Possibly a boneheaded AC question

[daxr]

I haven't done a lot of A/C work, and pretty much rely on our largely automatic machine, but one the other day may not have come out as well as it could, and I wonder if I could have done better. I know enough to have my certification, but not much more than that.

 

On a 2004 Hummer H2 with a 6.0 GM engine and running gear, the complaint was "not blowing cold enough". The compressor kicked in ok, but didn't blow very cold. The capacity was listed as 1.69lbs R134.

 

So I set it up and recovered .82lbs, went through the vacuum cycle and it passed the leak test. I recharged to spec, added 1oz oil, and got reasonably cold air on completion. The pressures when done were 50 low and 175 high, and the "reasonably cold" was explained as "proper operation" and we delivered the vehicle back. No further complaints from the customer, but do those reading really indicate optimal situation? Anything I might have missed or looked at to improve operation?

[Jsdirt]

My line of thought is that it did not need that extra ounce of oil. Losing .8 lbs over 10 years is hardly a large leak, so I doubt much - if any- oil was lost. Too much oil can cause poor cooling and higher than normal low-side pressures. A bad fan clutch will do that too, but then will cool normally once the vehicle is moving.

 

Don't know what your ambient was, but a high side of 175 psi sounds reasonable, so I'd rule out debris between the condenser and radiator (although I always check that out of habit). At that pressure you can probably rule out the fan clutch as well.

[Doug_Scott]

I got out of the field before R-12 was being moved over to R-134. I took a course(actually Firestone paid for it) that lasted 8 weeks, once a week, 2 hours per session. A big 16 hours. The course covered the generic a/c systems. No help on anything auto temp or model specific. Sessions were not from a tool vendor. It was actually done at the local college that runs the trade school for apprentice mechanics in Ontario. Surprising how this stuff sticks with you.

 

Before recovering the system you could have checked both sides of the fixed orifice tube(assuming it is touchable) to see if the temp difference is pretty high. One side of the fixed orifice tube is high side liquid, the other is low side liquid (changing to low-pressure gas after going through evaporator). The fixed orifice tubes will start to plug up over time.

 

Did the pressure readings you gave, 50 low and 175 high, mean the low side pressure was 50, and the high side pressure was 175?

 

I am not 100% on this part (refer to first sentence), but, I think the low side should be cycling from about 25psi to 40psi as the clutch cycles compressor on(at 40psi) and off(at 25psi). High side should be 225psi to 250psi. If those numbers are correct, then you likely have a partially plugged fixed orifice tube. Too late now to change it. The tubes used to very cheap, and simple to change if you have the puller to get the old tube out without leaving parts of it behind. Odd as it sounds, a plugged orifice tube will actually cause low pressures in system.

 

BTW, you can simulate highway speeds by spraying lots of water through the condenser and running the engine as a higher RPM.

[daxr]

Yes, those were 50 on the low side, 175 on the high side. I was thinking that the high side was ok, as the ambient temperature was 70 degrees of so. The low side reading maybe could have been better. I looked for frosting and didn't see any, but didn't locate the orifice or feel the temp differences. I would have done a better before and after check, but the first time it came in things came up, there was some miscommunication, and we had only recovered the system when we had to give it back to the customer. A few days later we had it back with more time and went through the full cycle, but still in a hurry.

 

I've gotten various advice over the years on adding oil. Years ago (when I first did some a/c work for a little while, r12 with manual gauges) we didn't add oil unless we replaced parts. The tech who showed me how to use the machine lately said he always adds about an ounce if the rig is older and may have been evac'd a time or two already. But possibly, then, too much oil (it would be nice to know, rather than to guess), or possibly the compressor was on the weak side?

 

I'll try to spend more time and learn a bit more on the next one, but its not always easy on flat-rate in a busy shop. In some areas - alignments, brakes, suspension, electrical and so forth, I'm perfectly comfortable, and after years of working in this industry I feel like a walking encyclopedia. In other areas I feel like a dumb kid, but I think some specific classes on the side are a good idea soon enough.

[Doug_Scott]

The nice thing about taking generic courses(not manufacturer specific) is that you learn all the concepts, not just the tricks for a Chevy. Concept is the same, regardless of manufacturer. I have always used the idea that if I understand how it works, I can fix it.

[daxr]

As a postscript, it turns out the likely culprit of the non-optimal AC job may have been our AC machine. It has had a quirk or two, such as a rig I put through the recover/vacuum/leak test cycle, which then failed the leak test. I thought that pretty unlikely as it had showed nearly a full charge, and its problem was a high-side kick off when hot - probably moisture in the system. I unhooked it under vacuum and let it sit for a day, then went to do a vacuum/leak test again, which didn't work as it found pressure in the system. Doing a second recovery the machine said it pulled out another .6 lbs of freon...That one did finally go through the cycle ok and worked fine afterwards, but then the machine took a dump. The service guy said it was all sludged up inside, had a couple internal leaks, and it was a wonder it worked at all.

[Jsdirt]

Yikes ... sounds like it might have got a dose of stop leak. Gotta love when that happens ...

[daxr]

I imagine the "sludging" was caused by stop-leak, as do-it-yourself ac charging and repairs seem to have grown in popularity. We've seen a steady stream of over-charging problems this summer, and the parts stores all sell those R-134 kits like crazy.

 

One bit of really good advice I've gotten is to not trust the machine we use, and I've gotten the best results lately by adding .25 lbs less freon than the specs call for. Probably the machine is weighing incorrectly. I did a newer Prius this hot morning, put in .41 kg where the spec was .42-.52 kg. Got 35 psi low side, 200-250 psi high side (cycling up and down as the motor and fans kicked on and off), and 40 degrees at the outlets.

 

In the afternoon another tech worked on a Honda and put in exact amount specified, got 40 psi low and 350 (and bouncing needle) psi high. I suggested he draw about .25 lbs off, and it settled right down to 35 psi low, 225 psi high, and worked perfectly. So our machine is operating, but we'll be adding a bit less than specs to account for its remaining quirks. Its easy enough, and probably a good practice, to add a bit less than called for and test the system, and top it up in tenths as necessary to get the best operation.

 

Going back to the original Hummer question, it seems most likely that, though I recharged to spec, the machine has an inaccuracy that inclines it to over-charge. I would probably have gotten a better low side and better cooling performance if I put in .10 or .20 less R134. The high side wasn't too high, but it was a cool morning and we didn't test it with a fully warm engine either.