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  • Pressures Good, No Cooling

    I thought that there was refrigerant loss, but the pressures tell me that is not the case.
    This has me baffled. I have spent hours looking online for a solution to no avail.
    Here are the details.

    1984 Mercedes 300TD
    Worked fine the first two years after system rebuild.
    R-12 refrigerant
    TXV
    ACDelco compressor
    Ambient 80 F RH is unknown guessing 50%
    Idle 32 PSI low side 250 PSI High side
    Revved up 20 PSI low side 260 PSI high side
    Vent temp 62 F at idle a little cooler when driving
    Low side gauge jumps about 50 PSI immediately after shut down.
    Slow to equalize pressure 75 PSI difference after two hours
    Auxiliary fan on
    There was oil at the low pressure port when gauge was removed.
    Good temp drop across the condenser
    Little temp change across the evaporator.
    There is some condensation dripping to the ground
    Return line is not cold
    Heater core lines cool

    Thanks for your help.

  • #2
    250 PSI= 155 deg which is dang hot for a condensing temp with the outside temp of only 80, what is the temp at the outlet of the condenser? I'm thinking that you have a condenser problem.
    Yesterday, it was 103 and the outlet of my condenser was only slightly warmer than the air Yours seams much hotter. You also didn't say what the static pressure is, but I don't think you are real low, can't say what the charge is by pressure alone.
    I would like to see much lower high side pressures, showing that the heat is being removed and the refrigerant is condensing to liquid. With outside air, or at least the air moving over the condenser, being 80 deg, a good high side would be a lot closer to 120 PSI or 100 deg condensing temp. Generally, 20-30 deg hotter than air temp is considered good for an air cooled condenser, but the closer you can get to ambient the better. IF you don't loose the heat at the condenser, you can't absorb more at the evaporator.
    Pressures are assuming you still have R-12 as the refrigerant.

    Comment


    • I hate working on cars
      Editing a comment
      The pressures seemed reasonable per the Mercedes manual chart. Do you know where I could look at another chart? Since you asked I ran it again. This time the ambient was 75 F with a strong breeze and outside rather than in the garage. The condenser was 150 in and 105 out. Is this a good temperature drop. I was thinking that the lower temp while driving was due to longer high engine speed dropping the low side pressure and thusly the refrigerant temp is cooler. But it is still not cold. The compressor is not cycling.

      I don't know for sure what the static pressure is because after two hours there was still 75 PSI difference. I think that this is a clue but I don't know what it means. Today after sitting for 20 hours, the low side was 102 PSI. I did not check the high side because it requires jacking up the front of the car,and setting jack stands to crawl underneath to the reach the port. I would guess that it was equalized.

      Yes, still R-12. What refrigerant are you using?

      I don't remember seeing oil at the low pressure port when the gauge is disconnected on other cars.I don't know if this is a clue.

      Another thing that I measured is the in and out temps of the TXV. 100 in 70 out. I have seen frost on this. This is strange because vent temp was 50.

    • Cornbinder89
      Cornbinder89 commented
      Editing a comment
      102 PSI static says the temp is between 90 to 95 degs. so I am a bit confused here, Also How are you measuring temp, Laser (IR thermometer?) I don't trust them, I have got readings that are way off, others have had a different results, and are happy with them.
      I am running 134a in a system designed for R 12 but with a condenser upgrade.
      I know some systems take a long time to equalize, and some Tx valves shut on pressure rise so I will not say that long time to equalize is necessarily a clue.
      I don't understand what you are saying about Tx valve temps, 100 in and 70 out but frost? Makes me question all temp readings.
      A condenser can fail in many ways, and being "clean" in the fins doesn't mean it is good. If the fins have lost their thermal bond to the tubes, the fins can be "clean", but not remove any heat from the tubes. If the outlet was truely 105 F at the outlet the pressure would be 100 PSI, your high pressure indicates that the R 12 is much hotter.
      Are you sure of the charge amount?

