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2006 MAC Truck CX688 Refrigerant charge amount.

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  • 2006 MAC Truck CX688 Refrigerant charge amount.

    Hi,my name is Nick and I am new to your forum.I am from Australia and I have been working in automotive AC industry for over 30 years.I am working on this MACK truck RHDrive cab AC only (no sleeper) with a history of evaporator freezing up ( owner has seen the pipes after Tx valve frozen),I have replaced Tx valve and R/Drier (supplied by owner) truck had new condenser fitted not long ago due to an accident. I have regas it with 1150gr of R134a (pressure and sight glass method) on 34C day and was fine. I need to find out the recommended charge of refrigerant for this truck (a lot of research on the net suggest 3lbs=1350gr of gas ???) before I go any further with my diagnostics ( owner say that it's still freezing up pipes after Tx valve) I have asked the owner to check evaporator to see if it's not clogged up with dirt and he said that looks clean ( I will check myself when I go back to the job).I hope that somebody here will know the charge amount for this truck (I have about 4 different AC charge charts but none have MACk trucks in them)and will be a great help for me,thank you.
    Last edited by nickyanc; 04-27-2020, 02:29 PM.

  • #2
    Welcome and trying from a mostly US site bear with me another here is really sharp with the trucks of the sort.
    I'd just like to clear one thing up please do know I don't speak (think that way) metric measures but my calculation is from your quote " (a lot of research on the net suggest 3lbs=1350gr of gas ???) "

    I came up with 5.25 OZ (Ounces) +/- decimal points. That's so different in a car (I'm better at) would ice up just for being huge capacity and low. Would snap out of it and come back and yes the ice melted so air could go thru. First sign might be airflow is dramatically reduced the vents like dirty as you suspects yet isn't.

    Hold on I really want to see what my associate here says just lots better with the "rigs"- "Semi truck or.tractor" as I would call them - forgive me both US English differences and my location has an accent (alternate whole words) that's messy on top,
    Tom
    MetroWest, Boston

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    • #3
      A quick look on the Big Mack site seams to come up with 3lbs for a day cab CX. Something in the back of my mind remembers another guy having a problem with frosting of the evaporator and there was a sensor, not a snap action switch in the evaporator to limit the evaporator temp. It was on a low cab forward model, so that would be different then yours but might be controlled the same. The Tx doesn't care how cold it gets, just that the inlet and outlet are close to the same temp. Obviously anything reducing the air flow, will reduce the heat load on the evaporator. Dog fur and dust can make a thick mat that can really cause a problem.
      I come up with 1360 gr for 3 lbs.
      If you are close on the refrigerant, got good heat rejection at the condenser and good air flow over the evaporator, that really only leaves the frost control not working and letting the evaporator and return pipes get too cold.

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      • #4
        Originally posted by Cornbinder89 View Post
        A quick look on the Big Mack site seams to come up with 3lbs for a day cab CX.
        I come up with 1360 gr for 3 lbs.
        If you are close on the refrigerant, got good heat rejection at the condenser and good air flow over the evaporator, that really only leaves the frost control not working and letting the evaporator and return pipes get too cold.
        Thanks for all your help,last time I had a look at this truck on a mild day 25C I only noticed the evaporator inlet pipe right at the exit from Tx valve was getting frozen but I had a good temp at the vents 6C on 3rd fan speed so the frost regulator didn't cut the power to the compressor,the evaporator outlet or suction hose was not freezing. All of this freezing was happening only while the big radiator fan was engaged,as soon as the the fan stops the frost disappear,so I was thinking that insufficient refrigerant may cause that (not enough refrigerant liquid pressure to make the Tx work efficiently) or 2 other things:
        1:I have a restriction somewhere in the liquid feed to the Tx ( liquid hose has a splicer joint maybe it was over crimped,reason for thinking that was that about 2 months ago I have come across on a new F150 Ford with R1234yf where the compressor has seized due to suction hoses being over crimped and the hole inside those fittings reduced from 10mm to 5mm ).
        2: wrong TX valve ( 2Tone supplied by owner and should be 1,5Tone.)
        To me it make sense at what the owner is saying that the evaporator pipes after Tx valve are freezing while he's driving because a good air flow at the condenser it's keeping the liquid pressure too low for the Tx valve to work properly. Probably next week I will go and look at this job again and will keep you posted.
        Last edited by nickyanc; 04-28-2020, 12:37 AM.

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        • #5
          Too big a Tx can and most likely is the problem. If it can't meter down low enough some liquid is boiling in the suction line. If the temps are ok, and no liquid is making its way back to the compressor to slug it with liquid, I wouldn't worry. Look at it this way, what if you turned on the A/C on a cool day? the condensing temp would be low as would the high side pressure, I guess I'm saying the high side pressure can't be "too low for the Tx valve to work properly". High side pressure is tied to condensing temp.
          Big truck systems are a little different in that the radiator fan doesn't turn or draw air over the condenser until the high side gets high enough to turn it on or road speed is enough to keep the head pressure low (enough air flow thru the condenser) So sitting still you'll see the high side climb, the fan turn on and pressure fall until the fan cuts off and the cycle starts again. hitting the fan overide will give a good indication of what would be happening at road speed, while sitting still.
          I guess I'm saying I wouldn't worry about a little frost on the outlet of the evaporator, if you have frost all the way to and on the suction port of the compressor, I would be concerned that you will get liquid entering the compressor.

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          • #6
            I wouldn’t have worried about that bit of frost either but the guy reckon that AC is not cold (when i test it was cold) when he manly uses it when is loading the truck or in the loading area where is too much dust so he close his windows and run the AC.
            I am going to see him next week and i’ll add another 150gr of gas and see what will happen,thanks for your help

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