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HomeOff Topics › Need some AC guide help....can't find the specs of pressure
01-13-2009 07:39 AM  9 years agoPost 1
InvertedDude

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I would like to ask for some guide on an AC issue on a 2000 blazer. This is the 5th time the vehicle will be fixed due to faulty techs that either take short cuts or just don't know what they are doing.

I have decided to tackle this project for myself. The truck developed a leak on the high pressure port and vented the R134a.

I went ahead and replaced the high pressure port line. $55.00 from GM

I purchased a set of gauges and 3.0 CFM AC pump.

I added 2oz of PAG spec oil due to leaking hose during replacement.

The system spec for the 2000 blazer will use 6 to 8oz of PAG oil. Will use 1.75 lbs of R134a freon.

Installed new orifice tube.

Pressure sitting idle with vehicle turned off with outside air temp of 55 degrees shows 65psi on low and high side.

I went ahead and fired up the AC system and it started blowing cold air...

Checked the pressure on the low side and it ranges from 45psi to 23psi and cycles the compressor off when it hits 23psi and climbs back up to 45psi to turn the compressor on.

Checked the high side and noticed that it stayed at 70 to 75psi. I thought it should be around 100 to 150 psi? I did noticed when the vehicle is started up for the first time the high side is around 100psi.

I checked the air temp in the vents and it is blowing at 29.8 degrees F. Yeah ...cold and all but I am suspicious of the high side so I counted the time for the cycle and it went 20 seconds ON time and 27 seconds OFF time.

Is my compressor bad or low on freon because I have no clue or reference to go from here.

Vehicle 2000 blazer LT model automatic climate control.

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01-13-2009 07:47 AM  9 years agoPost 2
mchammer

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It is cold outside so the system is probably cycling on the low pressure cutout.If you charged the system with the 1.75 lbs on the spec you will be fine.

Peace Through Superior Firepower!!!

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01-13-2009 07:48 AM  9 years agoPost 3
InvertedDude

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01-13-2009 07:12 PM  9 years agoPost 4
InvertedDude

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01-13-2009 07:56 PM  9 years agoPost 5
Brokenlink

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Oakdale

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Take the ambient air temp and subtract 35 from it. Then cross reference this number to a pressure/temperature chart for the refrigerant you are using and that number you find is what you should read on your low side at idle.
example: if it is 90 degrees outside, subtract 35 (because you are using a forced convection evaporator) that leaves 55. At 55 degrees the pressure for R134a will be roughly 51 psig. This is your low side reading.
For your high side you will take the ambient temperature and add 30 to it (froced convection condensor) then cross reference the number.
example: 90 degree day add 30 degrees = 120 degrees. At 120 degrees the pressure for R134a is roughly 170psig. This is yor high side reading.
So on a 90 degree day your gauges will read low 51 high 170.
On an 80 degree day they will read low 39 high 145.
Here is a good site to convert if you want it.

http://www.csgnetwork.com/r134apresstempconv.html

Jamie Griffith

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01-13-2009 10:20 PM  9 years agoPost 6
InvertedDude

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01-13-2009 10:46 PM  9 years agoPost 7
InvertedDude

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01-13-2009 11:53 PM  9 years agoPost 8
Brokenlink

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Well the chart is just going to be in the ball park.Did you run a vaccum pump on you system first?What should your low and high side pressure be? There is no magic answer for that question.There are too many variables,Compressor (engine) RPM and airflow across the condenser are always changing,also the engine speed is always affecting pressure.30 psi on the low side is just about right is because that translates into an evaporator temperature somewhere around the freezing point of water. Look at your low side R12 gauge and you'll see a temperature scale right next to your pressure scale. That low side pressure translates into evaporator temperature. Since moisture collects on the evaporator,you want to keep the evaporator temperature slightly above the freezing point. R134a low side pressure will be be slightly lower (27 PSI) at this temperature.

What should the high side pressure be?
With R12 systems, high side pressure is usually 1.8 to 2.1 times ambient temperature. That means on an 80 degree day, with moderate humidity,you would expect to see between 144 to 168 PSI on the high side. On hot humid days (with R12), you could say ambient temperature plus 100 PSI., and be pretty close.

With R134a it's common to see high side pressure between 2.2 and 2.5 times ambient temperature. On that same 80 degree day you would see between 176 and 200 PSI on the high side of an R134a system. The system operates in a specific range based on outside ambient temperature. High side pressure has a broad range relative to temperature because of heat load on the evaporator, humudity, airflow across the condenser, and engine speed.

Jamie Griffith

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01-14-2009 05:56 AM  9 years agoPost 9
mchammer

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The formulas are great for a ballpark figure as brokenlink stated.If you are trying to top off a system that is low and you dont realy know how much refrigerant is in the system already; this is a good way to do it.What you dont know are many variables such as condition of coils or if they are partially restricted from debris,compressors pumping efficiency etc.The bottom line is the mfg spec is the exact charge weight for your system. If you have evacuated the system and have installed the exact charge by weight per mfg spec that is as close to perfect as it gets.IMHO I will go by weight over pressures any day.Your system was engineered for this specific amount of refrigerant and if it doesnt function propperly with that amount you have other problems.

Peace Through Superior Firepower!!!

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01-14-2009 06:23 AM  9 years agoPost 10
kingair

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Lots of good advice here for you but IMO if you try to do this below 85 ambient you'll very easily overcharge the system.

There is a very fine line between "hobby" and "mental illness."

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01-14-2009 06:36 AM  9 years agoPost 11
InvertedDude

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01-14-2009 12:47 PM  9 years agoPost 12
Brokenlink

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You probably have it closer than some shops would get.I have worked on some that have just come from the shop and all they did was put a couple shots of dye check in to check for leaks,then a couple cans of freon and sent them off on their merry way.

But it sounds as if you got a handle on it though,you have it cold enough in there that you would have to be part snow Mexican (Eskimo)to ride in your blazer.

Jamie Griffith

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