About a month ago, I concluded Part XII of this series by saying, “So… there’s nothing left to do but evacuate and charge [the a/c]. Next week, we’ll see if the gods of decorum, publishing, and common sense conspire to allow me to polish this off and write an a/c recharge piece the week of Thanksgiving. In New England.”

I didn’t quite hit that window, but shortly after that piece appeared, there was a 65° day that had me thinking that charging up the Shark might not be a fool’s errand. So instead of publishing an a/c-recharge piece during the week of Thanksgiving, we’re going to do it six days before Christmas—which makes, you know, infinitely more sense. Imagine that it’s so cold inside my car that it’s a winter wonderland in there, with tinsel, candy canes, and cute little Dr. Seuss creatures peeking out from the dashboard vents. Work with me. ’Tis the season.

The issue, of course, is that on one hand, after spending months retrofitting the modified wing-cell bracket, the rotary-style Sanden 508-clone compressor, the biggest parallel-flow condenser I could fit in the nose, the biggest fan I could fit on the condenser, and the rebuilt evaporator assembly—plus rebuilding the heater box so the degraded foam wouldn’t let hot air pour into the passenger cabin, punching holes in the firewall for the hard lines and in the transmission hump for the evaporator drain, jury-rigging the wiring so that I could turn on the fan as both a condenser fan and as an aux cooling fan, fabricating the hoses, and leak-testing the system with pressurized nitrogen to be certain it was tight—you’ll forgive me for not wanting to wait until spring to know if the damned thing actually works or not.

On the other hand, works means “blows enough cold air that it actually cools you when it’s hot out.” And the degree to which one can tell the former depends on the latter.

Because of this, I really was quite content to let the a/c in the Shark sit until spring. A few people gave me a hard time when I said exactly that on Facebook, advising that it was unhealthy to let an a/c system sit uncharged. The issue here is that the refrigerant lubricants, PAG oil (used exclusively for R134a systems) and ester, or POE, oil (used for both R12 and R134a), are both hygroscopic, meaning that they absorb water, and the water reacts with the refrigerant to form acids which corrode the components. I had pumped the system full of dry nitrogen and leak-tested it twice, so I felt pretty confident that I’d displaced any humid air.

However, evacuation—pumping the system down with a vacuum pump—is better than simple displacement with dry gas. The idea behind evacuation is that by drawing a deep vacuum, you lower the pressure of the system, which allows any water left inside to boil and the gas to be drawn out. Evacuation is a necessary prelude to recharging.

When a 65° day was forecast for early December, I unearthed my vacuum pump, hooked it up, and ran it for the requisite 90 minutes. Then I shut it off, let the system sit for an hour, and observed that the vacuum held. Note that some folks use this as a primary means of leak-testing; I don’t. I’d already let the system sit pressurized for several days without the needle budging, so for me, this was simply a final sanity check. I would’ve been stunned if it leaked, and it didn’t.

Then I had to decide whether to stop and let the Shark sit with its a/c under vacuum all winter, or go for it and charge it up. It was not exactly T-shirt weather in the garage, but it definitely qualified as unseasonably warm—almost eerily so. I could almost hear a voice on the breeze: Go the distance… charge me!

Well, what would you do?

Several months back, I wrote a fairly detailed piece about refrigerants, comparing their pros and cons. In a nutshell, R12 (commonly called Freon) has incredible cooling capability, and was the original refrigerant used in the 635CSi, but it’s a pernicious ozone-depleting agent, and is a bit pricey. Contrary to popular belief, it is still legal to purchase R12 if you have an EPA 609 certificate, which I do.

R134a, the replacement for R12 that has been in every new car since the 1996 model year (and usually earlier), is readily available and moderately priced, but when used in retrofit applications, it simply doesn’t cool as well as R12. Whether it cools adequately in a vintage car whose a/c has been updated with a rotary-style compressor and a parallel-flow condenser depends quite a bit on the car and the climate in which it is used; I’ve never been thrilled with it in my 3.0CSi.

