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2001 Mack Truck

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  • #16
    It is a block type. Let me rethink this. I have poor cooling. The pressures are low. I added that 10 ounces and it didn't change the pressures at all. I am going to recheck the pressures again. I kind of want to add more until I get up to 30 low and 200 high. Where I want to see it at.

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    • #17
      DO NOT charge to pressure! Pressures ONLY tell you the temp at the point of measuring. Example: today was 90, the outlet of my condenser was only warm to the touch, inlet was hot! I would guess by "feel" my outlet wasn't warmer than 110 deg, which would be 140PSI. It might have been cooler. This is the reason my A/C works so well, it sheds almost all the heat from the refrigerant. If I added until I saw 200 psi it would be way overcharged!
      You've already proved that the system will get cold enough to frost over the evaporator. It can't get colder than that! Either you don't have enough air flow over the evaporator , or you have heat leaking into the system (duct work).
      Over charging a system will reduce the effective area of the condenser, and raise the pressure causing more heat in the refrigerant that can't be shed by the reduced effective area of the condenser! To put it short and sweet, you'll make the problem worse!
      It would be helpful to get a temp reading right at the evaporator with the fans running and the system operational.
      The real telling thing, is that with the frost switch disconnected it got cold enough to ice. I don't remember if that was with the correct 2lbs or with 2lbs 10oz overcharge?
      Many trucks have an air filter on the inlet air to the heater/A/C, does this truck? is it clear? Some trucks have dual blowers, are both functioning? It seams like either not enough air flow, or too much leakage in the air distributing duct system.
      I have seen both heater and evaporator coated with dirt, dog fur, plant material such that air flow is reduced to the point it doesn't cool.
      As to the low side, if you don't absorb heat in the evaporator, (i.E if there isn't sufficient warm air flowing over the fins) the outlet will cool, and the Tx will throttle back, and you will see low pressure. It just means there isn't sufficient heat load in the evaporator.
      The long and short of it is: Pressures will tell you what the temp of the refrigerant is but not if the charge is correct or why the refrigerant is at the temp it is. You must use other means to figure out why.

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      • #18
        I guess what I'm trying to say is: Your problem is not with the refrigeration cycle, but with the air handling, either air is not flowing over the evaporator (or not enough flow) or much of the air is bypassing and not being cooled. My advice is to restore the correct amount of refrigerant and look hard at the air flow thru the HVAC system, that is where I think the problem lies.
        Sometimes worn bearings/bushings in the fan motor can slow the blower. Also most squirrel cage blowers are directional, turning them backwards will not reverse the flow but will reduce the flow by 1/3 to 1/2. So that is one place to look. Another is seals and air control doors. The Evaporator must be sealed so not air can flow around it rather than thru it. Doors must direct the air and shut off alternative paths for the air to flow. It may take pulling the dash apart or the heater box.

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        • #19
          Originally posted by Mamrak76 View Post
          Back again! So after my much needed vacation and my truck having transmission repairs it is back in my hands. To fill anyone else in, I was trying to get my ac system up on my truck. Weird pressures which were caused by holes. So I ended up needing a condenser. I also replaced the txv and reciver dryer. And a couple of busted up hoses. So now we are in business, well sort of. I pulled about an hour and a half of vacuum. My truck calls for exactly 2 pounds. So I added exactly 2 pounds. That put me at 15 low and 150 high at 65 vent temp. Also I have a very nice fieldpiece thermometer and I trust it. I am not one to over charge but that seemed low at 80 degrees ambient. So I added 10 ounces extra and that brought me to 20 low and 170 high with a 55 degree vent temp. Only problem now is my Evaporator is freezing up. The vent temp stays around 50 but then after a while it goes up. It seems to be freezing when i am idiling. Any suggestions?
          Just a thought, - you did end up with the same spec condenser?? I know you have questioned the TXV you replaced, just wondered if the new condenser was 100% kosher?

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          • #20
            Originally posted by worzel gummage View Post

            Just a thought, - you did end up with the same spec condenser?? I know you have questioned the TXV you replaced, just wondered if the new condenser was 100% kosher?
            You missed the part at the end where the Evap was freezing, Right or wrong the condenser must be doing well to cool the evap that much, so not the problem.

