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  #1  
Old 06-26-2008, 09:10 PM
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Help with my a/c pressures...

I have rebuilt most of my system.
Installed a parallel flow condenser.
Put r134a in it and saw temps of about 47F at the vent.
I didn't like that so I took the r134a out and put r12 in.
I use BVA Auto 100 from ackits which states the oil is compatible with both r134a and r12.

My 84 300SD says it holds 2.9lbs r12. I have less than that in there right now and I'm seeing high side pressures of well over 300psi. My low is around 25psi at.

Ambient air temp is 85F.
I am seeing 48F at the vent on the highway.

I took it for a drive with my gauges taped to the windshield and the high side needle was jumping all over the place.

I put 3 12 oz cans and started a fourth one. So I have somewhere around 40oz or about 2.5lbs.

Why are my pressures so high when I don't have a full fill?
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  #2  
Old 06-26-2008, 10:31 PM
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The label attached to your car is no longer accurate for refrigerant fill since you changed the condenser to a different type. The new one is probably smaller inside, but that is by far not a sure thing.

You figured out that I wasn't joking after all, about taping the gauges to the windshield.

I suspect an airflow problem. Since you have a different condenser, I must ask this: how good is the seal between the condenser and radiator? And did you clean the radiator airway?
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  #3  
Old 06-27-2008, 01:17 AM
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Quote:
Originally Posted by Matt L View Post
The label attached to your car is no longer accurate for refrigerant fill since you changed the condenser to a different type. The new one is probably smaller inside, but that is by far not a sure thing.
Yes, but shouldn't it be a higher amount if anything? Since the pfc is more efficient I assumed it would have a higher capacity than a serpentine one. I only say this because my pressures seemed higher than they should be as I was adding the freon the entire time, not just towards the end.

After writing this thread I took it for another drive. The high pressures were lower, hovering around 250 and up to 300, sometimes spiking up to close to 400. The low side was anywhere from 20 to 30.

Perhaps I have too much freon, but I'll check elsewhere first.

Quote:
Originally Posted by Matt L View Post
You figured out that I wasn't joking after all, about taping the gauges to the windshield.
Yes, I did. It did seem a bit odd at first but when I ran it under the hood I found they would sit nicely on the windshield. I did tape them but it seems the tape was unecessary. Thanks for the tip.

Quote:
Originally Posted by Matt L View Post
I suspect an airflow problem. Since you have a different condenser, I must ask this: how good is the seal between the condenser and radiator? And did you clean the radiator airway?
Please explain more. This sounds like a good thing to check into.
So you are saying my condenser should be in direct contact with my radiator?? I don't believe they are but I'll have to check tomorrow.
I don't believe I cleaned out the radiator airway. I'll do that tomorrow.

Thanks for the help.
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  #4  
Old 06-27-2008, 08:34 AM
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an airflow problem, meaning you may not have fans operating properly pushing/pulling enough air to condense the refrigerant.
the pfc condensers are very efficient at removing heat, but they do not have a large capacity for liquid. I wonder if there is a larger reciever that can be used.
also, you had 134 in, then changed to 12... did you change the reciever too?
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  #5  
Old 06-27-2008, 10:48 AM
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Quote:
Originally Posted by vstech View Post
an airflow problem, meaning you may not have fans operating properly pushing/pulling enough air to condense the refrigerant.
the pfc condensers are very efficient at removing heat, but they do not have a large capacity for liquid. I wonder if there is a larger reciever that can be used.
also, you had 134 in, then changed to 12... did you change the reciever too?
I did not change the reciever. I did not open up the system so I did not think it was necessary.

My fan is operating. I thought Matt was describing more of a blockage of sorts, not a fan problem. Such as the air is stopping around the condenser because it is not in contact with the radiator or it is not in-sync with the radiator.
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  #6  
Old 06-27-2008, 11:23 AM
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It's normal for pressures to jump around. Two things which affect high side pressure are compressor output and airflow across the condenser. Both of these vary independently. Compressor output varies with engine RPM, airflow across the condenser varies (mostly) with the speed of the car. So it's normal for pressures to spike pulling away from rest - compressor output is high, airflow is low. In contrast, pressures should stabilize at a 40MPH cruise - compressor output is moderate and steady, airflow is good. What is your high side pressure under these conditions?

When you see 47 degree air temps, what is the fan speed. On high fan that is quite good for an old R4 car. If medium or low speed, not so much.

Have you looked into the fresh/recirculated air flaps? I'm not as familiar with the 126 setup, but if you are trying to cool fresh air rather than reciculating at least the majority, you won't get very good cooling or dehumidification.

- JimY
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  #7  
Old 06-27-2008, 11:41 AM
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Actually, compressor output for our cars varies with both engine speed and evaporator heat intake. We have variable-displacement compressors.

The condenser needn't be in direct contact with the radiator, but you should have some sort of shroud to help the condenser get all the air that it can.

As for the "capacity" of the system, the PFC has a higher capacity to dissipate heat, but probably a lower capacity for holding liquid refrigerant.
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  #8  
Old 06-27-2008, 02:28 PM
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Yes, you should have changed the reciever/drier. Also, did you replace the expansion valve? When you removed the r134, did you evec the system again?
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  #9  
Old 06-27-2008, 06:24 PM
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[QUOTE=Matt L;1895342]Actually, compressor output for our cars varies with both engine speed and evaporator heat intake. We have variable-displacement compressors.
QUOTE]

Not on this car you don't. This baby uses the classic Delco R-4. It's about 20 years too early for a variable displacement compressor on an MB.

- JimY
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  #10  
Old 06-27-2008, 07:06 PM
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[QUOTE=jcyuhn;1895674]
Quote:
Originally Posted by Matt L View Post
Actually, compressor output for our cars varies with both engine speed and evaporator heat intake. We have variable-displacement compressors.
QUOTE]

Not on this car you don't. This baby uses the classic Delco R-4. It's about 20 years too early for a variable displacement compressor on an MB.

- JimY
That's good to know. When did the variable-stroke compressor make its entrance?
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  #11  
Old 06-27-2008, 07:17 PM
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http://books.google.com/books?id=4rgbtVJ2-h4C&pg=PA284&lpg=PA284&dq=V5+air+compressor+introduced&source=web&ots=9rkbj2oTSu&sig=N_TMORn8mugtJB0Fy8-si-naRy4&hl=en&sa=X&oi=book_result&resnum=7&ct=result
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  #12  
Old 06-27-2008, 07:29 PM
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I should have been more specific. When was the variable-stroke compressor introduced in a Mercedes-Benz?
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  #13  
Old 06-29-2008, 02:07 PM
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Quote:
Originally Posted by Matt L View Post
I should have been more specific. When was the variable-stroke compressor introduced in a Mercedes-Benz?
It seems to have been done with new model introductions, rather than an across the board change. Every 210 chassis I have seen uses the variable discplacement compressor. Every 140 chassis as well. I don't know about the C-class, but would guess the 202 had it when introduced. Basically all new models introduced in the 1990s came with it.

- JimY
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