AC High Pressure side reads low pressure

[bdufpv]

I have a 2002 eddie baur explorer with the 4.0 sohc engine and rear AC.

the compressor was not kicking on so i added freon. I only had the low pressure gauge at the time. i charged it to 45 psi and the compressor kicked on and the low side pressure immediately began to drop and the compressor turned off at 17 psi (the ac low pressure switch doing its job). once it kicks off the pressure builds back up to 45 psi, the compressor kicks on and the pressure immediately begins to drop again. this process will continue until i turn the ac off.

i have since gotten the high and low side gauges.

with the engine off the
low side is at 50psi
high side is at 50 psi

with the engine at 1500 rpms, the ac on max cold and max fan and the ambient temperature 75 degrees and a fan blowing on the engine compartment…

the low side starts at 50psi the compressor kicks on and drops to 17psi and kicks off- the same scenario as described above
the high side goes up to 80 psi when the compressor kicks on and drops to 60psi when the compressor kicks off.


the low pressure on the high pressure side sounds like the compressor is wearing out. is this what is happening?

i am not sure why the low pressure side goes up and down. is it related to the compressor? is there air in the system? i put some uv dye in the system i haven't found the leak yet.


i will have to do research on what causes the compressor to wear out and what other parts need to be replaced if that is my problem. i am hoping you guys can tell me before i spend a lot of time researching the wrong topics.

thanks
brian

 

[esclamada]

there's a leak somewhere or just low on charge, check that UV dye you used. A dying compressor will eventually explode or make a noise before cracking.

here's some troubleshooting guide.
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[Donystoy]

Sounds like you have a leak, Common place on these vehicles is the side of the drier. Try to get your hand down the side of it inside of the black Armaflex insulation. If you feel oil towards the bottom you have a hole corroded through the side from the inside out. They are around $70 or less to purchase. Of course to do this you have to have the air evacuated prior to re-filling with refrigerant.

 

[bdufpv]

thank you for the replies. there is definitely a leak. i saw the dye but i haven't been able to locate exactly where the leak is. its by the compressor.

does the leak explain the rise and fall of the low pressure side when the compressor kicks on?

does the leak explain the low pressure on the high side?

thanks

 

[Donystoy]

The pressures you indicate above shows normal operation with a low charge. When the compressor is off the pressures on both the low and high side will eventually be the same and if they don't there must be some error on one of the gauges. A proper charge in warm weather will have considerably more pressure than what you indicate when compressor is off. Edit, when I say off that is after a few minutes as the system has to equalize its pressure through the metering device before they read the same. If you had a proper gauge manifold attached to the system you can speed this up by opening up both valves on the manifold at the same time thus equalizing through it. Just remember to bleed any air out of the hoses individually first so you do not contaminate the refrigerant with air.

Even back in the day I would put a standing charge of 70 pounds of nitrogen in the system prior to engaging the compressor just to test the integrity of the compressor.

When you do get the leak repaired and properly evacuated, the refrigerant gas should be weighed in to get it correct. An undercharge effects the cooling of the compressor while an overcharge causes high head pressures that can blow the condenser, lines etc. and also damage the compressor from ingesting liquid instead of only vapor. When I see those compressor blown up my first assumption is that they had way too much refrigerant gas.

 

[bdufpv]

at one point i had the low side charged to 70psi according to the manifold gauges. the manifold gauges appear to be working correctly.

with the engine at 1500 rpms, ac on max cool and max fan the pressure on the low side was still dropping to 17 psi and the compressor would kick off and the pressure would rise back up to like 60psi and the compressor would kick on…. and repeat until i turned off the ac. (after turning off the ac the low pressure side settled back at 70psi)
the high pressure side would reach 110 psi when he compressor was on and drop to 80psi when the compressor kicked off

i know the leak is still an issue, but is it causing my problems…

at 70 psi (not weighing the freon) on the low pressure side how is it acting like there is a low charge?

why doesn't the high pressure side reach appropriate levels? ~170psi

at this point i am getting close to taking it somewhere, i just don't want them to tell me i need to replace something that i don't.

