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  #1  
Old 06-05-2006, 06:58 PM
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A/C High Side Pressure 375 Too Much?

85 380SE approx. 125,000 miles converted professionally to R134a by previous owner about 10 years ago. System had never been touched again, never even topped off according to the PO and it was blowing cool when I got it. I topped off approx. 1.5 months ago when the weather was in the high 70s to low 80s, and pressures were all in line with max of approx. 250 to 275 on the high side.

I hooked up the gauges a few minutes ago to see if everything still checks out OK. With engine hot after a 45-min. highway drive, had my wife idle the car at 2,000 RPM in the driveway. Low side reads almost 50, high side reads about 370 (EDIT: I put 375 in the thread title, it is actually 370, but with pressures so high, who's counting?). At idle (650 RPM) the high side drops to approx. 250 and there is a high-frequency oscillation in the gauge needle that smooths out immediately once gas is applied. Ambient temp is approx. 94 degress with 42% humidity (according to both weather.com and the thermometer in my kitchen window).

Am I going to kill this compressor? Or are the pressures just unusually high due to idling in the heat (i.e. way less airflow through condensor versus when driving around). Should I evacuate some R134a out to bring high side back down under 300? I've never left a system with such high pressures before, and want to make sure I don't kill the compressor.

Does the vibrating needle indicate any kind of problem?

Incidentally, the A/C is quite comfortable, but not what I would call "cold" and yes, I am aware of the 80% efficiency of a converted system. It's nowhere near as cold as the R12 system in my 300E. The suction hose does not sweat and is not particularly cold...

Any thoughts? Thanks!
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Last edited by gmercoleza; 06-12-2006 at 11:51 AM.
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  #2  
Old 06-05-2006, 07:40 PM
carson356
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is the auxillary fan coming on? seeing if it is would be a good place to start. it should come on with pressure that high
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  #3  
Old 06-05-2006, 07:54 PM
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Yes, it's definitely on, no problems there.
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  #4  
Old 06-05-2006, 08:12 PM
carson356
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Quote:
Originally Posted by gmercoleza
Yes, it's definitely on, no problems there.
have you tried cleaning the condenser area with the water hose?, may have debris not allowing air to flow
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  #5  
Old 06-05-2006, 09:03 PM
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Condenser is new, was replaced just a few years back and is still clean and shiny. A few bugs in the fins, but nothing out of the ordinary.

Again, the question is not "why is the pressure so high" as much as it is "is the high pressure OK." I'm trying to determine if 375 is safe for the system, or should I evacuate some refrigerant out?

I guess I should just come out straight and ask what is the maximum safe (for the compressor) high-side pressure on this car?
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  #6  
Old 06-05-2006, 09:44 PM
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Sounds too high to me and seems like it is flooding the evap....
On a 134 conversion, you should still see bubbles at eye-sight w/cond. fan on . If the sight is clear, it is most likely over-charged
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  #7  
Old 06-06-2006, 01:59 AM
GVB GVB is offline
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If the system was stock, and it should be, you may have too much zone in it. Low should read about 30-35 at 2K, and in those temps 95 w/45% relative, should be cooling in the low 40's. But that is if you have the correct system. I would reccomend that you find a shop with real ac equipment that can flush the system out, and put R12 back into it. A real ac machine can flush the entire system by adding twice the amount of liquid zone in it, and pumping it through the system backwards for about two hours. 134 works on higher pressures and it tends to destory 12 compressors over time, and then add in a mix of the 12 and 134 oils that dont mix too well. A good flush can save your ac system. That and I laugh everytime I see a sight glass on a 134 system. You charge by a thermomiter and pressures, thats it. You look at the sight glass on a 134 system and charge it that way, you will have too much zone in it everytime.
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Old 06-06-2006, 08:58 AM
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<<You charge by a thermomiter and pressures, thats it. You look at the sight glass on a 134 system and charge it that way, you will have too much zone in it everytime.
>>

