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Old 05-05-2021, 04:25 PM   #1
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HVAC - Help - Low too low - High too high

Class A diesel, replaced compressor, did evac and recharged BUT.

The low side is just 15 lbs and the high side is 325 lbs on a 82dg F day. Fan runs on the condenser coil.

But the book low should be 40-50 lbs and high should be 276-345

If I try to put in more to raise the low I can get all the way to 400lbs and just 28lbs on the low side.

Even with bad numbers I get nice cold air at 46 dg F, full flow no icing.

Any Ideas?
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Old 05-05-2021, 05:00 PM   #2
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Step one is to charge the system by weight , then see what pressures you have. It is also important to have balanced the refrigeration oil level. I’m guessing an R134 system , and a correct charge is critical. Much more so than the older R12 systems. If the charge weight is correct , the most common thing that will cause low/low & high /high is a restriction - as expansion valve or drier. Occasionally a compressor failure will stop up the condenser tubes with particulate. Low heat dissipation, as in poor airflow across the condenser , or overcharging, will typically cause both sides to be high. If there is a restriction you will detect a temp change where it is.
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Old 05-05-2021, 05:04 PM   #3
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Step one is to charge the system by weight , then see what pressures you have. It is also important to have balanced the refrigeration oil level. I’m guessing an R134 system , and a correct charge is critical. Much more so than the older R12 systems. If the charge weight is correct , the most common thing that will cause low/low & high /high is a restriction - as expansion valve or drier. Occasionally a compressor failure will stop up the condenser tubes with particulate. Low heat dissipation, as in poor airflow across the condenser , or overcharging, will typically cause both sides to be high. If there is a restriction you will detect a temp change where it is.
Charlie
There is no specification for the refill amount. This is a DP not a ford clsss C.
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Old 05-05-2021, 05:07 PM   #4
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There should be specs for the fill amount and oil also despite being a class A. Mine was by the dryer on the front firewall. If all else fails reach out the manufacturer
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Old 05-05-2021, 05:30 PM   #5
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Correct, EVERY vehicle AC unit has a specified number of ounces for a correct fill. For example my Phaeton takes 32oz for a correct refill after being evacuated for at least one hour.
I would suspect you have a clogged receiver dryer based on the information given.
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Old 05-05-2021, 06:53 PM   #6
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This might be helpful, RV chassis refrigerant charge: https://www.proairllc.com/pub/media/...%20Systems.pdf
and RV air conditioning parts: http://acmeairparts.com/rvacheaterparts.php
Evans Tempcon diagnostic links: https://www.proairllc.com/design-eng...empcon-guides/
HDT dash air conditioning theory of operation: https://www.btrac.com/documents/svc-...i-feb-2011.pdf

I lost my link to refrigerant charge in lines by foot.
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Old 05-06-2021, 09:44 AM   #7
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Quote:
Originally Posted by gpounder View Post
Class A diesel, replaced compressor, did evac and recharged BUT.

The low side is just 15 lbs and the high side is 325 lbs on a 82dg F day. Fan runs on the condenser coil.

But the book low should be 40-50 lbs and high should be 276-345

If I try to put in more to raise the low I can get all the way to 400lbs and just 28lbs on the low side.

Even with bad numbers I get nice cold air at 46 dg F, full flow no icing.

Any Ideas?
Im assuming you mean 15psi on the low side gauge? IF the compressor is coming on, I would continue to add a couple more oz's of refrigerant to get the psi up to 45psi.

IF compressor is NOT running or cycling, you may need to jumper the low pressure cut off switch to force the compressor to continue running and pulling in refrigerant. I've also put the bottom half of can of R134 in a warm container of water to help speed the process up.
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Old 05-06-2021, 10:01 AM   #8
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Low side is wrong,along with high side being wrong, you have a restriction, evac, remove reciever drier, check it for restriction, and also check the expansion valve for debris, expansion valve might have a screen in it, should be NO metal particles anywhere in the lines if there is compressor is gone, clogging up the receiver/drier.
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Old 05-06-2021, 10:02 AM   #9
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Im assuming you mean 15psi on the low side gauge? IF the compressor is coming on, I would continue to add a couple more oz's of refrigerant to get the psi up to 45psi.

IF compressor is NOT running or cycling, you may need to jumper the low pressure cut off switch to force the compressor to continue running and pulling in refrigerant. I've also put the bottom half of can of R134 in a warm container of water to help speed the process up.
With the high side being too high in pressure, he certainly doesn’t to make it worse with more Freon. It sounds like there’s a restriction in the system. This is precisely why most laymen shouldn’t do AC work.
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Old 05-06-2021, 10:02 AM   #10
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46 degrees in the chute at 75 degrees ambient temp?

Call it good, until it is not.

That is what I do with a 30 degree differential.
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Old 05-08-2021, 11:20 AM   #11
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Update
I finally found a label with the fill quantity, 5 LBS for R134A.
So I evacuated the system and did a refill to the specification.
High side is normal now, about 200psi
Low side still at 15 psi, very low by the book.
But I get real cold air, 45 degrees on a 75 degree day, no signs of icing on the coil.

So it is working. Note the low pressure switch is not stopping the system so that is leading me to think the switch is up front on the dryer and my gauges are in the back just 2 ft from the compressor. Is the long hose affecting the low side reading?

If the system was somehow clogged causing a low reading would I get still get nice cold air?
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Old 05-08-2021, 12:35 PM   #12
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In my opinion, no. I would definitely not add more refrigerant, and assuming you are testing on a relatively high outside temperature day and given a good temp drop, coil sweating over the entire area, good hi-side pressure, I would button it up and go with it.
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Old 05-08-2021, 12:50 PM   #13
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45 lbs is waay too high! 15lbs is the right number to chase, particularly if the Schrauder valve is close to the compressor. As you can tell you’ve found the sweet spot!
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Old 05-08-2021, 06:04 PM   #14
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You said low “by the book”. What book are you referencing? If you are expecting similar gauge readings that you see on a car, then you will be off.

The extremely long run of hose from the evaporator back to the compressor drops the low side pressures lower than an automotive system. If you are getting 15 to 20 on the low, 200 to 250 on the high, and the low cutout switch is not cycling the compressor, then you are right where you want to be.

You were very smart to evacuate the system and start over again. You had too much refrigerant in it, driving the high side pressure into compressor fail territory.
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