoop (the relationship of gas, liquid and temperature) is called Enthalpy.

If you want to know more;
http://en.wikipedia.org/wiki/Enthalpy


By the way if you do have air in the system, air will not condense into a liquid, the air just takes up space as it circulates through the system, like the equivalent of a big bubble.
You can't get the propper amount of freon into the system so the head pressure (high side) pressure goes off the chart because won't condense in the condenser.

Thats why it won't cool very good with air present..

Hope that makes some since to you without getting too technical.
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Old 06-12-2008, 01:49 AM
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Wow, guys, thanks for replying. Basically, what I gather is that my car has a leak in its system, and the indicator is that air has infiltrated which causes the AC to take longer than normal to cool down the cabin.
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Old 06-22-2008, 08:07 PM
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I fear that we may have only muddied the water talking about air leaking in to the system. That is actually pretty unlikely, happening only if you loose all your freon and system pressure gets down to 1 atmosphere (which usually happens only if someone opens the system up to do repairs).

The only likely scenario for your car having any air in it is if the PO opened it up to make a repair and then just charged it with refrigerant from a can himself rather than paying a shop to pump it down to a vacuum before refilling it. That is unlikely.

More common problems are just low on refrigerant, or trash in the radiator fins, or too much refrigerant. I suppose that the compressor could be failing, but the compressors that I have seen fail have either pulled really hard on the engine or they started leaking around the shaft, or both.

My daughter told me this weekend that her '97 328 A/C works only when the car is moving at highway speed , not when it is stopped. I will check the switches and so forth, but if her electric fan is shot, she is just going to have to get used to it, because I have a list of car repairs waiting for $$ that are more important than A/C.
Old 06-22-2008, 09:15 PM
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There is a thread in E46 forum that needs attention if you AC guys have any suggestions for him.

AC not working
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Old 06-23-2008, 10:56 AM
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Air can get into the system if the system has a leak on the low side and is allowed to run in a vacuum. But that generaly won't happen unless somone has jumped a pressure switch out.
Also, not purging your hoses prior to charging is a good way to get air in the system.
But, Non condensables (air mostly)are very easy to get out, without dumping the charge, if you have the right equipment.
In reality the do it yourselfer is better off to just dump it, vacuum it (which presents another problem, getting a pump) and re-charge it.

On second thought, If you do have air, you can't really do it right without a substancial investment in tools and equipment, take it to a shop.
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Old 06-24-2008, 11:04 PM
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Thanks, Coolbear1. So, what you're saying is that the DIY kits will not do the job right; one must extract everything out of the system (freon and air), and then use special equipment to fill it back up because the Walmart kits will introduce air into the system. Is this right?

One thing doesn't make sense: How can air penetrate a system that is under pressure? Unless the R134a cans are a mixture of freon and air, how can air get in?
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Old 06-25-2008, 06:50 AM
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The DIY system can be used without introducing any air. You just have to purge the air out of the hose (you'll loose a little freon) before you attach the hose to the low side fitting.

There is no air in the can with the freon.
Old 06-25-2008, 08:52 PM
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Manolito, are you refering to the hose coming from the can of coolant and that attaches to the low side?
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Old 06-25-2008, 09:09 PM
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There is air in the hose if you don't purge it properly. What else would be in it when it is just laying around.
It is still going to be in it when you srew it to the car.
Then when you open the valve on the refrigerant can you force it in withe pressure of the refrigerant.

The proper way to charge anything is as follows;
1. Connect the hose to the can tap valve.
2. Screw on the can, but don't pierce the can with the valve yet.
3.Connect the other end of the hose to the low side connection on the car. ( this will pressurize the hose from the refrigerant in the cars A/C system.
4. Unscrew the hose connection at the can tap valve slightly.(just crack it untill refrigerant is comming out for a moment.

This will purge the hose, and youy can now safly tap the can and start adding refrigerant to the car without fear of air, or moisture contamination from the hose.


As far as air goes in most cases, the way to tell, is if you try to charge the system and you can't get the suction pressure up enough to cool the car good, without the high side getting too high, you most likely have air.
If you keep adding refrigerant to it, the suction pressure will come up, the discharge pressure will continue to climb, the car may cool somewhat at some point, but the compressor will most certainly eventually die.

If the thing charges up, cools good,(as is usually the case) and the high side is where it should be, don't worry about it. Life is good.

But, if you do have air, your options are;

1.Buy a $700. refrigerant reclaimer and a $60. reclaim cylinder.
Then you can pull $20 worth of R134A out of the car along with the air and put it into the cylinder. Refigerant is heavier than air so just crack the valve on the tank and the air comes right out the top.
You now have $20 worth of good refrigerant, and some expensive equipment you can sell on Ebay, or you may use every 3 to 5 years.

2.You can just let the refrigerant out of the car, but the air is still there.
It has no reason to come out.
Then you have to get it out with a vacuum pump, about $350.
Same problem. Just not cost effective to have at home in the garage.

3. Take it to a reputable automotive A/C shop. Around $150 total to vacuum and recharge, including a reciever drier.

I have all that stuff, I probably have worn out 50 vacuum pumps, and 5 recaimers, but I have been doing it for a living for 35 years.
Not cars though, homes and buisnesses.
The theory is identical in both.
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85 325e bought new, totaled after 20 years & 465K miles, trouble free service.
01 Ford Ranger 4dr stpsd 4wd. I drive,here
88 325i convert. only 98k miles,gone, sold
93 325i.here, 98 Z-3 roadster. gone, 08 128 cabro gone

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