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Old 06-09-2002, 08:46 AM
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Join Date: Apr 2002
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Question a/c hi side/ low side

My a/c compressor is bad. I bought a new one and a dryer, and an expansion valve. I also bought a R-134a retrofit kit to charge it with. Question, which is the hi and low side? Also, when I install these parts should I get it evacuated before I charge it? I am under the impression that all the moisture and air need to be removed or internal contamination will occur. Thank you for your help. 82 300 SD.
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Old 06-09-2002, 10:55 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I usually respond to such questions with my best answer figuring that there are many who could use the info even when the nature of the question screams that the best advice would be to stay away.

Anyone not knowing which side is high or low, should not be working on airconditioning. Since its obvious that such advice will go unheeded, I guess an old anarchist like myself can see good reason for legislation to keep fools from hurting ones self. Unfortunately that was not the reason for legislation on handlers of R12, because they made these toy kits and an even more dangerous (but evironmentally safer) refrigerants with no rules.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 06-09-2002, 11:35 AM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,878
I have to fall in here with Steve on this one. I am a pretty active diy'er and do not charge a/c systems (nor do I do alignments). Even if I have to change components out I will take it in to my a/c repair shop, have the system evacuated, change parts as necessary and take it back for re-charging. They have the tools and equipment to properly and safely charge the system and make it right.
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Old 06-09-2002, 01:34 PM
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Although I do extensive A/C work as a DIYer, I had refrigeration training many years ago in conjuction with some instrumentation work that I was doing. In addition to that, as a youth, I helped my Dad with air conditioning work. He took a three day course in about 1956 when there was a hot market for aftermarket auto air conditioning. He made a good supplemental income installing auto air conditioners after regular shop hours. I remember he said that he could clear about $50 and could usually install one in an evening after closing the shop. $50 was quite a bit of cash in 1956, when soft drinks and candy bars were a nickel. I was only seven, so I don't know the costs of anything else, except that one dollar was an enormous amount of money.

I believe that the flag that triggered Steves appropriate response was two fold; you were talking about using the changeover kit AND you didn't know how to identify the high and low side. If you have even a very basic knowledge of refrigeration, you can examine ANY refrigeration system and identify the high and low side.

I expect that most community colleges have some good continuing education courses on refrigeration. It's not a difficult thing to learn, but as Steve pointed out, it is important to know what you're doing for personal safety as well as to ensure that you don't make things worse on the equipment that you touch.

Good luck,
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Old 06-09-2002, 03:06 PM
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Join Date: Apr 2002
Posts: 7
Gentlemen, it was a simple question. I am an extremely competent DIY'er. I had it to a shop and it had a leak in the compressor seal. There is no refridgerant in the system. I had it overhauled just two years ago at an MB dealer for $1200. Not only did they fail to put the spacers in the mounting bolts, allowing the belt to run true, the following day, the belt broke, it took out one of the trans lines leaving my wife stranded on the side of the road. They did put the correct spacers and a new belt, but I had to pay the tow bill, trans hose and fluid. After this, I would not even consider letting an incompetent MB dealer mechanic change a fuse. Have a nice day!
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Old 06-09-2002, 06:34 PM
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Join Date: Apr 2001
Location: Baton Rouge
Posts: 485
I have to jump in here , Steve was correct with his saying that a DIYer who doesnt know the high side from the low has no reason messing with this system. Here is why . I worked for a ( mechanic) who had his own shop ( shadetree) when I was in high school. He payed me to pull engines ect. To the point , he was charging a system one day on a car and we heard a loud (pop). He didnt know his high side from the low on that certain car and he was charging through the high side when his quick charge bottle bursted. From then on he never worked on a car again due to he lost his left hand. From then on I took every precausion when working on systems whether it be SRS, or A/C. I love what I do and I want to do it until I die. But I dont want to be the fool that gets killed from stupidity, My advise if you dont know the system dont play with it . Money you can make to fix your car , but you cant fix what happens when you make that fatal mistake cause ( your not sure).
euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck
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Old 06-09-2002, 06:48 PM
jbaj007's Avatar
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Join Date: Jun 2001
Location: Santa Monica, CA
Posts: 2,053
Nick, hang out here for a while:
The high side Schrader valve is on the driver side of the condensor; the low side Schrader valve is on the inboard side of the driver side wheel well. The above referenced site is a wealth of MVAC info.
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)
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Old 06-09-2002, 07:10 PM
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Join Date: Apr 2002
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Thank-you very much, jbaj007! I will go to that site first next time.
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Old 06-10-2002, 02:30 AM
Posts: n/a

I hope you don't think we are trying to chastise you here. We are all just concerned with the safety of our fellow mshop folks. The is an excellent site for auto a/c advice. If you want to safely continue with your a/c DIY I encourage it. But I would suggest that you learn the basic theory of refrigeration for your own safety. The can help you with specific questions, but you really need to learn the theory first.

If you will read the first few chapters of a refrigeration book, I think you will see what I mean. Once you know the basic principle it will open a new door for you. and the will help you with specifics.

There is a basic circle or diamond shaped diagram that is used to understand any system. It starts with the compressor compressing, then the pressurized gas goes to the condensor where it returns to liquid state then through an expansion valve or capillary that "sprays" the liguid into the evaporator where the expansion at that point causes a cooling effect, then the gas with heat romoved goes back to the suction side of the compressor and the process starts over again. I suggest that you read up on this principle and then you'll be much better prepared to learn and do.

Best of luck,
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Old 06-10-2002, 10:42 AM
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Join Date: Apr 2002
Posts: 7
I appreciate that response Larry, the two things I don't usually mess with are the inside of an auto trans and A/C. But, screw me once shame on you, screw me twice, shame on me. I spent 20 years in the service and if something was broken, I would fix it, but only when I fully understood what I was dealing with. At this point, I would be the fool to take it back to the MB dealer. It was a total lack of competence and responsibility on their part. Sorry if I sounded like I was on the defensive, but I can't stand it when someone tells me I can't do something.
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Old 06-10-2002, 11:05 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,513
Nick - Noone here is trying to discourage you from working on your own a/c system. The message is more along the lines of "learn first, dive in second." A/C is not a learn as you go kind of proposition.

That said, getting a handle on the basics is not that difficult. Haynes publishes a generic automotive a/c manual that will get you started. It contains a good overview of the theory and practice, lots of service and troubleshooting tips, etc. About 10 bucks.

You'll need specialized tools to diagnose & service the system - a set of guages/hoses, can taps, and a thermometer at a minimum. These can be chased down for about $100. If disassembling the system, it needs to be evacuated before charging - a vacuum pump is a necessity. You'll need to budget $200-$300 for one, unless you can scare up a good used unit. An air compressor is also necessary if the system needs to be flushed - though a very small unit will suffice for this task.

So depending on the complexity/severity of the problem, the tools may cost more than simply paying for one repair. If you don't like the MB dealer, perhaps there is a competent shop which specializes in a/c?
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Old 06-10-2002, 07:24 PM
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Join Date: May 2002
Posts: 105
Could some one post photos of all the different locations that fill nipples could be located, both high and low pressure. Looking at my a/c system, the only 2 nipples that I see have the blue low pressure cap on it, I can't seem to find the red high pressure cap. I do know this system was converted over by a shop to r134a.
Thank you
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Old 06-10-2002, 10:10 PM
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Join Date: May 2002
Posts: 105
nm....I figured it out, had to look at the system and uuse gauges.
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