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2012 KW T370 with low pressures.

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  • 2012 KW T370 with low pressures.

    Hi all, FNG here in the A/C world. Mechanic for the last 18 but just started messing with A/C after taking a quick course thanks to Fleetpride. I had a truck with a dead compressor, leaking at the front seal. Replaced it, drier/filter, expansion valve, and a high pressure line from drier to expansion valve due to rust. In Vermont we like to put brine on the roads to rot everything in, on, and under the vehicles all in the name of saving some road side weeds.

    Anyway, flushed entire system, pulled a vac on the system for 50 minutes today, leak test for 10 and passed. All on a Robinair unit. Charged system with 3# of 134A per label on truck. Bad low pressure switch so replaced that as well. At ambient around 80-85°F about 65% humidity, my low was hanging at 20 pounds and high side was around 145, A/C on max recirculate windows open. I had 40°F in center vent.

    According to charts I've seen I'm low on both low and high side. If it cools this well can it get any better?

    At the 3# both lines at the expansion valve from evaporator were cold and wet. Just for a test I added another ounce. Now the return side of the evap started to frost up. Is that a sign there is almost liquid coming back? Just a tad too much? I didn't have my infrared temp gun to see where the condenser temps were.

    Thanks for any help, this was a dark art prior to last week but I'm trying to get the hang of it. At least it makes sense now.

  • #2
    I think you got it about as good as it will get.
    When testing or charging A/C don't use the recirculate mode, place it on high, but with fresh air, as this puts the maximum heat load on the system.
    Being it is a Tx valve system and not a Orifice tube, I'd leave the "extra" oz in there as it will do little to cause a problem and may be difficult to get just the right amount out. I should remain stored as liquid in the receiver and not cause a problem.
    I don't put much stock in those "performance charts" I look at the pressure/temp chart https://forum.aircondition.com/forum...pressure-chart
    It shows a condensing temp of about 110 deg or 30 deg over the air passing thru the condenser, not bad. The low pressure can be a bit more tricky. While it shows the boiling temp of the refrigerant in the evaporator (on a properly charged system) it can be effected by how long it takes for the evaporator to reach the equilibrium temp, where any more cooling will cause the frost switch to trip and any less will cause all the refrigerant to boil before it exits the evaporator. This can lead to lower pressures than you think it should have, but if left to run for 20 min or so will start to rise a bit.
    As far as I can see, everything looks good, you have good heat rejection from the condenser (something that can easily get damaged from brine/salt) and good vent temps.
    On my cabover it takes a good 20 min or so of running to bring the vent temps down to their lowest reading, where they will stay for the rest of the time the system is run. The system has to cool the heater core (on mine, the heater core is in series with the evaporator) and the heater box before cooling the connecting tubes to the vent outlets. If you base your diagnostics' before the system has cooled everything down, it can be a little misleading.
    In short, I think you have it fixed and would let it go as is.

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    • #3
      Thank you sir. The truck ran all day delivering LP without a hiccup. Driver was happy as a clam. On to the next job. Maybe I can convert my old Volvo over someday with a little more experience. Regards...

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