  • #3
    Also, cooler while driving also points to a condenser (or air flow over the condenser) problem. Try misting the condenser with water and see if the high side drops and the vents cool.

    Comment


    • #4
      Totally agree - spray/mist condenser with water. If back to expected performance completely has to be mostly an airflow problem maybe well hidden debris in between condenser and radiator?

      If all else right this car almost certainly has a sight glass for info on charge or what is flowing right there. May be dirty or hard to find but with R-12 very helpful to see it. If not sure what you are looking for there I can explain if you wish,
      Tom
      MetroWest, Boston

      Comment


      • I hate working on cars
        Editing a comment
        I can get a good sight line to see that the condenser is clean and no debris in between. The sight glass is dirty and hard to get to but I can see movement. I should take the time to remove parts so I can clean it.

    • #5
      Originally posted by Cornbinder89 View Post
      Also, cooler while driving also points to a condenser (or air flow over the condenser) problem. Try misting the condenser with water and see if the high side drops and the vents cool.
      I am thinking that cooler while driving was due to lower low side pressures from the higher compressor speed. There are two fans and there is no blockage. Today there was a 42 deg temp drop across the condenser. This seems good but I don't ave enough data to know.
      Last edited by I hate working on cars; 06-22-2017, 10:07 PM. Reason: missrd a word

      Comment


      • #6
        Something.

        Comment


        • #7
          See my comment above. I still think you have a condenser problem. Spray the front of the car with a hose and sprayer and watch the temps and pressures.
          Either your pressures or temp readings are not accurate.

          Comment


          • I hate working on cars
            Editing a comment
            Thanks, guys. A lot of points here. I'll try to cover all of in this post. Let me know if I miss something.

            I am measuring the temperatures with K type thermocouples (TC) attached to a dedicated two channel reader. The TC were stuck to the condenser tubes under foam tape to give insulation from the air. When using an IR device one needs to be aware of emissivity to get accurate results. Some of them can't measure small surfaces because the beam is too wide.

            The frost seen on the TXV is from years past not today.

            " If the outlet was truely 105 F at the outlet the pressure would be 100 PSI, your high pressure indicates that the R 12 is much hotter."

            The inlet temp of the condenser was 150 which is close to the 250 PSI reported.

            "Are you sure of the charge amount? "

            I weighed the R-12 when I filled it two years ago. I can't say if there is leakage or not. I suppose I should evacuate and weigh.

            I cleaned the sight glass and I think what I see is brownish foam with really small bubbles.

            You think that a 42 degree temp drop across the condenser is not enough?

          • I hate working on cars
            Editing a comment
            The 42 deg temp drop across the evaporator seems good to me and is supported in this topic.

            http://acsource.net/acforum/viewtopic.php?f=1&t=2545

            One guy wrote of a test he did where the pressure drop across the condenser was minimal regardless of the temperature change.

            I am using high quality USA made Robinair gauges that show no evidence of damage and the needle movement is smooth. I trust them.

            I use the ice-water method checking the calibration of the thermometers. All were within 2 degrees.

            Where is the conflict in temp/press that you see?

            You mentioned "102 PSI static says the temp is between 90 to 95 degs". I know that there is a relationship between temps and pressures but I don't understand it well. especially from what the guy in the above link wrote.

            I checked my other vehicles for static pressures and this is what I got for low side pressures. All are R-12. Ambient was 89.

            F-350 CCOT 97 In the shade

            C-10 CCOT 135 in direct sun all day

            300TD TXV 140 in the shade all day

            Is it normal for oil to spray from the LPP when removing the gauge connector?

            Thanks

        • #8
          I don't use, nor do I give much stock in charts. Pressure reading tell the story (as long as the gauges are accurate) You want the high side to condense and cool as close to ambient as possible. The higher the temperature difference between the cooling medium and the cooled fluid the more heat transfer, BUT the more total heat you can get out the better, so as the fluid cools less and less heat gets removed as it progresses thru the condenser. Mfg make a tradeoff in condenser size, it must fit in the area they leave for it, which is why you see higher pressures in the "Charts".

