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  #1  
Old 06-06-2007, 09:32 AM
ksing44's Avatar
1995 E320 SE
 
Join Date: May 2002
Location: Downingtown, PA
Posts: 658
Full AC charge in pounds of pressure?

I checked the pressure in my AC system on my 1995 E320 with one of those cheap gauges on a can that my friend had for trying to fix his car. My pressure was 30 pounds and that was in the green on the gauge. From searching and reading a number of posts, I know the cheap gauge may not be callibrated, that more pressure isn't always better, and that the appropriate way to charge is to evacuate the system and add a specific amount of 134a. I am still curious, however, about what the operating pressure typically ends up to be with a full charge. I just couldn't find that number in any of the posts.

How many pounds of pressure is a full charge? There must be some rule of thum that the shops use to say your pressure is low and you need to get charged.

My AC is cool, although I have not read the actual temperature of the air coming from the vents. It just seemd that my compressor was coming on more than I remembered and I checked the pressure. Now I want to know if I could just add a bit to make things better.

Thanks for your help

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I just couldn't give up on my 1995 E320.

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  #2  
Old 06-06-2007, 09:48 AM
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It doesnt actually work that way... The low side pressure that u are reading will change with the temp of the inside of the car, the outside temp etc.. however on an 80f deg day after the inside of the car is cooled down, and the blower motor has kicked down a bit, the pressure u see is about right.. You really need to have a high side gauge to really be able to tweek it just right. More freon is not always better. More freon = more stuff to carry the heat from the inside of your car to the outside, but more freon = higher pressures and higher pressures mean higher temps. So its a balancing act.. there is a happy medium there and you really need a full set of gauges to get it right. If your AC seems to be working well, I would be tempted to leave it alone. Just adding freon could just as easily make things worse..

Bill
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  #3  
Old 06-06-2007, 09:55 AM
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30 psig with the system off is too low, unless it's very cold out.

My car reads about 75 psig (maybe a bit more) at about 75 degrees with the compressor shut off.
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  #4  
Old 06-06-2007, 10:39 AM
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Should always be checked with the compressor running. Compressor off is really not a good check. 35 psi with System is not too bad.
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  #5  
Old 06-06-2007, 10:53 AM
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Pressures vary with rpm's , too

i always check mine at 1500 rpms... if i am at 30-35 on the low side, i am happy.

i don't really deal with high side pressures.
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  #6  
Old 06-06-2007, 11:20 AM
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I just look at the high side to make sure it's not too high. It can be a great indicator of an improperly working aux fan, plugged expansion valve, or faulty compressor. But as stated earlier, just try to stay around 30 to 35 on the low side and you'll be OK.
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  #7  
Old 06-06-2007, 11:42 AM
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Checking the low-side pressure with the compressor running is not a good test at all. It would be valid on a fixed-orifice system, but we don't have one of those.
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  #8  
Old 06-06-2007, 11:56 AM
ksing44's Avatar
1995 E320 SE
 
Join Date: May 2002
Location: Downingtown, PA
Posts: 658
Temperature of air from vents?

I am getting the picture about it not being as simple as just measuring the pressure on the low side.

For the record, I measured the pressure on the low side with my car running at idle and the AC running. It wasn't especially hot outside and my car wasn't especially hot either, but the compressor was engaged when I read the 30 pounds of pressure with cool air coming out of my vents.

Next I am going to check the temperature of the air coming out of the vents. Any comments about how cold that air should be on a relatively hot day when the system has been cooling for a while?
__________________
I just couldn't give up on my 1995 E320.

I think it might be like always going back to that same bad relationship with an ex girlfriend.
You feel you love them too much, or you are just too stupid to know any better.



Flickr slideshow of my 1995 E320
http://www.flickr.com/photos/24145497@N06/sets/72157616572140057/
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  #9  
Old 06-06-2007, 11:56 AM
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Quote:
Originally Posted by Matt L View Post
Checking the low-side pressure with the compressor running is not a good test at all. It would be valid on a fixed-orifice system, but we don't have one of those.
I agree.
I would trust the eyesight before using low side pressures to determine charge capacity.
You can have a large high side variable with very little change to low side pressure. I would even use aux fan cut-in to verify charge before using just low side pressure readings.
Both gauges tell the systems working, but low side only can be very misleading.
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  #10  
Old 06-06-2007, 11:59 AM
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what is a fixed-orifice system?

I'm not sure what a fixed-orifice system is or how it would matter, but the air coming out of my vents was definitely on the warm side and so I hooked up a gauge to the low side, with the engine idling and the AC on max cool (all of this is per the instructions that came with the gauge) and it read low. I forget how low, but I think was less than 25 pounds and I remember it was definitely in the "charge needed" sector of the gauge. I proceeded to attached a 12 ounce can of 134a with the engine and compressor running and it emptied something more than half (i'm guessing) of the can and now it works fine.

If i remember correctly, he low side should read about 75 psi when the compressor is off but i agree that i think that information is not helpful to determine whether or not the system is low on refrigerant.
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  #11  
Old 06-06-2007, 12:06 PM
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Quote:
Originally Posted by Arthur Dalton View Post
I agree.
I would trust the eyesight before using low side pressures to determine charge capacity.
You can have a large high side variable with very little change to low side pressure. I would even use aux fan cut-in to verify charge before using just low side pressure readings.
Both gauges tell the systems working, but low side only can be very misleading.
I can attest to that. When I checked mine yesterday, the high side went from 150 psi to 250 psi and the low side only bounced back and forth between 30-40 at idle.
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  #12  
Old 06-06-2007, 12:16 PM
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The argument one gets is guys say that the directions on these KITS use just the low side reading , so that should be fine...BUT, on our systems , we are dealing with an AUX FAN that has SPECIFIC specs for proper operation and that means we need an accuraate charge for all systems to work in conjuction of one another ..you wind up with no low fan b/c you are short just a couple of ounces and you can easily wind up with an over-heating engine temp.
The correct way is to evacuate and recharge by capacity spec, but both gauges for topping is fine , as is eyesight if need be..certainly not low side reading only.

These low side only KITS are for generic a/c systems that do not have to contend with aux fan cut-in specs and balances of system pressures as Benz uses.

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