Recharging AC - What pressure?

[Shdwdrgn]

I picked up one of those recharge kits with the gauge that adjusts based on air temperature. This weekend it was about 95F outside, so I had the dial spun all the way up. That puts 45-50psi right in the center of the 'ideal' range for recharging.

So I rolled down my windows, cranked the AC to max fan, and put in about 3/4 of the bottle. Every time I let go of the trigger, the gauge dropped back to 43psi. So how do I tell when the system is fully charged? Am I just seeing the standard pressure on the back side of the pump and this is an indication that my system was nearly empty? Should I keep adding more coolant unti I see an actual rise in the pressure reading?

The coolant I added did make a noticable difference -- the air blowing out is actually cooling down the truck again, although I don't think it's still quite as cold as it used to be. I don't want to put in too much coolant and over-charge the system, but at the same time I would expect the pressure to change at some point indicating I am actually fully chraged... What should I do?

 

[Texan]

How many ounces of Freon were in the bottle(can)?
You really need a manifold set (low & high pressures)
to analyze the system. I would be inclined to add the
remaining 1/4 bottle to the system and see how it
preforms.

 

[Shdwdrgn]

It's this one -- a 20oz bottle. The AC has been bone-chilling cold for the last several years, but this year it has been taking awhile to build up. Coming home one day I was half way home before it actually started getting cold, so I stopped and grabbed this refill kit. I was expecting to see the pressure go up as I added coolant, but since that isn't happening it's hard to tell if I'm getting close or still need another bottle of coolant to top it off.

 

[meerschm]

you risk too much if you put more in. (and damage to the system, and if you are unlucky, pain for yourself----I watched the guy across the street do a DIY R134a add to his car, and the hose blew. just lucky he was not looking at it when it blew. oil everywhere.)


you need at a minimum, a full set of gauges, to watch the high side pressure, as well as the low side.

the proper way is to empty, and fill by the weight.

 

[Texan]

I would not add the stuff you bought because it
has a "leak sealant" in it. Find somebody that
has a set of gauges and see what the high pressure
reads. High side will be around 200 psi at around
90 degrees ambient temperature.

 

[Shdwdrgn]

Texan: As mentioned, I have already put most of the bottle in already. And nobody I know has any tools for working on cars, so I'm out of luck there. But really, how hard can this be? If thousands of people recharge their own AC every Summer, then there must be some reasoning to the method of reading the pressure from the low side.

 

[gmcman]

When the A/C is running, the high side builds as the low side drops, then when the compressor shuts off or cycles off, then the low side rises and the high side drops as the pressures begin to equalize before the compressor kicks on again. This is probably why your reading on the gauge may fluctuate, also, there's more pressure in the can to force the charge into the system so the gauge reading will change when the trigger is pulled.

So I agree it's not that difficult to turn the dial or set the low side to the corresponding ambient temps, you will get real close to your target charge. But like others have mentioned, it's not that simple when you are getting close to what the charge should be which is why gauges are essential in fine tuning the system.

The entire system is designed to efficiently run on a set amount of freon, adding more freon doesn't necessarily allow it to cool more, it just overworks the compressor and can cause damage.

 

[Shdwdrgn]

I understand you're trying to help, but I didn't say anything about the readings fluctuating. I have exactly the opposite problem -- the reading has not changed at all as I have added new coolant. Yes the needle bounces to max when you hit the trigger, that's why you only take your reading after you release the trigger. The AC is not kicking on and off, it was running full blast with the windows down on a 95-degree day.

As I said before, I emptied about 3/4 of the bottle into my system and the pressure reading has not changed one bit. It *always* reads 43psi when I let go of the trigger. The system was obviously low on coolant because the AC is working pretty good again (just not as great as it did last year). I understand that adding too much coolant is both detrimental to the ability of the system to cool, and dangerous if I put too much in. That's all fine and dandy, but it doesn't answer the original question... If the target charge at 95F is about 50psi, why is the reading on the gauge not changing as I add the coolant?

Additionally, I checked it again tonight when I got home. As before, windows down, fan on max, AC blasting, however today the temperature was only 82 degrees (and thus the target pressure should be closer to 40-45 psi). I would have expected to see a difference in the system pressure, but it was once again right at 43psi. I gave the system a couple more squirts of coolant, but the gauge still goes right to 43 every time. Of course judging by the table, today I'm right at my target pressure, but getting the same reading at two different ambient temperatures only deepens the mystery.