          Again, the pressures and therefor temps in the system say your not getting the heat out. Misting the condenser will tend to cool the r-12 and you should see a drop in high side, and better cooling.
          The old tube and fin condensers tend to be strong mechanically but are fairly poor at removing heat. They were cheap to produce and worked ok with R-12 but are far from optimum.
          If a misting test show that the condenser is not removing heat (or not enough) I would look for a generic condenser that can fit the area but INCRESE the BTU capacity. I would go with the biggest capacity in BTU's that you can fit. I would want at least 2 ton (24,000 BTU) condenser.
          I am running a 32,000 BTU condenser and it is why, in 103 deg temps, my high side is low, outlet of the condenser, temp is just above ambient, and even at these high temps, the outlet of the evaporator is right at the frost point. In other words, the evaporator is as cold as it can be, and not ice over.
          This is on a system designed for R-12 and is running 134a.

          Comment


          • #9
            Originally posted by I hate working on cars View Post
            I thought that there was refrigerant loss, but the pressures tell me that is not the case.
            This has me baffled. I have spent hours looking online for a solution to no avail.
            Here are the details.

            1984 Mercedes 300TD
            Worked fine the first two years after system rebuild.
            R-12 refrigerant
            TXV
            ACDelco compressor
            Ambient 80 F RH is unknown guessing 50%
            Idle 32 PSI low side 250 PSI High side
            Revved up 20 PSI low side 260 PSI high side
            Vent temp 62 F at idle a little cooler when driving
            Low side gauge jumps about 50 PSI immediately after shut down.
            Slow to equalize pressure 75 PSI difference after two hours
            Auxiliary fan on
            There was oil at the low pressure port when gauge was removed.
            Good temp drop across the condenser
            Little temp change across the evaporator.
            There is some condensation dripping to the ground
            Return line is not cold
            Heater core lines cool

            Thanks for your help.
            *****************************************
            Arggh: I see why you chose that username! This isn't adding up.
            * Pressures don't equalize for 2 hours off by 75 psi? That's totally locked/blocked?
            * There is some condensate? Are you so sure it's really A/C water off of evaporator? Can't be.

            It's old and a tricky vehicle from the start the combo is a total challenge. It worked since a redo for a couple years so where's the issue now is the question and why these observations?

            I suggest it's time to know that the tools - gauges, thermometers used and what types. CB89 doesn't trust IR thermos and I use them all the time (two) to check each other or check each on something known just to know they are having a good day and don't in wind or with air movement around very well. Plain touch and feel when possible and put those away when acting up.

            Gauges? Many types of manifold sets now still working with R-12 really need to know they are working. Assorted types hoses backwards lock flow or don't depress vehicle's Schrader if wrong or backwards perhaps not lining up properly on fittings should both be same threaded size and spit. I was said no oil was noted at that alone there's a clue there.

            Is this also hooked up to a refrigerant bottle like a 30lb one or what all while checking or just that yellow hose parked and staying sealed (mine does) on the manifold?

            This is probably so full of incorrect info, pressures, temps plus possible old failed gauges and throw in if the R-12 is really R-12 or some other concoction sold as the real thing?

            If it can't equalize and can prove that I think that's problem ONE.

            Nothing is flowing so drop in condenser temp would if it was just wide open with no refrigerant as a dead weight with air flowing over it or static locked tight with whatever is in it.

            There are so many unproven observations all thru this all of this, tools and equipment needs to be verified as working properly or compared to same stuff that is known to work or we'll get no place with this.

