CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

AC repair and recharge questions

,

@MCBMW Will your air compressor supply 4.2 CFM @ 90 PSI? That’s a whole lot of air. That’s what Harbor Freight says you’ll need. Unless you have a heck of a good compressor, I doubt it. Those Harbor Freight venturi things also shriek like a small jet plane.

I also doubt you can buy genuine R-12 near St Louis legally for $20 a can. If you can, sign me up. Freeze 12? No problem to buy, but there are others. One problem with freeze 12 is that anyone who might work on that car’s A/C system after you are done with it will suck contaminated refrigerant into their service equipment. Who knows what Freeze 12 + R12 + r134a add up to.

As for your compressor working with modern legal R-134a, never fear, it will compress it just fine. It doen’t care what the gas is. You will find that R-134a will eventually leak through the original R-12 hoses. R-134a hose has an internal barrier that is different from R-12. The molecular structure of the R-134a will allow it to eventually seep thru the old specification hose. You will probably have to recharge it every season or so even if there are no other leaks. If you do get a new hose made up using your old fittings, be sure they use the proper hose. If it were me, I’d sure take this opportunity to retrofit the system for the new refrigerant.

Good luck.

@Tester The pump should work actually. I plan on running it for up to an hour. In reality, with temperatures outside hitting 100 like clockwork around here, I don’t need it to pull a full vacuum, but rather just drop me a bit bellow one atmosphere. I’ve attached a handy dandy water phase chart. As you can see, dropping below 1 atmosphere at 100 degrees is enough to force water into the gas state and remove it from the system. I’m sure it’s much faster to do it with a full vacuum, but not the only way to go about it.

Incidentally you can find other interesting stories about this relationship between pressure and temperature in the NASA training programs. There was an incident where an astronaut in training was accidentally exposed to something like .15 atmosphere due to a hole in his suit. He commented that one of the last things he noticed before passing out was the saliva in his mouth boiling.

@MG McAnick Actually yes, my compressor will put that out. I didn’t mention my love affair with air tools did I? Not looking forward to the noise though, but I’ll see how mine sounds and take it from there.

As for my refrigerant selection, it’s still up in the air. I’ve been trying to get one of the many people who claim to possess R12 legally to simply dump two pounds in my car. I’ve offered cash and even my gauge set which I will no longer need in addition to cash, but so far no luck. I’m contemplating some Freeze 12 if I can find it.

Not to sound flippant about the next owner, but again, keep in mind that this will be my last summer… at least depending on this vehicle and it is a 27 year old machine. A fairly nice looking and very nice running 27 year old machine, but a 27 year old machine none the less. Perhaps in the future when I’m looking for some projects to keep me busy I’d do a full retrofit. I’ve actually been wanting to rebuild an AC compressor ever since I found the instructions online. For the moment though, any down and dirty approach will suffice and I’ve gone to much greater lengths than I previously planned to anyway.

Should be finishing up this job today. I’ll report back with my results. I’m pretty much defining success as having any manner of cold air coming from the dash that I can expect to last for any period of time.

Why not repair the existing line? Cut out the bad section, take it to a parts store, get a foot of the correct line + 2 unions + 4 a/c hose clamps and hook them up. O’Reilly lists them on their web site.

Here are pics:

You need to look at one minor detail on your chart, the cross point at 1 ATM is at 100°C, which is 212°F, AKA, the boiling point of water. I don’t have a detailed chart in front of me at this time, but it seems that I recall that at 100°F or about 38°C, you need to be down to about 200 Torr which is about 0.2 ATM. I’m sure someone here will correct me if I don’t recall correctly.

Just reaching the phase change point on the chart is not enough. All that means is that any moisture in the system is now converting to a gas. Depending on how much moisture got into the system, the time for all moisture to convert to gas is determined.

But even then, the moisture remains in the system until a further vacuum is pulled or known dry air is pumped into the system at the opposite end and the moisture is drawn out at the vacuum pump. As a general rule, using vacuum only, you have to get down to one half torr, which is about 0.005 ATM and you have to hold that for a half hour.

Those specs are for transformers though, I’m pretty sure the specs for an AC system doesn’t call for that much vacuum, it would probably crush the AC system. One of the AC specialists here can probably give you the specs for processing an AC system.

@keith you are correct sir, I did miss that the chart was in Deg C not Deg F. In any event, I’m about to slap the replacement hose I just picked up on, as well as the new reciever drier, and pull a vacuum on it.

Once I start running the vacuum pump I’ll report back with info about how far down it was able to pull the system since I have the gauges for it. Again, I’ll be letting it stand for about an hour, so hopefully it will come down far enough for long enough to be sufficient. I can’t imagine Harbor Freight could sell a pump that simply couldn’t do the job.

Ok, as stated before, I am reporting back in. My new bits are installed, and I ran the vacuum on the car for about 30 minutes and then decided to give my compressor a break something like 20 or 30 minutes ago. The vacuum is holding steady at just under 20 in HG vacuum.

