I worked on a number of other things that were more important to me in a fair-weather convertible than air conditioning, but eventually I got around to it. It is nice to be able to blow cold air on yourself on a hot day, after all.
Upon examination, I found that the belt, the idler pulley, and the compressor's clutch were all missing. Well, this really didn't have working air conditioning, no sir. Moreover, when I tried to spin the York compressor over by hand, I found that it had thrown a rod. Oh yeah, it's broken all right.
Not knowing what else might be broken, I was not interested in spending any significant money to fix the already identified problems. Which eliminated any option of having things fixed professionally. It also pretty much limited me to using used parts, preferably from the local U-Pull where prices are very reasonable. I had never worked on an air conditioning system before, and I knew nothing about them, nor did I have any of the special tools required.
However, we have a lot of older cars, and I've always liked spending my money on tools to fix a problem rather than hiring it done. So, given that I'd pretty much have to do it myself, and probably more than once, it became obvious that I'd need some tools and information. So I started shopping around.
At a pawn shop I found a used vacuum pump, name brand even. (JB Industries.) It looked a lot better than many I'd seen, and was $100. I was able to download a manual from their internet site, as well as a guide to vacuuming systems. At another pawn shop I found a used R12 gauge set, also a name brand. (Bluepoint/Snap-on.) I think it was around $50. At an auto parts store I found a Haynes manual on auto air conditioning. At another pawn shop I found a different manual on auto air conditioning, more of a textbook from a trade school. It also contained detailed instructions for rebuilding various (older) compressors. With all this junk in hand, I started reading a lot. Also pored over were the ACC manuals for our 107 and 126 cars. And plenty of internet surfing for information.
Well, so now I'm an expert. :-) And now I need some parts. At the U-Pull I grabbed another York compressor with clutch, from an MB even. $15. I also grabbed the belt ($0) and idler pulley ($1). Note: removing a York compressor is a real PITA. There's a wraparound bracket that you have to remove from the engine first. Then you can remove the York from the bracket.
Installing the new York was the first hurdle. I got the compressor installed, but the new idler pulley was a problem. The idler is mounted on a dogleg arm, so that you can tension the belt. The arm was too long, and the idler pulley wasn't in the same plane as the other two pulleys. A new correct arm is available, but expensive. So I took a hacksaw to the arm to lose 5/8" of height from its spacer end. Then, I screwed the bolt into the other end and chucked the bolt end into my drill press. Finally, I lowered the cut end of the arm into a file placed on the drill's table. This 'lathe' made the cut end smooth and parallel to the other end. Good thing it was aluminum and not steel. Scary, but it worked.
With the new compressor in place I was able to begin testing. When the dashboard AC switch was enabled, the clutch pulled in and the York began pumping. A beginning! (Note that this vintage of system doesn't have a low-pressure protective switch. The York would run even with no charge at all. A York, at least, is less likely to be damaged by this. It actually has a crankcase containing lubricant.)
Next step: Vacuum. I put the vacuum pump on the system, through the R12 gauge set. (Normal procedure.) When turned on, no vacuum would build up. Great, the system leaks. I hooked up a propane torch bottle to the system to supply moisture-free pressure. Hissssss. There's quite a leak at the condenser, it seems that the piping right at the hot end is cracked, down into the fins where it is not accessible (to me).
Back to the U-Pull for a condenser. I found one condenser that looked like it would fit, but it had a hole in the hot end pipe. Looked like somebody had given the car the coup-de-grace with a .22! (Or maybe they stabbed it with a screwdriver.) But this hole was in the bare pipe, where I could reach it.
So I tried to repair this (aluminum) condenser. The short version of this story is: don't bother. I wasted vast amounts of time trying to patch this hole using various techniques. Every one was difficult to do at best, and leaked anyway. By the time I was done this condenser was totally ruined. In the end I did what I should have done in the first place: I took the original condenser to a radiator repair shop. $35 later I had a leak-free condenser. (Or I could have been very patient and waited until a good condenser showed up at the U-Pull. Hah! Since this was done I have acquired another good condenser, in case the repaired one should fail.)
After this I was able to pull a vacuum on the system. And it held. To eliminate any harmful moisture from the system, I put a heat lamp on the receiver/drier, and put the system under vacuum overnight. Conventional wisdom dictates that you put in a new receiver/drier at this point, to ensure that you have a nice fresh bag of desiccant in the system. I did not wish to do this, because these things are not free and I had no idea whether or not the system would be worth such an investment. As the harm caused by moisture takes time to develop, not doing this at this point was warranted. Next step: putting in some refrigerant.
To save money during this testing phase, and to avoid any question of environmental harm, I used propane from a torch to put a small amount of pressure in the system. It held. With the compressor engaged, I got a pressure differential across the compressor. Things are starting to work!
Propane is a good refrigerant, but it has approximately the same temperature/pressure curve as R22, not R12. If you try to use propane as a replacement for R12 you will have pressures that are much too high, and will probably blow a safety valve or a hose, with a small attendant risk of a fire. However, in small amounts it works well to test out the system. Essentially you run with a severe undercharge so that pressures remain safe. There will be only slight cooling, because there is not enough mass in the working fluid to transport much heat. Along with this, there is probably too little working fluid under these conditions to transport the lubricating oil, so you only want to run for short periods of time to avoid destroying the compressor.Also, propane in bulk cylinders (hamburger gas) is not always dry enough for use in an A/C system. That found in Coleman torch cylinders is reputed to be drier than most, so it is recommended. You don't want to introduce moisture into your system.
See here for more information.