            Probably can't trust a vacuum and charge to start all over to possibly find compressor even if engaged and quiet is just throwing total sludge blocking up whole system might have contaminated product used. Lots needs to be verified on the first posts whole list of observations,

            Tom
            MetroWest, Boston

            Comment


            • #10
              High side pressure is determined by at what temp the vapor condenses to liquid. Pressure is a very accurate measurement of refrigerant temp. SO I believe (as long as the gauges are accurate) that the 250 PSI reading means 155 deg condensing temp, which is way to hot, which brings us back to the condenser not doing its job. The inlet temp to the condenser has nothing to do with high side pressure, that is determined by condensing temp, so yes the inlet is hot, but so is the outlet, because the R12 is not cooling below 155 deg. You could have a 300 deg temp coming out of the compressor, but with a good condenser, the high side could be 100 deg (after the condenser) and a high side pressure of 120. As soon as the hot gas hits the condenser tubes it will loose heat, and continue to do so down to and below the condensing temp, Liquid takes up much less space than hot gas, so the pressure is determined by the temp at which the gas turns to liquid. The pressure is the equilibrium point where if the pressure is any lower, liquid will boil to gas, and if it is any higher, gas will condense to liquid, for a given temp
              2nd I am concerned about the "brownish foam" in the site glass. R-12 will tend to foam white or lightly off-white, Foam means not all the refrigerant is condensed to liquid (again pointing to a bad condenser). Mineral oil tends to be amber, and I wonder if you have contaminated oil? I guess it comes down to "how brown" is brownish? Could just be seeing oil moving in the system, or could be something else discoloring the R-12. Anyway you cut it, a properly charged system that is working correctly shouldn't have foam, may be a few small bubbles every now and then, but not foam.
              Again, I would do the "spray test" on the condenser while watching the pressures, and if it confirms the condenser, I would replace with something better than the original design. While the system.is open, I would drain and replace the oil from the compressor, drier and lines, starting fresh with new oil of the correct viscosity.

              Comment


              • #11
                Originally posted by Cornbinder89 View Post
                I don't use, nor do I give much stock in charts. Pressure reading tell the story (as long as the gauges are accurate) You want the high side to condense and cool as close to ambient as possible. The higher the temperature difference between the cooling medium and the cooled fluid the more heat transfer, BUT the more total heat you can get out the better, so as the fluid cools less and less heat gets removed as it progresses thru the condenser. Mfg make a tradeoff in condenser size, it must fit in the area they leave for it, which is why you see higher pressures in the "Charts".

                Again, the pressures and therefor temps in the system say your not getting the heat out. Misting the condenser will tend to cool the r-12 and you should see a drop in high side, and better cooling.
                The old tube and fin condensers tend to be strong mechanically but are fairly poor at removing heat. They were cheap to produce and worked ok with R-12 but are far from optimum.
                If a misting test show that the condenser is not removing heat (or not enough) I would look for a generic condenser that can fit the area but INCRESE the BTU capacity. I would go with the biggest capacity in BTU's that you can fit. I would want at least 2 ton (24,000 BTU) condenser.
                I am running a 32,000 BTU condenser and it is why, in 103 deg temps, my high side is low, outlet of the condenser, temp is just above ambient, and even at these high temps, the outlet of the evaporator is right at the frost point. In other words, the evaporator is as cold as it can be, and not ice over.
                This is on a system designed for R-12 and is running 134a.
                Your situation is unique. I need the charts. I don't have the time, desire or money to do a condenser change. Do you not agree that the 42 degree drop across the condenser is good?

                The misting test is not scientific enough for me. There is no way to know how much heat the water is removing. It could be 100,000 BTU or 1000 BTU. If it was 100,000 and I used this to buy a 32,000 BTU condenser, I'd be right back were I started.