 

[Donystoy]

You apparently have a leak which should be the first item addressed. A system that you have to re-charge all the time will only end up costing more in the end. The fact that the suction side is going down to 17# and causing the system to shut down on the low pressure control says that you do not have enough gas in the system. 110# on the high side is way low. I do not know what the ambient temperature is where you are (assuming that it is a hell of a lot higher than ours right now) but I think the charts supplied by esclamada in an earlier post will give you an indication as to what they should be. From his charts if the ambient temp. is 80deg F the low side will be around 40#-50# and the high side 175#-210#. With a hot interior these pressures will likely be even higher until it is down to a comfortable range.

If you take it somewhere make sure that you let them know that you want the leak repaired, evacuated and re-charged. The industry is full of shops that will tell you that it is only a tiny leak and all you need is a re-charge. All they are interested in is for you to come back when it quits again so they can get more money out of you. You indicate that there is already dye in the system so the leak should be easy to find. Any competent mechanic with an electronic leak detector and soapy water should be able to find it. It only gets complicated when none is found and the evaporator inside the vehicle is suspected.

If you say the leak is by the compressor, you might have to have a hose assembly replaced. Have you checked out the receiver / drier as I mentioned in my first post? It is at the passenger side of the rad and is covered with black insulation which might be held on with tye-wraps. It is the unit that your low side gauge goes to. Cut the tye-wrap and push down the insulation so that you can examine the aluminum cylinder. Very common to have these corrode through about 3/4 ways down and will be all oily. This item should be replaced anyway if the system is opened up and major surgery is done.

Just an added note, Whenever items are replaced, oil has to be added to make up for what has been lost via the leak. The oil is constantly moving through the system with the gas. Due to this when the system is new, an overcharge of oil is put in the compressor as it is known that soon some will be circulating throughout the system. A certain amount will end up in the receiver / drier so when ever I have replaced one of these I would cut the old one in half and remove and measure the amount of oil in it. I would then pour this amount of new oil into the new drier and add a little that was lost due to the leak over time. If I remember correctly these vehicles use PAG oil in their A/C systems. Sorry for the rambling post!

 

[esclamada]

at one point i had the low side charged to 70psi according to the manifold gauges. the manifold gauges appear to be working correctly.

with the engine at 1500 rpms, ac on max cool and max fan the pressure on the low side was still dropping to 17 psi and the compressor would kick off and the pressure would rise back up to like 60psi and the compressor would kick on…. and repeat until i turned off the ac. (after turning off the ac the low pressure side settled back at 70psi)
the high pressure side would reach 110 psi when he compressor was on and drop to 80psi when the compressor kicked off

i know the leak is still an issue, but is it causing my problems…

at 70 psi (not weighing the freon) on the low pressure side how is it acting like there is a low charge?

why doesn't the high pressure side reach appropriate levels? ~170psi

at this point i am getting close to taking it somewhere, i just don't want them to tell me i need to replace something that i don't.

Your readings fluctuate because of a leak in your system, it can't stabilize the pressure. If for example you have a very small leak (that last for a month) and it's low on refrigerant, initially the low and high pressure will fluctuate until both of them stabilizes. At an ambient temp of 75, the low pressure should remain constant at 45 (max) and high pressure at 170 (max). This is the time you stop adding refrigerant.

If the leak is large enough, you'll never reach this pressure and end up wasting refrigerant.

IMHO A/C servicing is not a DIY thing, just go to a shop they'll handle it for you easily and not waste those r134a cans.

If you're going to do a DIY route, you at least need a vacuum pump and an AC manifold gauge. Since the refrigerant already leaked there's no point in evacuating it.



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[bdufpv]

i have since replace the AC hose assembly which had a leak in it and replaced the accumulator. i vacuumed the system for 60 minutes and it held the vacuum for 60 minutes- there is no leak in the system.

when recharging the system i was only able to put about 10 ounces of freon in when the same scenario as above began to repeat…..

with the engine at 1500 rpms, the ac on max cold and max fan and the ambient temperature 75 degrees and a fan blowing on the engine compartment…

the low side starts at 50psi the compressor kicks on and drops to 17psi and kicks off-
the high side goes up to 80 psi when the compressor kicks on and drops to 60psi when the compressor kicks off.

i am confused why it took so little freon and why it has the same problem. it should be able to stabilize pressure now that there is not a leak in the system.

i even took the low side upto 70 psi to see if that would keep the compressor from reaching the low pressure threshold and it still shut the compressor off because it reached about 17psi on the manifold gauges.

any ideas?