 

[meerschm]

http://gmtnation.com/forums/gallery/image/215-photo419/

Here is the chart from the manual for the '03.

you are on the right track that the test conditions are parked in the shade, windows open, system on max cool and full fan. then wait for five minutes or so for stability.

there is a range of values, and you have to see the high side as well as low side.

the readings vary with not only the heat, but the humidity.

as described before, the clutch cycles on and off under these conditions if it is too cold, or if there is not enough R134a in the system.

if you have steady state operation, and the clutch stays engaged,

and you are too cheap to order a set of real gauges, be happy with what you have.



also look at how clean and free from damage the AC condenser is (radiator-like thing in front of the actual coolant radiator) if it is full of gunk, the system will not operate well)

you also can look at airflow inside the dash, and make sure all the temperature regulator doors are working properly.

 

[gmcman]

What happens when you or have someone hold the throttle at about 1500 RPM?


The compressor may not cycle at idle, but things change when the engine is above idle. The pressures may not even move at idle.

Many things affect your cooling...proper cooling fan operation, clean or dirty condenser coil, do you have a cabin filter...is it clean? Not sure what years had them.

 

[NJTB]

When partially charging the AC, you won't know how much freon is actually in there, theres no 'full' indicator. The air gets warm because there's not enough freon to keep the evaporator cold enough. To be sure, an evacuation/recharge is necessary.
The 43PSI is because thats what the orifice tube (in low pressure line near accumulator) is reducing the high side pressure to. That translates to about 43 degrees F., and will be the temperature of the air coming out the vents in the car.
The problem comes in when there's too much freon added to the system. The freon doesn't get a chance to turn back to vapor, and enters the compressor as a liquid, damaging the compressor.
I think the label inside the engine compartment says 2.2lbs of freon (check, please). If the discharge air is too warm, add until it starts to get cold (while running), take it for a ride to see how cold it gets. If good, stop there, if not, add a little more. Be careful.

 

[Shdwdrgn]

I hadn't heard of letting it idle for 5 minutes to stabilize... Is that assuming from a cold start, or does it also apply after the vehicle has been driven?

Last time I looked, the condensor was nice and clean, very few fins bent... but it has been a year since I looked, certainly worth checking again to be sure. There's no cabin filter, and there is plenty of airflow through the vents, no apparent obstructions anywhere.

Well at least it's actually working now. We'll see how long this lasts. If it loses its charge again this season then I probably have a leak somewhere and would need to take it to a shop anyway. Ugh.

 

[meerschm]

manuals posted someplace for download should explain the test conditions.

I sold my paper manuals.

all the variations is why the real spec is to fill by weight.

if you really wanted to check, you would have to use high and low gauges, and check the temperatures in a couple places. (subcooling and superheat) and then, the best path would be to compare with known good values on that vehicle in similar conditions.

 

[Mounce]

Manuals can be found in Mooseman's signature for free download.

 

[Mooseman]

Also pinned at the top of the tech section

 

[Trios]

These cans of 134a were probably thought up by someone in the vehicle HVAC repair business. You'd think that letting the customer do the recharge themselves would be bad for HVAC shops, but that's really far from the truth.

Vehicle HVAC is not something where you can just go up to a vehicle, shoot a couple ounces of R134a into the low side port, and call it good. If you do that, you run a (fairly significant) risk of damaging the compressor, requiring replacement of the compressor and likely the condenser as well. This means $$$ for the HVAC repair guys.

The only way you can recharge a partially-charged system is with a set of manifold gauges and the FSM pages which indicate operating pressures and associated temperatures. Any other method ('charge until arrow is in green area!') is faulty guesstimation.

You might not believe me based on this, but I don't actually do auto HVAC repair, but I am in the HVAC business (building automation controls) and I work with refrigerant mechanics. I do work on my own vehicle's auto A/C, under the instruction of a refrigerant mechanic and with a proper set of manifold gauges, a vacuum pump, and a refrigerant scale. The best thing to do IF YOU HAVE ALL THOSE PIECES is to evacuate the system fully, check for leaks, if no leaks purge with nitrogen and check for leaks when pressurized with nitrogen (DO NOT USE AIR, skip this step if you have no access to pure, dry nitrogen), evacuate again, and then charge with exactly the specified amount of R134a and no more (using a refrigerant scale).

