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Air Conditioner Low Pressure On The High Side


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#1 FriendlyMan

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Posted 25 June 2011 - 07:15 PM

I bought some gauges (and AC book) and refrigerant for my truck and added refrigerant according to the manufacturer's specs (from Haynes manual).

The compressor will turn on and cycle at 45+ psi, but doesn't stay on. It's rated and charged at 60psi (low side), which is normal.

The high side pressure is low, at around 120 psi when it should be much higher in the 400 275-300 range.

The pressures do equalize when the engine is off. When the compressor comes on, the low side measures 0 psi and high side 120 psi.

Ambient temp is 95 degrees F here.

There is no cold air.

Do I need a new compressor or could it be something else?

EDIT: Not 400 psi, but 275-300 psi - according to the table Quest Manifold Gauge and Hose Set Instruction.

Edited by FriendlyMan, 26 June 2011 - 02:55 AM.


#2 doug_scott

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Posted 25 June 2011 - 07:38 PM

I bought some gauges (and AC book) and refrigerant for my truck and added refrigerant according to the manufacturer's specs (from Haynes manual).

The compressor will turn on and cycle at 45+ psi, but doesn't stay on. It's rated and charged at 60psi (low side), which is normal.

The high side pressure is low, at around 120 psi when it should be much higher in the 400 range.

The pressures do equalize when the engine is off. When the compressor comes on, the low side measures 0 psi and high side 120 psi.

Ambient temp is 95 degrees F here.

There is no cold air.

Do I need a new compressor or could it be something else?


Not sure what book you looking at, but high side pressure should be two times ambient temp. Expecting 400psi means it is approx 200 degrees where you live. If the low side is truly zero, then you need to research how to properly fill an empty system. If that haynes manually actually says it needs to be 400psi on the high side, throw that book away and get a better manual. You do not fill based on pressures. If it were me, I would put more in via the low side.
And btw, if you have low pressures on a properly filled system, it means there is a restriction on the low side somewhere.

#3 asc08

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Posted 25 June 2011 - 10:04 PM

The low side should be in the 20-30 psi range, and the high side on a hot day will be in the 150 - 200 psi range (NOTE: do not charge to these numbers, there are several variables that go into this, but this is just an order of magnitude, or a reality check). If you run the system up to 400 psi, you will be replacing the entire system.

What year truck do you have? I am assuming that it uses refrigerant R-134a.

#4 FriendlyMan

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Posted 26 June 2011 - 02:51 AM

I bought some gauges (and AC book) and refrigerant for my truck and added refrigerant according to the manufacturer's specs (from Haynes manual).

The compressor will turn on and cycle at 45+ psi, but doesn't stay on. It's rated and charged at 60psi (low side), which is normal.

The high side pressure is low, at around 120 psi when it should be much higher in the 400 range.

The pressures do equalize when the engine is off. When the compressor comes on, the low side measures 0 psi and high side 120 psi.

Ambient temp is 95 degrees F here.

There is no cold air.

Do I need a new compressor or could it be something else?


Not sure what book you looking at, but high side pressure should be two times ambient temp. Expecting 400psi means it is approx 200 degrees where you live. If the low side is truly zero, then you need to research how to properly fill an empty system. If that haynes manually actually says it needs to be 400psi on the high side, throw that book away and get a better manual. You do not fill based on pressures. If it were me, I would put more in via the low side.
And btw, if you have low pressures on a properly filled system, it means there is a restriction on the low side somewhere.


Ah yes, I am sorry that was a typo there. I should correct that. If the high side should in fact be twice that of ambient temperature in degrees F, than ~190 psi should be the closer to the correct pressure? The low side pressure reads 60 psi when the compressor is cycling, but when it's shut off.

I have read quite a lot in the Haynes AC manual, and I have a hypothesis that air is not properly flowing on the evaporator core. Maybe because the air valve is not working. I'm going to take a look at it to see if the evap core is even getting cold soon.

I think there could be moisture in the system causing the restriction when the refrigerant line gets cold and it freezes up and I need to get it emptied and cleaned too, but I don't have another system to test or compare anything on.


The low side should be in the 20-30 psi range, and the high side on a hot day will be in the 150 - 200 psi range (NOTE: do not charge to these numbers, there are several variables that go into this, but this is just an order of magnitude, or a reality check). If you run the system up to 400 psi, you will be replacing the entire system.

What year truck do you have? I am assuming that it uses refrigerant R-134a.


I mistakenly said 400 psi. According to my AC manifold gauge papers, for my conditions (95 degrees F I should roughly have 50-55 psi low side / 275-300 high side. It's a year 2000, and it takes R-134a.

but I realize now this is probably incorrect and I'm looking for a better explanation and specifications on this AC system.

Also the AC clutch will not engage unless there is at least 45 psi there. I have replaced with switch twice with the same results.

Thank you both for helping out.

Anyone with a similar year/truck with AC working know the normal high and low side pressures?

#5 asc08

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Posted 26 June 2011 - 07:48 AM

Don't really know what the issue is, but if you feel the evaporator is not getting airflow, you can try cleaning it. there are a number of products available for that. Is there a cabin air filter that needs to be changed?