At this point, I flanged up a way to put a can of butane/propane blend camping fuel into the system. I had read that an Isobutane (R600a)/ Propane (R290) blend was a good functional replacement for R12, using about a 30/70% R600a/R290 blend, in about 35% of the amount of a full R12 charge. I thought I was using Isobutane, but I believe that this particular brand of fuel was really regular Butane (R600). This didn't really seem to matter during this testing phase, but it's something to watch out for. The danger of regular Butane is that it can be in a liquid state at the input to the compressor, which can damage its reed valves. Isobutane has a more suitable temperature/pressure curve.
With this in place I'm starting to get some cooling from the system. But it's feeble. I try more charge. I try adding some propane. I worry that I've got too much charge in there, so I vent the system and start over. Repeat. I experimented a lot here with charges, fan speeds, etc. Just trying to see if I could get any real cooling out of this system.
Eventually, I got to a point where suddenly I got very cold vent temperatures with the fan on low. Below 20 °F. But the pressures on the gauges were wrong, and suggested that there was a blockage in the system. (Very low pressures on the low side, and high pressures on the high side.) And there was no significant cooling capacity, the low temperatures only were gotten on the low fan setting. With the fan turned up, the system pretty much stopped cooling. So I started disassembling the system looking for the blockage.
Under the dash is the thermostatic expansion valve. Its job is to regulate the amount of liquid refrigerant entering the evaporator, ensuring that not too much or too little refrigerant flows. It does this by measuring the temperature of the output fitting of the evaporator. It's a sensitive little beast, with a small variable orifice in it. To protect this orifice from clogging, there is a screen in the input side fitting. I found this screen clogged with a gritty substance. It had come downstream from the receiver/drier. I don't know if this indicates that I must replace the receiver/drier, or if this was something that can be ignored for awhile. Well, I suppose I'll find out. If this screen clogs again I'll know what to look for.
At this point I resealed the system, and repeated the vacuuming steps. The system appears tight, and it is able to pump refrigerant around. I tried a new technique, wherein I put in one can of Isobutane/Propane (30%/70%) mix. This is an approximately correct charge of Isobutane for a 2.2# R12 system. But, of course, it didn't cool well. Then I started adding propane, keeping an eye on the pressures. (With the system set for maximum cooling, and the engine at 2000 rpm.) My trick was to monitor the temperature of the pipe next to the high-side fitting using an infrared thermometer. As I slowly leaked in Propane, I watched the R12 temperature scale on the high-side gauge and the actual temperature of the fitting. When they were close to each other I was done. The idea was to mimic the (proven successful) R12 pressure/temperature curve. I also watched the absolute high-side pressure to ensure it was not too high, and I watched the low-side pressure to make sure it was still in the 20–30 PSI range. You have to make changes slowly enough to let temperatures and pressures reach equilibrium so that you get accurate readings.
Done this way, I was able to get vent temperatures under 40 °F, which is very good. Driving around town it was very nice to listen to the purr of the York under the hood, while having a nice cold blast of air to sit in.
It is my belief that the Propane is doing most of the cooling. The Isobutane is there to keep the pressures down. It provides enough circulating mass to transfer both heat and lubricating oil, and has a temperature range that keeps it from being a liquid in a harmful place. The proportion of the final charge that is the Propane is an 'undercharge', because if it weren't the pressures would be too high. But it's still enough to provide sufficient cooling, and the Isobutane is filling out the ranks. But because the system is still in a sense 'undercharged', it is very sensitive to leaks. A typical R12 system is actually 'overcharged' in that it is designed to be able to lose some portion of the charge (by inevitable slow leakage) without impacting performance. This blend doesn't have that luxury, so once some charge is lost the performance will suffer. Fortunately it is cheap and easy to repair! Because the components of the blend can leak at different rates you don't know what proportions are left once you've lost some charge, so you should evacuate the system and start from scratch rather than try to top it off.
I understand that this blend will work very well until ambient temperatures get above 100 °F. Because Propane has a critical point lower than R12, once its pressure gets too high it will stop condensing altogether. Then the only cooling is provided by the Isobutane, which has a higher critical temperature, and cooling performance is minimal. This can happen when it's really hot outside. If you live in such a place, stick with R12. (And you probably wouldn't be happy with R134a's inherently lesser performance if you were to convert.)
Conversion. Yes, at this point I had proved that the AC system was functional and relatively leak-free. At this point one could consider refilling it with the correct R12 ($150+), or converting it to R134a. From what I have read, R134a conversions are a bad idea. First of all, at best R134a systems are only 80% as effective as an R12 system. (Designed-for-R134a systems are larger as a result of this.) Secondly, R134a has much higher pressures than R12, something that a retrofit system might not tolerate too well. Thirdly, R134a does not tolerate the mineral oil that is used in an R12 system. You have to add a different oil, which is immiscible with the mineral oil. If you don't first remove all of the old oil, it can sit there like a lump coating the pipes, reducing the effectiveness of the system even more. Flushing all the oil out of a system is a tedious process. Lastly, the oils required by R134a are very hygroscopic, which means that it is very difficult to ensure that the system is totally moisture-free. And R134a, like R12, is a chloro-fluorocarbon, and will form acids in the presence of moisture. So it is understandable that as part of the conversion you must fit a new receiver/drier.
So, R134a conversion is not very attractive. Expensive and time-consuming to do right, and when you're done it never works as well as it did, and is harder on the system due to the higher pressures.
Keeping R12 is attractive except for the price. If something goes wrong, you're out a bunch of money again.
On the other hand, the test refrigerant is working pretty well. It's supposedly actually even more efficient than R12. It's tolerant of moisture since it contains no chlorine or fluorine to form acids with. It's cheap. It's environmentally friendly. The only down side is that it's flammable, but then again, so's R134a! So, at the moment that's what I'm using. As I've had to drain the system a couple of times since I got it working, to work on other things, it has proven (so far) to be a wise choice.