Checking AC system for leakage

**DeathTongue** · 07-16-2015, 04:36 AM

I'm about to evac and refill my ac system, but I've noticed that some people use pressure when testing the system for leaks. For instance, one guy on youtube hooked up the compression side of the vacuum pump and pressurized the system to about 150 psi to see if it leaked, then did the regular check with the vacuum. Is there a reason to use a pressure check in addition to the vacuum leakdown check? Or a reason not to? thanks in advance for any replies!

**pajeff02** · 07-16-2015, 05:47 PM

No, no, no...

Best way to test the system is with a vacuum pump. You should do this anyway to evacuate any moisture from the system as it is the enemy of AC components. Draw the system down and run the pump for around 30 minutes. Close the valves, turn off the pump and watch the gauges to see if you lose vacuum over time. If the system holds vacuum for an hour it is probably good. Any loss of vacuum is a sign of a leak.

Out of curiosity, why are you evacuating the whole system? You can simply add refrigerant to what you already have.

**DeathTongue** · 07-16-2015, 05:56 PM

Originally Posted by pajeff02

Out of curiosity, why are you evacuating the whole system? You can simply add refrigerant to what you already have.

The short version is, I recently had the compressor replaced and it was not blowing cool enough to suit me. So, I asked around and everyone says my mechanic didn't replace enough parts and my new compressor is probably already junk. Rather than get a new compressor or go back and complain at this point, I want to do what I can with it. I've got some mastercool gauges and a vacuum pump, and I'm always up for buying more tools whenever I need them. I've got some experience as a mechanic, but ac systems are new to me, I barely know what the parts are, except the compressor, but I'm trying to learn. I'm going to purge the system, do the vacuum test to check for leaks and if it holds vacuum, charge it with a couple of cans of R134a and 2-3 ounces of oil (as recommended by another mechanic), then see how it works.

**pajeff02** · 07-17-2015, 03:16 AM

If the old compressor fragged it may have circulated trash throughout the system and your orifice tube could be plugged up. Or, the system could simply be low on refrigerant, however, your AC gauges will tell you that. If you do refill the system, go by the spec for how many pounds of refrigerant are required. There are some great vids on YouTube on how to do this.

**pajeff02** · 07-17-2015, 03:18 AM

I haven't messed with the AC on our T/A, however, here is a thread where I am working on converting our Monte SS to R-134a with pics of the tools in use: http://www.ls1.com/forums/f70/r-12-r...ersion-183938/

**RONS98TA** · 07-17-2015, 03:49 AM

NEVER use an air compressor to put air into the a/c system!! Compressed air has moisture in it.
Just like pajeff02 said, its a NO NO. The moisture will make acid in the a/c system and cause vital parts to fail inside the a/c compressor. Vacuum is the only way to go.

**Firebirdjones** · 07-17-2015, 04:43 AM

If it were mine, since you have no idea how much oil might already be in the system, plus the fact that you had a compressor die previously.....I'd completely disassemble the system and flush everything out first.

Blow all the oil out of the evap with non chlorinated brake clean and compressed air, clean out all the lines and set the compressor up to drain the oil out. Then start fresh with new o-rings and reassemble with a NEW dryer, and replace the orifice tube.

Add the recommended oil. Most will go in the compressor, the rest can be shot in through the charge if you wish when the time comes to charge it up. Suck it all down with a vacuum pump for a good 30-45 minutes. If it holds vacuum you are set. Most of the first can of charge (whether oil or freon) will be sucked in as soon as it's hooked up because the system has a vacuum on it. After that, I then turn on the AC system and finish charging to the recommended amount.

I usually do it this way if there are some unknowns that way you know what you have when finished. Especially the oil content. Too much oil and the system won't blow as cold as it could, and not enough oil is detrimental to compressor life. So when in doubt, I flush it all and start fresh.

**DeathTongue** · 07-19-2015, 07:18 AM

Well, I finished my operation on my GTO AC system, and it seems to be a success, with ice cold air and reasonably good readings on the manifold gauges. Whether or not it continues to work is still a question, and I'll report back if my compressor craps out or anything else happens. For the record, here's the procedure I used:

First, I pulled the vacuum for about 45 minutes with a rented autozone pump, and it quickly got the vac down to about 28 inches. After the pump ran for 45 minutes, I closed off the high side valve at the manifold, then closed the low side valve and turned off the pump at the same time. After disconnecting the pump, I waited about 30 minutes to see if the system held vacuum, which it did, the needle didn't budge on my mastercool gauges. Since the compressor had just been replaced, someone on another site (a GM Master mechanic on a mechanic's forum) advised me to just add 2-3 ounces of oil to the system to make sure it wasn't over-oiled. So, I added 2 ounces of oil into the yellow hose right before I hooked the hose up to my can valve, already attached to a 12 ounce can of r134a. I started the engine at this point and turned on the AC with the fan set to the second speed setting and a thermometer in the central vent, which started out reading 140. Then, I opened the can with the can valve, and loosened the yellow hose at the manifold for a moment to let the air out of the hose, then opened the low side valve to let the coolant in. I started out with the can just sitting upright, and let it charge slowly, but after a while I would shake the can a little and rotate it to the sideways position, as the can emptied I was eventually holding it up over my head upside down to make sure I got all the r134a out of the can. I also let the cans sit in the sun for about 20 minutes first. Once the first can was almost empty the compressor started to cycle and it was coming on and going off normally. I added the second can, and again got all the refrigerant out, both times when I took off the old can, hardly anything came out after I closed the low valve and loosened the yellow hose at the manifold. I weighed the cans before and after, and I ended up getting 23.3 ounces of refrigerant, which is pretty close to the proper charge of 24 ounces. After everything was settled down and running the low side pressure was around 40 and the high side around 220, which doesn't seem too far off normal. The temperature was 93 degrees with 42% humidity, and the thermometer in my vent registered 50 degrees at the second fan level. When I turned the fan speed up to 3, the temp was 56, which is still pretty good, slightly better than what our Grand Caravan can do. So, I guess I'm an AC expert now. NOT!

**DeathTongue** · 07-19-2015, 07:31 AM

One other point, and I'm not sure I made this point on other sites either, it wasn't actually the compressor itself that went bad, the clutch was making a grinding noise, so the whole compressor and clutch assembly was replaced. I'm not sure if that makes a difference in which parts have to be replaced, but thought I should mention it to see if that changes any recommendations. thanks for your patience with my long winded thread!

**pajeff02** · 07-19-2015, 08:40 AM

Sounds like you got it right. If the clutch failed, you are probably fine then as it wouldn't circulate any trash or debris. It is when the internals on the compressor go that you can have issues. In the event your charge leaks down, you'll need to grab some UV dye and a flashlight to locate the problem area. Only issue is that you have to purge the system to add the dye. I think some newer GM's actually came with dye in the system from the factory.

**Firebirdjones** · 07-19-2015, 03:10 PM

Right on Jeff,

If it's just a clutch issue, that can simply be replaced without even sucking down and charging the system. You just need a clutch tool to remove and replace the clutch. I set the clearance at .015 to .020 and call it good.

The fun part is trying to get to the compressor on the 4th gens and whether or not there is enough room to get the tool in there to remove/reinstall the new clutch. Mounting those things on the bottom of the car wasn't the brightest idea.

Any time you crack the system open for any length of time the dryer should be replaced, that's why I mentioned it. I also mentioned flushing the system because it wasn't clear whether you had a compressor issue or not that might have passed trash through the system, plus the fact it seemed we weren't clear on the amount of oil that may already be in the system, critical to get that right. That's why if there are any unknowns, I just prefer to start fresh.