As far as moisture in the system, this is pretty unlikely, unless the system had a leak, ran out of refrigerant, and then got warm, moist air into it (or was partially disassembled, left that way for a while, and then closed back up). Otherwise, as a sealed system, it has refrigerant and oil under positive pressure. If it does have moisture in it, it will need to be evacuated and put on a vacuum pump to get rid of the moisture and air, prior to recharging.

Is the system original, and untouched, or has it been hacked into in the past? As above with the moisture opening up a system may introduce dirt and contaminants that may cause a restriction in a screen or orifice, but typically they don't just clog up, unless there is a compressor issue coming, and things are starting to let loose.

Before you start buying a lot of parts, and refrigerant, you may want to get an AC guy involved, and see if you can look over his shoulder. HVAC is one of those areas that has a lot of art involved with all of the science.

#6 doug_scott

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Posted 26 June 2011 - 10:46 AM

The truck has a receiver/dryer on it, that will take the moisture out of the refrigerant, but, it can only hold a finite amount of moisture. Back in the day, it was recommended that if the system was left open to the atmosphere for any significant length of time, you should replace the receiver/dryer.
You can check to see if they system is at least working by feeling the high side line (it is actually the low side at this point, but easier to find if you call it high side) right after the fixed orifice tube location, it should be cold. The fixed orifice tube is where the high side becomes the low side.
The proper way to fill the system is to do it by weight. You should apply a vacuum to the system for a min of 30 minutes to boil out all the moisture (water under a vacumm boils at less than room temp), then add the specified weight of refrigerant. Sometimes it is just easier to take it to a shop for this.

#7 FriendlyMan

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Posted 26 June 2011 - 07:40 PM

I am planning on taking the housing apart and measuring the exposed evaporator's temperature. I thought that maybe the air bend flap was not working.

I took the cover off today and adjusted the temp from the AC controls to see if I could hear any motion and feel any difference in the air temperature. Sure enough, I can hear it move and the heater setting blows out much hotter air. So that must be working.

I am going to clean the outside of the evaporator with compressed air and check for filters the next chance I get. If that does not work, I am thinking about either getting a service station to empty the refrigerant I put into it and having them vacuum pump it all dry. - I am thinking about doing it myself if I can safely recover the refrigerant.

I do not know if the AC system was untouched or not prior to getting the truck. I got it as is and I have had it for about 4 years and the AC never worked (I have never really used it, but I did run it from time to time).

I really wish I knew an AC guy, I might stop by a shop and see if I can get any advice from someone. I have heard that it does involve a lot of art before.


How can I tell if the dryer is bad without taking it apart? Is there any way to do that?

I do notice a temp difference on the low side line, but it's only very noticeable if I bridge the pressure switch connection to keep the compressor on. The clutch cycles on and off continuously, what is always noticeable is the heat on the high side.

I will try and take pictures and maybe a video of the compressor and pressure changes next chance I get.

Has anyone used a venturi pump like this one? http://www.harborfre...tors-96677.html

Thanks for the help and wisdom.

#8 FriendlyMan

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Posted 28 June 2011 - 03:46 PM

The evaporator is not getting cold. I noticed that there wasn't a filter where the filter goes in front of it when I took the cover off.

I checked in the Haynes manual and the AC system is specified to hold 2lbs of refrigerant. So after it is vacuum empty, it needs 32 oz? I was adding to try to get an appropriate measure of pressure, but obviously I cannot put too much to where the pressure is too great.

#9 FriendlyMan

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Posted 01 July 2011 - 02:31 AM

Alright so I found a solution to my problem.

First thing I learned is that the compressor has to be on and stay on for accurate pressure readings. Mine had cycled on and off and I was reading the pressures while it was on for a few second than shut off. This was confusing me. Being the first time I've worked on AC and not had something to compare it to was a puzzle.

Second thing is that when you're filling the system with refrigerant, the pressure when the compressor is cycling, that is when it is on for a few seconds, the pressures on the low side are much higher at first (and high side pressures are lower). It isn't until the compressor finally begins to stay on that it evens out to pressures that you'd expect to see.

I had gotten my refrigerant removed than went to harbor freight and bought a 2.5 CFM vacuum pump for about $89 and ran it on the system for 30 minutes. It was already 95 degrees F outside and my engine was still hot. I removed all of the moisture in the system. And began to fill it up with the 2lbs of refrigerant my truck's AC system called for.

I bought 3 12oz cans of R-134a (2 lbs = 36oz). I put two in there, the compressor came on and ran steadily, and I checked the air temp and it was arctic cold. Checked the temps, 95 deg F outside, it's blowing out 47 deg F air. Checked it again later when it was 88 deg F outside and it was blowing out 40 deg air.

Also, I realized that it can take 15 mins before that can is empty, and you don't need to turn it upside down until it's almost empty.

That's just about all I can think of, I considered cleaning out the lines if they had oil in them and checking the orifice tube but I didn't. I only put two cans in.

The pressure reads 50 psi on the low side and 270 psi on the high side for me, with slightly less refrigerant than it calls for. It's ultra cold, and After driving for 2 hours down the highway it was wonderful.

I hope my experience teaches somebody else out there thinking of learning about AC systems., and that they write about theirs.

All total, I spent:

$50.00 on AC manifold gauges
$50.00 on three 12oz cans of R-134a (and only used two)
$89.00 on 3.5 CFM vacuum pump

Might sound expensive, but the investment in tools is a great one in my opinion.




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