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  • Machine work done on shop pickup AC

    Wellllll---at least we machined an adaptor for connecting/evacuating the AC.

    Evacuated and recharged AC system using 27 oz. of R134a, producing 40 F at dash register but low side pressure still short of optimum, according to chart. System quoted to hold 33.44 oz. Added 12 oz for a total of 39 oz. installed, but low side still 5 psi or so below optimum spec.

    AC working great, ie. 38-39 F at dash outlet register, 31 psi low side and 70 F ambient.

    Going into the desert now-------Is the 'overcharge' of 5 oz. going to bite us when the ambient approaches plus 100 F?

    --G

  • #2
    One of the several add-on units I installed in the early '70s worked fine for several months of spring and early summer driving, but blew a hose when I drove it on a 100+ degree day.

    So, higher ambient temps can cause the system pressure to go up, but whether it blows up depends on many factors.

    I'd suspect overcharging will reduce the cooling capacity of a system, since it reduces the pressure differential across the capillary and through the evaporator.
    Any products mentioned in my posts have been endorsed by their manufacturer.

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    • #3
      I work on electrically driven freon systems on aircraft. Many of them will tell you to under charge a system if extremes of temperature are expected,
      eg. > 100F. High side pressure becomes such that the electrical load will blow the circuit breaker, or you get belt slippage. I think a 30 degree delta evap/ambient is pretty good though.

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      • #4
        +1 for a slight under charge.
        I just need one more tool,just one!

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        • #5
          Going into the desert now-------Is the 'overcharge' of 5 oz. going to bite us when the ambient approaches plus 100 F?
          More than likely to cause you some issues...sooner or later. If the system is over-charged expect a noisy compressor first followed by system failure.
          Much safer and easier on the system to slightly under-charge than over-charge. Efficiency will also be less when over charged.

          System head pressures will virtually double between 70°F and 100°F!
          This with the correct charge of R134a.

          Have a look at the chart below.

          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories

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          • #6
            yes leaning towards minimum is much easier and safer on everything - plus i hate spending extra money to ruin things - that's just salt in the wound...

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            • #7
              Agree with the others, slightly under is always better than over, just from a seat of the pants point of view, R-134A has always seemed a little more sensitive to overcharge than R-12 to me, as far as comfort. I doubt you will have any trouble if the system is in good shape.
              James

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              • #8
                Doing the same thing

                1996 F350.
                New Denso compressor, accumulator, orifice tube.
                Had it evacuated and charged at a shop.
                I get it back and the output air is 65 degrees F.

                Ambient is 100 deg F

                Low side is 58 psi
                High side is 245

                Is this system overcharged?

                Comment


                • #9
                  I'm guessing yes, it is overcharged. Even for the ambient temperature, the head pressures are saying overcharge. The big question is; do you have too much refrigerant in the system, or too much oil. The system is designed to hold a specific amount of oil and refrigerant. Too much of either is going to cut cooling efficiency, even though some experts claim an automotive A/C unit that can pull the duct temperature down 40 degrees from ambient is working properly.
                  Did whoever did the compressor flush out the excess oil in the condenser & evaporator, before it was evacuated and recharged? I've seen as much as 8 ounces of excess oil in the condenser and evaporator, especially on Fords.
                  You should be experiencing a laboring compressor, possibly a noisy one, too. R-134a is particularly sensitive to over-under charging. Sometimes an extra ounce or two of oil can raise head pressures noticeably and increase duct temperatures inside the cab.

                  I used to maintain a whole fleet of police cars and road department trucks, and we're out in the desert, where A/C isn't a luxury, it's the difference between life & death at times. Good A/C performance was usually one of the first and loudest things a driver would complain about (and you don't want to piss off guys who carry guns & drink a lot of coffee)

                  Undercharging has ruined more compressors than anything. In the "olden days" guys used to recharge A/C systems with those little 12 ounce cans of freon. Unfortunately, it was widely believed that one can was a pound of refrigerant (a pound is 16 ounces)....So, a lot of A/C systems got seriously undercharged. And, a lot of A/C compressors got murdered. When the industry went to R-134a, the frequency of A/C compressor failures went off the scale. Manufacturers of A/C components scrambled to discover why.
                  What they found was; by nature, R-12 and R134a both carry the lubricating oil through the A/C system. When the charge of refrigerant is low (undercharged) the refrigerant has a tendency to "superheat" and the oil drops out of the refrigerant, and accumulates in the low parts of the system. Thus starving the compressor for oil....The compressor soon fails, and everybody stands around shaking their heads and wondering about the quality of the re-manufactured A/C compressor.

                  Charging ANY A/C system requires a clean, and completely empty system to begin with. Oil is carefully measured and added to the accumulator, evaporator, condenser, and compressor as prescribed. Only then is the system evacuated, and recharged. During recharging, the refrigerant must be measured very precisely or the system performance will suffer. R-134a is especially sensitive to the quantity in the system.
                  Referring to your truck, Rex, I've done similar trucks, when I was working, and usually had pressures and temperatures as follows: Ambient 100+
                  low side 33 psi....High side 270 psi...inside duct temperature 39 degrees F.
                  You may see a slightly higher duct temperature due to humidity.
                  I'd take it back and "pitch-a-bitch".....
                  Last edited by saltmine; 06-30-2011, 08:05 PM.
                  No good deed goes unpunished.