                R-134a needs a lower temperature than R-12. Mercedes has a temperature switch on the receiver drier that turns on the condenser fan at 144 F.
                Last edited by I hate working on cars; 06-24-2017, 07:52 PM. Reason: Adding more information

                Comment


                • #12
                  Pressures don't show a 42 deg temp drop they show a 155 deg condensing temp. Misting is a quick and easy test. Your car, your choice on what to do. I fear there is no quick and easy answer to your problem. A little corrosion on the tubes and the condenser will not shed heat and the thermocouple will not transfer the heat reading properly
                  I don't see how my situation is unique, it is just using thermodynamics to optimize the system as best as I could. By removing as much heat from the refrigerant as possible, the evaporator size (capacity) becomes the limiting factor in the system. The oversized condenser has the added benefit of reducing the head (high side) pressure, reducing the load on the compressor.
                  Most Tx used in automotive use are either 1, 1.5 or 2 ton valves, so there is no way the system can move 100,000 BTU. 2 ton units are mostly in trucks and vans, I don't have a listing for a Merc Tx valve, but I doubt it would be over 1.5 ton. A 2 ton condenser would eliminate the condenser as a limiting factor.

                  Comment


                  • #13
                    *****************************************
                    Arggh: I see why you chose that username! This isn't adding up.
                    Yes, but I pretty much hate everything.

                    * Pressures don't equalize for 2 hours off by 75 psi? That's totally locked/blocked?
                    This is a clue to me as is the oil at the LPP.

                    * There is some condensate? Are you so sure it's really A/C water off of evaporator? Can't be.
                    My theory here is that there is enough refrigerant to cool the first few inches of the evaporator but is warmed up by the time it exits.

                    It's old and a tricky vehicle from the start the combo is a total challenge. It worked since a redo for a couple years so where's the issue now is the question and why these observations?
                    I am thinking that I may have two problem that are messing up the diagnosis. Would low charge and a blocked TXV give this problem?

                    I suggest it's time to know that the tools - gauges, thermometers used and what types. CB89 doesn't trust IR thermos and I use them all the time (two) to check each other or check each on something known just to know they are having a good day and don't in wind or with air movement around very well. Plain touch and feel when possible and put those away when acting up.
                    Vintage high quality America made Robinair gauges no evidence of damage. Needles move smoothly. No IR. Dial type thermometer for vent and ambient. Tested with ice water and found to be within two degrees. Thermocouples also tested to .2 deg.

                    Gauges? Many types of manifold sets now still working with R-12 really need to know they are working. Assorted types hoses backwards lock flow or don't depress vehicle's Schrader if wrong or backwards perhaps not lining up properly on fittings should both be same threaded size and spit. I was said no oil was noted at that alone there's a clue there.
                    Shrader valves are fine because of the movement of the needles. I think that oil at the LPP is a clue also but I don't have enough experience to know if it is normal. There may be oil in the sight glass but it is hard to see. I'd describe it as brown foam.

                    Is this also hooked up to a refrigerant bottle like a 30lb one or what all while checking or just that yellow hose parked and staying sealed (mine does) on the manifold?
                    I have a 30 lb bottle but it is not hooked up.

                    This is probably so full of incorrect info, pressures, temps plus possible old failed gauges and throw in if the R-12 is really R-12 or some other concoction sold as the real thing?
                    The about six pounds was recovered from a professionally maintained restaurant freezer. About 10 pounds was recovered from 14 oz cans marked Refrigerant12, Dichlorodifluoromethane and CCI2F2. About 3 pounds was recovered from the F350. Both the F350 and C-10 have excellent AC and were charged from the same bottle.


                    If it can't equalize and can prove that I think that's problem ONE.
                    I agree, but what does that tell us?

                    Nothing is flowing so drop in condenser temp would if it was just wide open with no refrigerant as a dead weight with air flowing over it or static locked tight with whatever is in it.
                    I don't know if it is flowing for sure. I would think that it was blocked there would be no temperature drop across the condenser and the high side would be much higher. Low side much lower.

                    There are so many unproven observations all thru this all of this, tools and equipment needs to be verified as working properly or compared to same stuff that is known to work or we'll get no place with this.
                    I don't understand why you are skeptical. I am very thorough and I have good tools. What observations are suspect?