That was my point and I agree.
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  #9  
Old 06-06-2006, 09:24 AM
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Quote:
Originally Posted by GVB
If the system was stock, and it should be, you may have too much zone in it. Low should read about 30-35 at 2K, and in those temps 95 w/45% relative, should be cooling in the low 40's. But that is if you have the correct system. I would reccomend that you find a shop with real ac equipment that can flush the system out, and put R12 back into it. A real ac machine can flush the entire system by adding twice the amount of liquid zone in it, and pumping it through the system backwards for about two hours. 134 works on higher pressures and it tends to destory 12 compressors over time, and then add in a mix of the 12 and 134 oils that dont mix too well. A good flush can save your ac system. That and I laugh everytime I see a sight glass on a 134 system. You charge by a thermomiter and pressures, thats it. You look at the sight glass on a 134 system and charge it that way, you will have too much zone in it everytime.
I am well aware of all of this, and am not attempting to debate the merits of R134a vs. R12. I agree returning the system to the original R12 is preferred, and I just might do that eventually since I have all the proper equipment, am 609 certified, and have plenty of R12 on hand. As we all know, converting a system introduces another set of variables so it isn't as easy as looking up data in the manual. My question was whether a high-side pressure of 375 is too high, and that question has been answered. I will evacuate a small amount to bring pressures under 300, as I thought I should have done to begin with. Thanks to all who confirmed for me.
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  #10  
Old 06-06-2006, 09:58 AM
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Quote:
You charge by a thermometer and pressures, thatís it.
I would have to disagree. I charge by weight. I have added refrigerant to systems that are not completely empty by taking into consideration the pressures, ambient temperature and center vent temp. but I think charging by weight is preferable, since it's the only way to put in the exact amount of freon your system calls for.

Consider posting on board in link below, Iíve found it to be very helpful.
http://www.autoacforum.com/categories.cfm?catid=2
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  #11  
Old 06-06-2006, 10:07 AM
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Weight is correct for spec , but can't be used when topping/adding to a system .
I prefer evacuation/weight recharge on any system b/c you do not know if there is air in the system or who/what else has been there before you.
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  #12  
Old 06-06-2006, 10:07 AM
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Seems overcharged to me. Both the high and low sides are reading higher than desired. Overcharging also reduces capacity; that would explain the cool but not cold situation. I would pull out refrigerant until the low side shows ~35PSI while running the engine at 2K RPM. Low side pressure is what controls the evaporator temperature.

In my experience Mercedes specs steep high side pressures. Still, 375 is darn high. You're probably close to triggering the high side cutout. While I'm not familiar with the 126 chassis, the 123 & 124 service manuals would state that ~300PSI on the high side is expected under those conditions.

So long as the high side is acceptable after setting the low side to the proper pressure I wouldn't sweat it. R-134a is going to run higher high side pressures than R-12. My gut feel is that anything below 325PSI under your stated conditions would be OK.

I see we are going to have 98 degree highs later this week; guess you'll get to try it out real soon.

- JimY
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  #13  
Old 06-06-2006, 10:24 AM
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I wouldt worrie about blowing a hose with those pressures. If I am after a hard to find leak, and cant find it, you can evacuate the system and pump it up to 300 and start sniffing. Can't find it then go to 400, you'll find the leak then. If it fails, shouldn't be in the car. Needs new.
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  #14  
Old 06-06-2006, 10:44 AM
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Quote:
Originally Posted by jcyuhn
Seems overcharged to me. Both the high and low sides are reading higher than desired. Overcharging also reduces capacity; that would explain the cool but not cold situation. I would pull out refrigerant until the low side shows ~35PSI while running the engine at 2K RPM. Low side pressure is what controls the evaporator temperature.

In my experience Mercedes specs steep high side pressures. Still, 375 is darn high. You're probably close to triggering the high side cutout. While I'm not familiar with the 126 chassis, the 123 & 124 service manuals would state that ~300PSI on the high side is expected under those conditions.

So long as the high side is acceptable after setting the low side to the proper pressure I wouldn't sweat it. R-134a is going to run higher high side pressures than R-12. My gut feel is that anything below 325PSI under your stated conditions would be OK.

I see we are going to have 98 degree highs later this week; guess you'll get to try it out real soon.

- JimY
Thanks Jim, I always appreciate the insightful manner in which you post. You have confirmed all my thoughts. In the back of my mind I was thinking 35/300 would be ideal but with these converted systems you can never be positive. You have educated me with regard to the low-side controlling evap temp; I didn't know that. I guess I have always focused on the high side pressures out of concern for the compressor more than anything, and tended to disregard low-side.

Interestingly, I checked out the specs on the binary switch for my car which shows a high-pressure cutout of 30bar or about 435 psi. It seems to me that such a high pressure might damage the compressor, but if MBZ thinks 435 is OK, then it must be. I will evacuate this evening after work to bring the pressures more in line with 35/300. You're right - the temps around 98 will be a good test! Thanks again.
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91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280
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  #15  
Old 06-06-2006, 11:06 AM
GVB GVB is offline
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Your r12 high side in a normal range should be around 250-275 with the aux fans on. 12 shouldn't be in the high 300's
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Old 06-06-2006, 11:06 AM
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