I know its been posted but i cant find it. what should the low side pressure be. also is the pressure supposed to drop as the rpms increase? if so what rpm should i check it at?

 

Good question. I recently bought a set of gauges and would like to know what exactly I'm checking for as far as pressure goes. I have always went with the "hand feel" technique of checking to see if I need more charge or not, and using gauges all i have ever done is is used them to hook to a freon keg and add freon to the compressors that have the low side service port running directly into the compressor to prevent washing out the oil while filling. I have never really hooked up to both the low and high side to check pressures. I do work on ammonia refrigeration systems at work, so i understand the principles of refrigeration, however an ammonia system is far more complex than an automotive system. Two completely different setups I mean. My current method has always worked, but I got a heck of a deal on these gauges used, so i might as well play with them a little bit. :D

-Justin :cheers:

 

r134 reada basically the same as 12, I use my 12 gauges and jinx the plumbing to work....I don't bother with spending money I don't have to...allthough the price of r134 gear is down a LOT from when I started with it some 6-8 years ago.....today it's not worth the bother....but then again I do household units also....so I can swing either way....

on the suction, LO side, large pipe out of the evaporator....the readings are the same as R12 for all practical purposes....meaning look on the gauge for a coil temp that is not lower than 32-30F temps....if it is...it will ice up, and that's not good....cuts airflow, and hard on the compressor.....
the high side with r12 reads around 250-300 lbs on a hot day.....about 50 lbs higher with r134....so the compressor is working harder, that's why newer cars have larger condensors in front of the radiators...by about 25%?? something like that.....
measure at say 2000 engine rpm's.....I do it in the driveway and just block the throttle up a bit with a screwdriver....cardboard....anything....

another thing, you have to put only about 2 12 oz cans of r134 in...maybe possibly 2.5 cans....otherwise the system will not cool worth crap, and I don't know WHY, just that it's said, and I have proven to myself it's true....do NOT OVERCHARGE.....only about 80% at most of the R12 quantity....

GENE

 

I would expect the pressure to change, but don't know what exactly the pressure should be. What is it that you are checking?

I looked at the web site for the manufacturers of the cans of R134a that I used to charge my system: National Refrigerants and Genetron. There is a partial chart which shows the pressure and boiling point temperature for R134a. Keep in mind that R134a boils at -15ºF with a vapor pressure of 85 psi where it is not in a container, i.e. it is allowed to expand, and is a superheated gas at room temperature (72ºF).

This site: http://www.wc101.com/guides/refridgeration/page7.php says that the low side should be around 30-40psi and never more than 50psi. This makes sense because the low side should be a gas. And, the chart on the National Refrigerants page shows a boiling point of 10ºC (50ºF) at 32.8psi for R134a which is about what I would expect the temperature of the gas to be at after passing through the evaporator. I would also expect the temperature of the R134a gas to be at around 0ºC (32ºF) when it enters the evaporator and thus result in around a 4ºC (40ºF) temperature out of the vents which I have already verified in my system. I would figure if your low side is about 40psi then the temperature out of your vents would be somewhere around 20ºC (70ºF) and not cool enough to do any good.

So, with the analysis finished I believe the low side should be around 30psi optimum and less than 35psi or it won't do any good.

 

My system went empty over the winter. I put in a 3oz can of just the dye, a 14oz can of dye and R134a and 3 12oz cans of just R134a with no dye.. I couldn't get the compressor to stay running so I kept adding R134a. My kit came with a color low pressure side guage. When pressure drops below 25 (green zone) the compressor cuts out. Give it a couple seconds and pressure builds up, compressor cuts in, pressure drops and compressor kicks off. The last can bought the system up to 65psi with compressor on. That's the high side of the yellow and I get really cold air. Never thought about using a thermometer to see actually how cold but I've had to turn it down cause it's too cold. So far it's been about a month and I still have cold air. I think I'll check it tonight just to make sure the system is holding the R134a.. Hey, maybe that leak sealer stuff really worked.. :thumbs:

 

Please keep in mind when you are rolling down the road and through traffic, the discharge air temps can vary a LOT with a convertible...depending on the air tight integrity of your car....meaning air leaks and such...coups too, depending....fire wall air leakage...etc....the closest temp drop across any proper functioning coil on the cold side is 20f....and that's about IT....
so if the car starts out at say 90f, when your turn on the system, it goes to 70f fairly quickly...then processes the air in the car...some cars are so super tight, you find yourself feeling chilly if not freezing your azz off and you turn down the blower speed...typically....just causes the system to cycle differantly, that's all....
so now as the car interior eventually cools down...depending on leakage and speeds, and whole LOT of variables...your discharge air temp drops quite a bit....don't forget it's only a 20f temp drop at any passage over the evap coils....

GENE

 

Eeehh... That's REALLY bad. The compressor is SUPPOSED to switch off and on. Your system is called CCOT, or Cycling-Clutch Orifice Tube. Note the "Cycling Clutch" part... With that much R134a in there I bet your high side pressure is sky-high. You're risking blowing a hose at anytime.