                    Probably can't trust a vacuum and charge to start all over to possibly find compressor even if engaged and quiet is just throwing total sludge blocking up whole system might have contaminated product used. Lots needs to be verified on the first posts whole list of observations,
                    If there is too much oil, evacuation may remove it. The compressor is engaged or I would not get the pressures reported. What would you like verified? I thought that I was very thorough. Let me know if there is something I have not covered. I do hear a vary faint clunk once in a while.

                    Comment


                    • #14
                      The 42 deg temp drop across the evaporator seems good to me and is supported in this topic.

                      http://acsource.net/acforum/viewtopi...f=1&t=2545

                      One guy wrote of a test he did where the pressure drop across the condenser was minimal regardless of the temperature change.

                      I am using high quality USA made Robinair gauges that show no evidence of damage and the needle movement is smooth. I trust them.

                      I use the ice-water method checking the calibration of the thermometers. All were within 2 degrees.

                      Where is the conflict in temp/press that you see?

                      You mentioned "102 PSI static says the temp is between 90 to 95 degs". I know that there is a relationship between temps and pressures but I don't understand it well. especially from what the guy in the above link wrote.

                      I checked my other vehicles for static pressures and this is what I got for low side pressures. All are R-12. Ambient was 89.

                      F-350 CCOT 97 In the shade

                      C-10 CCOT 135 in direct sun all day

                      300TD TXV 140 in the shade all day

                      Is it normal for oil to spray from the LPP when removing the gauge connector?

                      Thanks
                      I copied and pasted so we can keep the replies straight with which post they go with.

                      Temp vs. pressure chart is here: https://forum.aircondition.com/forum...pressure-chart This is the only chart I put any stock in, all others can be effected by humidity, temp across the condenser different than ambient etc.

                      Pressure in a system is like LP in a your house tank. The pressure depends on temp of the fluid. The high side pressure is determined by the temp at which the gas condenses into liquid. Change one thing and you effect everything else.
                      If little heat is removed at the condenser, the pressure will rise until it hits the pressure that it will condense at the temp of the refrigerant. High side readings tell you the condensing temp, low side tell you the boiling point in the evaporator.
                      The more heat you can remove at the condenser, the more heat can be absorbed into the refrigerant at the evaporator.
                      Again, your high side is showing a 155 deg condensing temp at 80 deg ambient temp, or 75 deg above ambient temp, that is way too high.
                      Oil circulates with the refrigerant so it would not concern me to see oil at a port.
                      In an ideal system the high side would condense at ambient temps (refrigerant gives up all it heat to the air) and the high side pressure would correlate to the outside air temp. Low side will be aprox the boiling point in the evaporator so around 28-32 PSI but since the compressor speed can vary, the low side can dip below the boiling point, The Tx should be making sure that all the refrigerant has boiled by the time it leave the evaporator, so whlie the low side pressure can drop below 28 with the engine rev'd up, only gas is being moved, in this instance the low side pressure, while accurate, doesn't exactly equate to boiling point.
                      Everything points to an ineffective condenser, a mist test will show a change in high side pressure, confirming it. It may not be what you want to hear, but that is what the symptoms point too.
                      In that link you posted to the archived site, you will note that very little pressure drop is noted across the condenser, only a temp drop, and as stated above, pressure = temp.
                      Last edited by Cornbinder89; 06-24-2017, 10:06 PM.

                      Comment


                      • #15

                        I know that there are some Tx valve that shut on a rising evaporator pressure, this is to prevent a rapid heating or defrosting of the evaporator when the compressor cycles off. I have no idea if Merc uses such a valve, but they do exist, esp in commercial refrigeration. So if that is the case, slow equalization times would not concern me. The only thing that is way out of normal, is the 75 deg difference between ambient air and condensing temp.

                        Edit: With this type of Tx, the valve will not open until the low side suction line pressure is below a set point, This would explain the rapid rise to 50 psi and then seam to "lock off" at that point.
                        Last edited by Cornbinder89; 06-24-2017, 09:51 PM.

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