 

[budwich]

Depending on your year (although I think most TB), you can probably "skip" buying the gages and use the torque app since the AC system monitors the pressure of the AC as it will among other things shut down the compressor. So if you have access to the torque app or are planning to go that way, I would try that out first in combination with the "cheap" low pressure gage that comes with the can. Of course, as mentioned a "full and proper AC service" is best but so is winning a lotto.

 

[Tiggerr]

Torque doesn't show the ac pressure on mine.

 

[Mounce]

Never seen an option for ac pressure in torque.

As far as I know the system doesn't monitor it's pressure, just has a low and high (I think it has a high one) pressure switch. It don't see numbers, just a yes or no.

 

[Tiggerr]

Never seen an option for ac pressure in torque.

As far as I know the system doesn't monitor it's pressure, just has a low and high (I think it has a high one) pressure switch. It don't see numbers, just a yes or no.

I've seen the option for it in torque. Not supported tho. At least not by my '04. Or my adapter maybe. I have the bafx one

 

[budwich]

works fine on 2008. According to schematics, the "info" appears to be feed to the pcm which then "decides" on what to do with it.

 

[budwich]

Never seen an option for ac pressure in torque.

As far as I know the system doesn't monitor it's pressure, just has a low and high (I think it has a high one) pressure switch. It don't see numbers, just a yes or no.

I think if you do a scan of the some of the schematics, there is NO high pressure switch. I do believe the PCM does all the "work". Whether its "reportable" seems to be determine by GM obd data PIDs.

 

[Texan]

Bought my 03 TB new, 12 years ago. Now has 138k miles
and has been best vehicle I have owned, knock on wood.
All maintenance done by the book. Have not had one problem.
I changed the thermostat, radiator hoses and antifreeze last
week. Original GM 12572988, new GM 12620112 made in
Mexico. Both have BEHR in casting. While doing this project,
I noticed that the AC accumulator is surrounded by what appears
to be Styrofoam insulation in the bracket. This would prevent
my old school way of determining the level of Freon by looking
for or feeling the sweat back line on the accumulator vessel.

 

[6716]

It *always* reads 43psi when I let go of the trigger. The system was obviously low on coolant because the AC is working pretty good again (just not as great as it did last year). I understand that adding too much coolant is both detrimental to the ability of the system to cool, and dangerous if I put too much in. That's all fine and dandy, but it doesn't answer the original question... If the target charge at 95F is about 50psi, why is the reading on the gauge not changing as I add the coolant?

Those gauges are crap. Browse the reviews on Amazon for products like the one you bought. The gauges are basically packaging, since they don't work. So many people way overfilled relying on those.

I bought a separate hose with a gauge, and individual cans of r134. The gauge on that hose was crap, too. I may as well have just gotten the hose, which they sell. Ultimately you are shooting to have the correct number of ounces of refrigerant ... like 30 ounces, I believe on mine. I was bone dry, added two 12-ounce cans, and even 6 ounces low I have blown cold-ish enough for a couple of months now.

http://acpartshouse.com/refrigerant-and-oil-capacities/chevrolet-light-truck

 

[Trios]

I was bone dry, added two 12-ounce cans, and even 6 ounces low I have blown cold-ish enough for a couple of months now.

http://acpartshouse.com/refrigerant-and-oil-capacities/chevrolet-light-truck

If your system was 'empty' because it leaked, that means it is actually full of air. You are going to destroy your a/c system, it's now a matter of when and not if.

Get your system evacuated properly and the leak fixed and you may be able to save the compressor.

 

[6716]

If your system was 'empty' because it leaked, that means it is actually full of air. You are going to destroy your a/c system, it's now a matter of when and not if.

Get your system evacuated properly and the leak fixed and you may be able to save the compressor.

I know. The best guess is the leak is in the dash. I've had the dye test and no one seems to see any, but clearly there's a leak. Haven't torn into the dash yet. If the compressor goes before I get to that repair, I'll just add it to the list. A/C leak is currently trumped by: transmission shift kit, pads and rotors soon, O2 sensors, a coil, a ball joint .... Fortunately I live up north.

And the gauges on those slick refrigerant fill bottles are still crap.

 

[c good]

Q.D. fittings and Schrader valves are source of most leaks. Be sure to check those first with a soapy water solution. If there are bubbles there's your leak. HTH c good