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            • #21
              Well I appreciate everyone's comments. I have not tried to recharge because I was waiting to get my recovery machine. Which I now have! Any comments on a Robinair 17534 for r134a systems. I just played with it but now can pull out the extra 10 ounces.
              Corbinder89, how would you explain duct temps of 70 degrees with correct charge of 2 lbs . then with 10 ounces of overcharge it is down to 59 degrees?

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              • #22
                The thing I keep coming back too, is the icing of the evaporator. If the evaporator can get cold enough to "trip" the frost switch, it is cooling the evaporator as much as possible in A/C. The question remains, if the evaporator can get that cold, then it is not getting heat out of the air flow. At 80+ degs one would expect the it would struggle to reach the frost level. Another telling thing is how much the temp rise was when the disconnected switch was re-connected. It tells me a large portion of the air moving thru the system is bypassing the evaporator and not being cooled, and mixing with the cool air. Your low, low side and good/ normal high side says you are not at the capacity of either condenser or evaporator. It tells me the Tx valve is throttled back because the outlet is cool enough. One possibility is a defective evaporator, but again icing tend to point against that. If the refrigerant wasn't flowing thru the tubes, they wouldn't ice. There have been cases where the evaporator was made wrong and the refrigerant just flowed in the end tanks and not thru the tubes, but little or no icing of the tubes and fins would result.
                Duct temp doesn't equal evaporator temp, it is the temp of the air in the duct only! Your readings tell me you are ending up 20 deg above freezing, to gain that much, air has to be bypassing the evaporator.
                Overcharging reduces the (effective) condenser capacity and adds more heat (and pressure ) to the system, which reduces capacity.
                It is your system, and you are free to do as you want, but it is my opinion that you are focused on the wrong part of the system, I think your problem lies in the air handling part of the system.

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                • #23
                  Look, I have no dog in the fight, I am giving you my best, thought out opinion. I have designed and built my own complete systems, and the A/C in my cabover hits the frost switch temp with outside temps in the mid 90's. It gets so cool inside, I have to turn it down (warmer). I WANT you to solve this, and I think you are barking up the wrong tree messing with the charge amounts.
                  Overcharging a system 1st fills the receiver with liquid, and if the Tx valve is not passing large amounts, it backs up into the condenser. So the high side pressure goes up along with the refrigerant condensing temp, with part of the condenser filled with liquid, it reduces the area available to shed heat from the vapor, All this means there is more heat retained in the refrigerant and takes more hp at the compressor, You put more effort in for less cooling capacity.
                  The refrigeration side of the system is reacting the way it was designed for the heat load it sees. "sees" is the operative word here. You are not getting the heat load to the evaporator. Solve that and I think you will solve your problem.
                  It might not be easy, but it will likely be cheaper than anything you can do on the refrigeration side throwing parts at it, like an new evaporator, TX or condenser.

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                  • #24
                    Ok I am starting to see the light. What are suggestions for fixing bypass problems? Get a tube of silicone and starting hitting cracks? The evap cover is clean and in good condition. It has inlet holes at the bottom. The fan motor has 1 squirrel cage on each side of the box. Could I use duck ("duct") tape on the "ducts?"

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                    • #25
                      Is this to say "Duck" tape can actually be used for "Ducts?" Just funny - note there is real metal duct tape you have to ask for to tolerate wild temp extremes. Assorted peel and stick products.

                      Other stuff is "Removable Caulking" one sold by Mortite (spelled that way just read off a box) is like a roll of spaghetti you can shape to fit gaps tolerates wild temperature ranges and can be removed clean if needed later is great if back there again or if another product is better it's gone. Hardware store product really meant for a sash window mostly that's old and loose. Link to pictures of what it looks like > https://www.bing.com/images/search?q...D5891951BFDB9E

                      If leaking the cooled air out I would hope you could feel for it and find where.

                      Some sort of "blend" door like a choke plate might not shut tight. Do remember that in a fight heat wins.

                      BTT ( back to top) was said this "Big Rig" truck listed just 2 lbs of refrigerant and adding 10 oz markedly dropped outlet temps. Icing when low common to me and that comment suggests the spec is all wrong just a thought. Regular pick up trucks only hold 2 lbs. for a single system w/o rear A/C or second evap types.