Ok, edit, just fired it up again and it went down to 25 inches and is holding. Sounds like either my pump or compressor is letting me down a little, but I’m giving it another break and as things have been going I’m imagining it will hit 30 then. I’ll let it sit for a little and run the pump some more after that, then it might be charging time.

Update on charging, I’ve decided to just go ahead and run the R134 for now. Just doing the down and dirty approach. I may try to get this better in the future, but for the moment it seems just as well to have it run R134 and some ester and not get caught in an EPA sting or spend another week looking for mineral oil, so ester and 134 it is.

Ok…

So… that didn’t work.

I’m not honestly sure why, but it isn’t working and I’m done throwing money away on it until I have a larger source of said money to tear the whole thing down and do a retrofit.

I will say this though, I’m not sure I have any idea what is going on with the damn thing as nothing seems consistent. So I charged it last night, or rather attempted to and the pressure on the low side stays low. It comes up a little, but stays pretty low. Also, the High side seems to be pretty steady between 150 and 200, which sounds reasonable based on what I’ve read but who knows.

In any event, I can get the pressure in the system to where it seems about correct both high and low side, and I have near frostbite from removing the gauge fitting from the high side which seems to suggest something is sort of working. There doesn’t seem to be any blockages, nor leaks.

Really crazy thing, I went down tonight and hooked the gauges back up and now it’s telling me that it’s over charged, around 70 to 90 on the low side and 200 on the high. I let it run for a bit, still no change. I go to remove the High pressure side and while doing so a little vents off while I’m unscrewing the connection and then the low side drops to 10. So I add another can and it only comes up to about 20 to 25 on the low side. Air seems somewhat cool, but I’d hesitate to actually call it AC.

So, that’s that. I’m done, I give up until I have the funds to throw a proper conversion on there, which I may do as I’ve driven the car a bit in the past few days and remember what a pleasure it is despite being an old sort of ugly little thing.

Thanks for the advice everyone, but I guess I just can’t win them all.

A 150-200 on the high side is a bit low. It will vary based on a number of things but generally 225-250 is more like it.
The low side at 70-90 is way high and there’s simply not enough of a pressure drop from 200 to 90 (using the extremes) for refrigerant to get that cold.

The evaporator temperature is roughly equivalent to the low side pressure so ideally what you want on the low side is 30-40 PSI at elevated RPMs. Low side pressures of 70-90 PSI puts you in warm weather territory.

Offhand, I’d say there’s a problem with the expansion valve or the compressor is worn and suffering some internal leakage.
Good luck on this.

“First, you replace the ruptured hose AND the receiver/dryer. THEN you have a professional repair shop evacuate and recharge your system, adding the proper amount of PAG oil or whatever they use when doing conversions…OR you will continue to waste your money on failed attempts to get the system operating again and risk a nasty injury which you just narrowly escaped…”

Caddyman, July 27, first reply to your opening post…

@ok4450 I was kind of thinking compressor myself. It seems to get intermittently cooler in spurts. Later in the evening it seemed like it was pulling the low side down, but I still wasn’t getting any real effect at the dash. I’ll work on it again at some point in the future, and when I do I’ll be flushing the whole system and replacing the expansion valve, all the O rings, and either replacing or rebuilding the compressor.

@Caddyman Look, I know you’re doing the “I told you so” thing but you’ve got a really awful attitude about it and frankly I’ve seen your comments elsewhere and they take the same general tone. You seem to be, what we Internet folks call a troll. Also, for the record, I did in fact take all the steps you recommended and replace all the parts you mentioned. I even proceeded to get a quote from a local BMW specialist on a retrofit who mentioned no plans to replace the expansion valve and who surely wouldn’t have known the compressor might be bad until the system was charged and tested. So basically, I’m still on the same path your recommended, I’m only “out” a couple 10 buck cans of refrigerant.

In short, your posts are neither helpful to me, note anyone else. I’m all to familiar with troll behavior, and I’m well aware that you will continue, but for whatever it’s worth to you and to people that see your comments in the future you are kind of an idiot. Say something helpful or say nothing at all.

Caddyman can be a bit rough around the edges, but he is not a troll. He has come up with some good solutions on occasions and he has been here a long time. I think you were a bit rough on him. If he were a troll, we would have driven him off some time ago, we tend to do that here.

I’m stuck leaving short posts again so please bear with me. My ISP is not very good and I’m about to fix that. Anyway, you replaced your dryer, was there any evidence that the old dryer came apart and allowed desiccant into the system? That will make fixing this one a lot more difficult if it did.

BTW, you are right that you can’t use PAG oil with ester oil, at least that’s what I heard too. I think there is an ester oil that can be used with R134a and will mix with the older oil. I did the Q&D conversion to an 86 Toyota using the R134a with the ester oil in it. The Toyota had broken a line too and I had to get one made at a radiator shop.

You mentioned that you got near frostbite removing the high side connection. The high side should have been hotter than a Saturday Night Special (aka $3 pistol). The cold side should give you the frostbite. I assume that was a typo?

Refrigerant expelled through the high side service port can cause frostbite because while the refrigerant may be extremely hot inside the line, the minute it exits the service port it become sub-zero and will freeze something in a nano-second due to the rapid pressure drop.