AC systems should not be charged by pressure. Always do it like the pros: By weight. If you are converting from R12 to R134a, the proper charge is 70-80% of the R12 charge.

 

i understand but i am wanting to check to see if the system has held properly for the past year since it has not been used.

 

Then you can pretty much ignore the high side. Turn the AC to MAX, fan high and look at the low side pressure. You should see it cycle: Starting at 45-65 PSI, compressor ON, pressure slowly decreases until it gets to 20 or so, compressor OFF, cycle repeats.

If the cycle is short (only 3-5 seconds) and the pressure never goes up much above 30 PSI the charge is low. If the cycle is normal (about 15-60 secs, depending on ambient temp), it's fine. Also check the temp out the vents. When everything is working well you should see no more than 50F on a 90F day.

The high side pressure will vary wildly depending on a number of factors, but mostly the air temp across the condenser. I've seen as low as 150 PSI high, and as much as 300PSI. Both were correct, but conditions were different.

 

How long should it take to cycle off? Mine was cycling off within seconds and not building up cold air. Now the system starts at 65psi and comes down at the same rate as before but stops around 25psi. and continues to run. I don't remember the system ever cycling on and off that quick when I first changed over the system and had it charged at a shop.. Cycling on and off that quick can't be good.. :confused: If I'm between the reccommended psi on the low side why would you think I will blow a hose? And if reading just the low side isn't a proper reading they why does the guage they sell with the recharge kit onloy test the low side?? I appreciate the input but ya got me worrying now.. :confused:

[/QUOTE]AC systems should not be charged by pressure. Always do it like the pros: By weight. If you are converting from R12 to R134a, the proper charge is 70-80% of the R12 charge. [/QUOTE]

Now you got me wondering if the shop knows what it's doing. I watched as they charged another car. The problem was that the compressor was cycling on and off.. He just added more R134a until the compressor stayed running.. I believe he said he added a pound?? Wasn't much and I don't know what type of system they had but his approach was the same one I used. :confused:


[Modified by Koz, 8:31 AM 7/8/2003]

 

Depending on conditions it could take anywhere from 15 seconds to over a minute for it to cycle off.

You may not even be overcharged (even though I suspect you are). Only way to know for sure is to check the high side pressure. If you see 350+PSI on the high side you're overcharged.

 

Almost forgot: First time you turn the AC on it will take longer for the compressor to cycle off as it has to pull the evaporator down from high temp to freezing. After that it will start the regular cycle at 15-60 seconds.

 

thanks for all the info zwede. next, since my system has the dreaded VIR is it going to cycle like that also?

 

So how do I correct if I don't have a high pressure side guage.. The kit only came with a low pressure side. I know you said you don't charge via pressure BUT should / could I turn the system on.. My guess (since I haven't put the guage on in a couple weeks) is that when running my guage will read about 27psi.. off it jumped to 65psi.. so if I start the sytem and she stays at 27psi just start releasing R134a until it cycles off??

 

Yes, that's what I would do. Since it cools well, you probably basically have a sound system, except for the charge. Start it up, block the revs to about 1500, and let it run for a few minutes. Then release a little bit of the charge at a time until the compressor starts cycling.

BTW, you can pick up a full gauge set (high+low side) from discount places such as harbour freight for about $50.

 

No, the VIR and the earlier TXV systems do NOT cycle. Only CCOT systems cycle. The TXV and VIR have a throttle suctioning valve that basically throttles the compressor so it only does as much work as it has to. The CCOT system run the compressor full tilt until the evaporator is ready to freeze, and then turns off.

With a TXV or VIR system I look for low side pressures slightly below the throttle suctioning valve setting which is 29.5 PSI for R12 and a few PSI lower for R134a. IF you have about 20-25 PSI low side you're fine.

 

I was just thinking.. How do you release the gas.. My first thought was to put the guage/valve on the sytem with no can attached to the valve and just open the valve to release BUT isn't that the suction side of the sytem..? Do I have to purge from the high pressure side?

 

Koz

Find yourself a friendly local shop and have them check the high pressure side. Most mom and pop shops will do it for free in hopes that you will come back if you have future problems. Chain stores normally charge to inspect the system.

 

Thanks.. Getting to the shop is the tough part. Most places want you to leave the car. I just tested the car.. Still holding 65psi and drops to 25psi in about 5 seconds and stays at 25.. It goes from just before the red which is overcharged acording to the guage to 25 which is just above the undercharge line.. Full swing of the "charged" section of the guage. I think if I remove any gas it will be cycling on and off every 5 to 10 seconds. That doesn't sound right either.. May be time to get the high pressure side guage. I changed over the sytem last year and paid a shop to charge the unit in hopes that I wouldn't have to keep dinking with it.. Imagine that.. :lol:


[Modified by Koz, 11:02 AM 7/8/2003]

 

Either side works. Even the suction side has more pressure than the atmosphere in it so stuff will come out. Be aware it is illegal to release R134a and R12 into the air, though...