                      Just that suggests the charge is all wrong for this for some reason probably not the only reason,
                      Tom
                      MetroWest, Boston

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                      • #26


                        I wouldn't jump to that conclusion just yet. 2 lbs is low for a big truck system, but this is a different sort of truck than an on road tractor. ( Think trash truck) I think we've seen that spec in more than one place. This truck has one evaporator.
                        I'd like to see a lot better cooling and pressures that show that, before messing with the charge. Right now the pressures tell me there is very little heat load making it into the system. Remember the recent post of the Subaru, where his pressures were "low" until he got the system operating correctly, then they came up to normal ranges, with no change in amount in the system.
                        A Tx valve is an adjustable orifice, they don't totally shut or totally open. they vary thru their designed metering range. When you over charge, you raise the pressure across the valve (High side gets higher for the reasons posted in previous post), if the heat load is still low on the evaporator, you end up "forcing" the Tx to operate outside its design range, meaning it can't throttle down any lower and the pressure across the valve is higher, so more refrigerant gets to the evaporator than it can boil, which leads to boiling in the suction lines and frosting of the Tx and assorted low side piping as well as "low" pressure readings. None of this increases the capacity of the system, or have much effect on duct temp.
                        I think the problem with the system will lie in the air handling, but would not rule out a defective evaporator but that would be evident if you run the system and observe the evaporator temp across the surface ( look for even temps). I would not change the evaporator without an observed problem with it. I.E cold on one side and warm in the center or opposite tank.

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                        • #27
                          CB89 - I can't imagine the layout of this - your thing. My own truck - C/K 2500 short cab could cool a lot more cab than it needs to at just about 2lbs charge no problem at now over 20 years old blowing 42F out center vent. Noted side vents up that about 3F just it hits center first. Hot days too as I don't care for me until 90s or more,
                          Tom
                          MetroWest, Boston

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                          • #28
                            I have an unrelated question. When I got my Robinair machine I made sure and reset and zeroed the scale. I then bought a brand new 30 pounder from bumper to bumper. Why did only recover 26 pounds from it? My machine put the 30 pound can under vacuum so I know it was auctually empty and I also put my refillable cylinder under vacuum.

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                            • #29
                              I have zero experience with that or most other recovery machines, so can't answer. I pump my used stuff into a tank but either use small cans or a scale to charge.
                              Might want to post your question again in the tools and equipment section, you might get more traffic on that there

                              Comment


                              • #30
                                Oh no! Nothing wrong with the name brand Robinair by itself. We have a problem with this A/C to figure out and if weights and temps are not known correct the problem is the equipment or products before we can even get going at all.

                                A 30lb container new just with the box should weight on any scale about 35.5lbs for the box and metal. I know that varies by makers of the container and condition of the box as I will weight empty stuff and compare it just to know. Metal bottles of assorted brands can't verify one to the next very well new in a row by insignificant amounts.

                                This just took what should be 30lbs and measured 26lbs came out.

                                This is total trouble that's too much to be off so have to get some things known. There's strong motivation to short product amount or worse it's not pure product. Knowing this for your new or know equipment doesn't much matter now if we are talking buying cold cuts at a deli or what.

                                This has to be known. Cheating or mistakes absolutely known to happen.

                                Screwed: Have to know your product and equipment is right as compared to something else known right it that's going to be the discussion.

                                ****************************************
                                Have a scale I trust, gauges I trust and try to compare anything new (just plain anything never mind A/C) to be correct.

                                Temps: Have at least 20 thermometers! Happy when they are within 1F all things the same.

                                Weights: Spot on for this no tolerance to be off never mind that's over 10%!!!!

                                Question now becomes how to we "verify" accuracy? I've used distilled water known to weigh 8lbs per gallon minus tare weight.
                                Do the math from there with known stuff.

                                Is the problem the equipment at your end or the product maker? If more than one thing - anything is off you simply have to adjust to knowns if possible or you have a complaint with a product.

                                With this truck performance was far enough wrong IMO we don't need nor anyone wrong temps and weights or products.

                                I don't have one universal answer or way for you to know your own equipment new, by brand names have to leave that up to the person to verify their own,




                                Tom
                                MetroWest, Boston

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