R134a pressure question (2024)

Post: #1

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Post: #2

is that running at 1000 rpm on max cold?

Ideally you want 15-20 psi low side and 150 ish high side while running at 1000RPM max cool. If you let it sit and equalize it will be very close if you take whatever temp it is outside in Fahrenheit and thats what you pressures should equalize too.

Excessive low side means overcharge, low low side means undercharge or a plugged orifice tube or Expansion valve. High high side means overcharge, or the fan/cutout isnt working, or the condenser is blocked. Low high side means undercharge, too much cooling, or a stuck expansion valve.

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Post: #3

Don’t put too much stock in those numbers. It looks like you’re working on a 379/389. I run a few of them. If it’s an orifice tube system it should cycle about once per minute at 85 degrees ambient with the engine idling at 1100-1200, the engine fan on, and the dash fan on high with the controls set on fresh air.

High side will be roughly 150-175. The low pressure should fluctuate between 25-40. When the compressor is running the low side should slowly draw down to 25 when the low pressure cutout switch will cycle the compressor off until the pressure rises back above 40.

At 85 degrees ambient this should occur about once per minute. The higher the ambient temp the less the compressor should cycle. Ideally at 100+ ambient the compressor should run continuously at 30-35 on the low side.

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Post: #4

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Post: #5

The only testing literature a have is from a KW diagnostic on how to test and what center temp should be given ambient temp/humidity. Those numbers in the picture were at 1400RPM, max blower, fresh air, fan on, 90deg ambient with 50% relative humidity. The center vent temp was 72deg. The coldest I can get it in similar conditions while operating the truck at lower speeds/normal is 62-65.

This is a '14 389 with a 36" sleeper without rear air. It has a two-speed Horton fan clutch. Condenser & evap cores were cleaned prior to testing. Numbers at idle were 50 low & 150ish high.

I just feel it can do way better than it is. center vent temp will rise to 70-75 deg at idle and after a few rises like that it just becomes woefully inadequate. As far as I know it is all original. What bothers me is even if I replace the TXV, compressor & switches it seems that this 389 won't do much better than this but my KW could get down to 40-45 degrees.

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Post: #6

(06-09-2019 )Unilevers Wrote: iIdeally you want 15-20 psi low side and 150 ish high side while running at 1000RPM max cool. If you let it sit and equalize it will be very close if you take whatever temp it is outside in Fahrenheit and thats what you pressures should equalize too.

Is that the ambient times two plus 50 method for the running high side? Or the resting pressure?
I don't remember what the static pressure was...

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Post: #7

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Post: #8

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Post: #9

(06-10-2019 )Wiseman Wrote: Ok , that’s good info you gave.
I’ll admit that I’m not familiar with KW hvac box . But if heater core and evaporator are in the same box, and use one fan and blending doors, first thing I’ll do is to stop the coolant flow thru heater core and test again. Let stabilize and write numbers . Than put a good size shop fan in front of condenser and write numbers again.
This test will give you fairly clear picture what’s going on .
Let’s see and we’ll go from there.

The valve was already off for the test in the first post. It is a combined box with blend door and bypass door. I checked all the doors for range of motion while I had it apart cleaning. I'm not sure how a shop fan can out do the rad fan on lock at 1400 but I'll try it later this weekend.

(06-10-2019 )iceman Wrote: Is your Evaporator core clean?

Yes, it along with the cabin filter, heater core, condenser core, CAC & radiator while I was in the mood.

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