                  Comment


                  • #10
                    Saltmine, thanks for the detailed answer.

                    The system was not flushed, as no debris was found at the orifice, and the compressor did not fail in service
                    New compressor was shipped with oil in it.
                    Additional 2 ounces of Ice-32 was added at the accumulator.

                    So, excess oil does sound like the culprit.

                    Still they should not have given it back with the pressures wrong and the temp not cool enough.

                    Rather than deal with incompetents again, I'll likely re-do it myself. I have all the equipment

                    thanks again

                    Rex

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                    • #11
                      Originally posted by Guido
                      ...Going into the desert now--------- .
                      Anybody heard from Guido?
                      Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                      ~Practitioner of the Electromechanical Arts~

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                      • #12
                        As I mentioned numerous times, Rex, the automotive industry is rapidly running out of skilled technicians (mechanics). Factory and independent shops refuse to pay most of them anything that resembles a "living wage", and they tend to do one of two things: quit, or continue on, carelessly performing the work, and not being properly paid for it. Unfortunately, the customer is to guy caught in the middle. Trade schools have attempted to fill the gap as older, more experienced guys retire or drop dead in their stalls. But most trade schools are hard pressed to keep up with technology and their business model is to get the "raw meat" in the door and get their money...first, then try to train them on old manuals, outdated equipment and instructors that haven't been near any of the newer vehicles, let alone worked on them. The finished product is invariably a large group of poorly trained kids, with a piece of paper and a huge debt to pay off. These poor souls are tomorrow's automotive technicians. Which is probably why your truck left the shop with the pressure readings all screwed up and nobody checked the temperature gradients or system performance.
                        BTW, without flushing, old oil still exists in the low points of any A/C system. Experienced A/C guys either will flush out the residual oil, or compensate for it during charging. Working in the fleet shop, I had a guy who was reputedly believed to be a Ford "expert" (he had 44 Ford training certificates)
                        He called me over to look at a Ford Taurus he was working on, that the A/C refused to produce cool air. He had evacuated and re-charged the system many times during the day, and the A/C performed worse every time....so...as a last resort, he asked me to look at it (BTW, I don't have ANY Ford training certificates) After examining it, I evacuated the system, and removed the lowest line on the evaporator.....and drained a quart and a half of refrigerant oil into a catch pan. After draining another pint of oil from the condenser, the system was evacuated, and recharged.....and magically started working like new....

                        I think I saw Guido...driving along with his windows all frosted up.
                        Last edited by saltmine; 07-01-2011, 11:31 AM.
                        No good deed goes unpunished.

                        Comment


                        • #13
                          Saltmine, your assessment of the industry is spot on. My POV is from being in the parts aftermarket all my life.

                          As for flushing, what's the best way to do it, and what fluid? We sell specific flush solvents, expensive stuff, and I suspect it's mostly alcohol.

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                          • #14
                            Originally posted by Rex
                            Saltmine, your assessment of the industry is spot on.
                            Absolutely.

                            At one time I had worked with a couple of (recent) tech school graduates that could not remember which way to turn a wrench. They knew how to turn pages in a book though...

                            Another problem is even though they may be making good money (flat rate) a very few, take riduculous "short cut's" by leaving things out. Things like bolts removed that were hard to access, NEW parts that never get installed, claiming to have performed services that never got done, etc.

                            Worse yet is the management that turns a blind eye to it all, because they are they shop's "top producers"!

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                            • #15
                              You're right , there, Hightower. Most of these "90 day wonders" I've had the misfortune to work with are either way too detail oriented, or are trying to short-cut the jobs by leaving fasteners out, or not installing parts they were paid to. One of my brothers has a Saturn sedan (his wife's car). Recently, he drove over to Arizona for a visit, and brought his tribe along...in the Saturn.
                              When he exited the freeway near the motel he had booked, the engine began rattling and performance dropped to zero (barely enough power to pull the car) They got to the motel, and two of us were hastily called to assess the problem (my next youngest brother and I have all of the family expertise) The engine in the car is a Chevrolet 2.2L EchoTech four cylinder. Known normally to be incredibly durable and long lasting.
                              A quick check showed there was hardly enough oil in the engine to wet the dip-stick. We did a few basic tests and discovered the cam timing had slipped, and the lifters were all almost collapsed. We did replace the missing oil, but didn't make it run any better. The Saturn returned to California on a tow dolly. My brother eventually got the opportunity to open it up and discovered the chain tensioner had two sealing washers (seals) on it, instead of one, and the chain had slipped, retarding the cam timing quite a bit.
                              During the tear-down he also found a couple of bolts missing from the cover, which allowed the oil, over time, to leak out. When questioned, my other brother admitted they had taken the car to the dealer for a timing chain tensioner/chain guide upgrade, Saturn had issued. The tech that was supposed to replace the timing chain, tensioner, and guides only replaced the tensioner, and had mistakenly installed two sealing washers on it, moving the tensioner away from the chain an additional .125".
                              My brother replaced the chain, guides, and tensioner, re-timed the cams and when the key was turned, the 2.2 barked to life.....good as new. All because a Saturn tech had done a flatrate recall repair. He also replaced the missing bolts the guy at Saturn never bothered to install.
                              I guess it's true, "If you pay peanuts, you're gonna get monkeys."
                              Sadly, it's going to get much worse before it gets any better.
                              No good deed goes unpunished.

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