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Need advice on charging Red Dot AC

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  • Need advice on charging Red Dot AC

    Have a John Deere 8970 tractor with a Red Dot 9800 back wall unit. I added the unit as the factory evaporator was bad. I am using the existing condensor which was meticulously cleaned. Put a brand new Denso 10PA17C compressor on. The problem is that the I keep getting the evaporator and the outlet line coming out of evaporator box frozen. I hear people say its undercharged and overcharged both. Can it be either or can someone point me in the right direction! I purged everything with nitrogen and pulled a vacuum with an almost new Yellow Jacket SuperEvac for over 4 hours. When system was pressured up with nitrogen I had no leaks (leak tested everything). This is a TXV system btw. I weighed in 7 pounds which I did via vent temp and the pressure chart. I got the high pressure reading that the chart suggested but my low side never came up above 20. Didn't know if I should keep pumping R134a in or stop there so I stopped there. Blows cool until system freezes up. Red Dot tech support is literally nonexistent. I am open to any advice!

  • #2
    Never seen a system outside of a motor coach that took much over 4lbs, so at 7 I would guess your way over charged. In my opinion, those "performance charts" do more harm than good.
    I'd start by charging to what the stock system called for, that should be close and you could make adjustments from there.
    There either needs to be a "frost switch" in the evaporator coil to sense when the fin temp gets close to freezing or an evaporator pressure regulator to keep the evaporator above freezing.
    The Tx valve DOES NOT control the evaporator temp, it controls the amount of refrigerant allowed into the evaporator such that the outlet is close to the same temp as the inlet of the evaporator. It doesn't care what the temp is as long as it is close to the same.
    On automotive systems, a frost switch is a cheap and easy way to control evaporator temp. Ag equipment has its own set of problems, and I am no expert on it. I do know they use chafe guards on the clutch, and they may opt for an evaporator pressure regulator in the outlet of the evaporator so the clutch doesn't have to cycle while out in the field, with the risk of chafe getting in the clutch and possible slipping of the clutch and setting chafe a light.
    Without one or the other system, it will continue to drop the evaporator temp below freezing and will ice the evaporator.


    • #3
      Not my game just the concepts. That's a lot of refrigerant but IDK this machine. It's blowing cool till iced up is the clue. NO it shouldn't but if you are even close with weight of any known listed charge it would behave. With evaporator new now is or did that fit in properly for you and position of temp sensing bulb where it belongs? Cornbinder is going to be much better at this is all guesses from me.

      How well did you seal the case back up again and tube to bulb can't have warmer air getting in or ices up. Just ideas if you were in trouble sealing it I have some ideas if tough to find and need this ASAP will discuss it then if the issue,
      MetroWest, Boston


      • #4
        Tom, it is a whole "cabinet" that mounts to the back wall of the cab, Made for trucks without a sleeper, mounts between the seats and to the back wall of the cab, he bypassed the original evaporator and used this unit with its own fans and evaporator.
        I'm thinking that what ever was used to prevent iceing isn't hooked up or working properly.


        • #5
          It's all yours. I didn't catch that just icing isn't part of the game. Another and I'm done is "residence" time of air flow is so general just an FYI you can't trap a unit without air to move. That's it, carry on.....
          MetroWest, Boston


          • #6
            I know that 7 pounds is a lot of refrigerant. The factory system called for 5 pounds. A lot I imagine because the evaporator is about 20 plus feet away from condensor. If it is way overcharged wouldn't my pressures have came up before I got too much in? Neither pressure ever came up close to chart specs until got this much 134a in and even now only the high side is close. My low side never comes close to what chart says. Now at 20 on low side is highest it has ever been. Im thinking I STILL don't have enough in but I could be all washed out! The line going back to compressor from evaporator sweats about 4 feet up the hose away from evaporator but that is it. On my F250 with the correct charge weighed in it sweats all the way to the compressor. That is an orifice tube system though so it could be different. Just thinking out loud here.


            • #7
              At this point if any pressure in system worth a static pressure sniff out all over for leaks new, used just rule that out as best you can. AG you'd probably have trouble measuring how much came out by weight? If you can do so.

              Start over? It's just not supposed to freeze to me usually just so low on charge it's evaporating inside evaporator and freezing. That ice isn't cooling air well. If seen wouldn't be dripping "water" but flood when shut off and work again later - several minutes.

              IDK location you are working could be screwing you up also? Put a charge in after a vacuum if all else is testing fine and stop when it's first blowing cool. Drive this thing around - that matters not just in one spot let oil and system run thru what's in it and add known amounts till you get the temps and nothing is frosting.

              That 20 feet doesn't add up to double the charge. One line is vapor doesn't weigh much and high line is smaller.

              Off exact topic + vehicle: Did HUGE motor yachts not a chance in the world to find capacity or even see where lines went thru a maze of wires, tubes and engines, batteries, tanks and who knows what just all over the place. Hanging upside down gauges hooked up from a vacuum just watched for the frost to quit a little at a time. THREE units, electric using water for heat exchange and evaps with drip pans you could see frost disappear. Ice maker and two other units. Just watching what was happening. They worked all season and here most short of ships are pulled out for off season/winters couldn't test out of water anyway.

              Just that - start over this time go slow I think charge is just all off or leaked out some at connections 134a like others works just more fussy. I was working with R-22 which wasn't on the yacht NOT my thing but only chance of things working,
              MetroWest, Boston


              • #8
                Forget the pressure performance charts! tear them out of the manual if you have too! Start with 5lb charge if that is what the OEM system called for.
                Tom, these Ag systems don't use outside air, they only use recirculated air to prevent drawing in dust or farm chemicals.
                High side should only be as high as necessary to condense the refrigerant to liquid, If the condenser is clean and doing a good job, with good airflow over it, and the air flowing thru it is not too hot, it can be as low as 130 psi! If you add more refrigerant to bring it up to "what you think" it should be, you are just filling the condenser with liquid refrigerant and for all intents making it smaller, less area to cool the hot gas to liquid!
                Low side can be a bit more tricky, esp because the system is re-cycling inside air.
                Start with a 5 lb charge and see how it does out in the field.
                As long as the evaporator can get enough liquid refrigerant such that it all just boils by the outlet, nothing is gained by adding more, you REDUCE the system capacity when you over charge. If a block type Tx valve is used, the tubes going to the evaporator from the Tx valve should be close to the same temp, if they are no adding of refrigerant is going to help. If the outlet is warmer, you are under-charged.
                Last edited by Cornbinder89; 09-09-2019, 09:03 AM.


                • #9
                  Nicely said C-Binder! OP - look for the temps at pressures when a known refrigerant would be a liquid. Takes good thermometers of what's going on where. Plain charts of what to see as said aren't going to cut it the conditions are too unique each time it's used.
                  For thermos: I like fast, wired in-out thermos usually for home use throw sensor out a window AND infrared touchless ones, do practice with those on what surfaces are working and my own sensitive to wind.

                  I hadn't factored a dust scene OF COURSE an issue to overcome so design for it. So far I still think this is 99% a matter of correct charge for the set up you have but do need to know temps. I'll post a chart is here somewhere if you want just ask,
                  MetroWest, Boston


                  • #10
                    I wonder if he didn't hook up the frost switch in the replacement backwall unit? Without it, the evaporator could ice and there would be nothing connected to the compressor telling it that it is too cold.