In addition to R12 and R134a, I’ve had a number of people ask me about R152a, which is what’s used in cans of keyboard duster, so I looked into it. R152a, R1234yf, and CO2 are EPA-approved refrigerants intended to replace R134a. Although R134a is not an ozone-depleting agent, is a potent greenhouse gas.

While R1234yf is currently being used in certain new cars (including the BMW i3), R152a is not being used yet, and the statement that it is an EPA-approved refrigerant is a bit misleading. R152a is classified as “moderately flammable,” and is only approved for use in new cars, not for retrofit application. And even in new cars, R152a needs to have certain usage conditions satisfied; engineering controls are supposed to be in place that either have sensors that detect leakage in the passenger compartment and vent the gas safely away, or use a “chiller” system (like an ice rink, where not refrigerant but salty brine is pumped under the ice) so that potentially flammable refrigerant doesn’t pass into the passenger compartment at all.

R152a is manifestly not approved as a drop-in replacement for R134a, even though YouTube is full of videos showing people doing exactly that.

Now, if I tell you I used R12, I’ll be perceived as a hold-out and a lawbreaker; if I tell you I used R134a, I’ll be perceived as safe and boring; and if I tell you I used R152a, I’ll be perceived as a Yankee redneck idiot, so I’m going to take the unusual step of not telling you which refrigerant I used. Seriously, I don’t want my choice to sway anyone—particularly since, let’s face it, when it’s 65° out, I can perhaps judge a/c functionality, but I certainly can’t judge actual performance.

I won’t know how well the thing performs until late spring when I’m hopefully heading to Southeast Sharkfest and/or the Vintage and hit some real heat.

Once a system is evacuated, there are four ways to charge it up. (Well, there’s one way to charge it up, but four ways to judge how much refrigerant to add.) If you have a completely stock a/c system running its original refrigerant, you can charge it by weight; that is, if the manual says it takes 24 ounces of R134a, you shoot in 24 ounces of R134a—done. But because very few do-it-yourselfers have a 30-pound canister of R134a and a refrigerant scale, from a practical standpoint, you wind up doing it by volume. So again, if the spec is 24 ounces of R134a, you go and buy yourself two 12-ounce cans, shoot both of them in, and you should be good.

However, if you have a modified system, or one that’s no longer running the original refrigerant—or, more to the point, both—neither the original weight nor the volume is likely to be correct. The rule of thumb for R134a is to use 80% of the Freon volume, but that’s just an estimate. The way I was taught to do it is to look at the liquid line (the one from the condenser to the evaporator) and charge the system until it begins to sweat—that is, until that line is cold enough that condensation begins to form on it as it heads through the firewall and into the evaporator.

The fourth way is to use a vent-temperature thermometer and charge the system until the vent temp looks good, then stop.

In practice, on a modified system, you wind up using a combination of all of these—at least I do. In preparation to charge the Shark, I needed to find my vent-temperature thermometer. I own two (having misplaced one and purchased a second), but in a cruel twist of fate, I could find neither—only the two red sheaths that hold them.

I briefly wondered if I should consider this an omen: Perhaps it was against the laws of man and god to recharge an a/c system in December. But I gave the omen the finger, took the refrigerator thermometer out of the mini-fridge in the Winnebago Rialta RV sitting in my driveway, put it in front of the a/c vents in the Shark, and forged ahead.

The actual mechanics of charging an a/c system aren’t that complicated, but they require care and understanding. The following is not intended to be a complete guide (although my upcoming book, Just Needs A Recharge: The Hack Mechanic Guide To Vintage Air-Conditioning, expected out in the spring, is intended to be a complete guide). I am not a professional air-conditioning technician, so take everything that follows as my opinion only.

When charging a system, you should use a manifold-gauge set. If you’re using R134a, you can buy those cans with an integrated hose, charging fitting, and gauge, and connect them directly to the low side, but using a manifold-gauge set is vastly preferred, as you can see what both the low and high pressure sides of the system are doing.

First, close both knobs on the gauge set (rotate them clockwise), then connect both charging fittings on the gauge set’s hoses to the car, with the blue hose connected to the low-pressure side and the red one to the high-pressure side. If you have an R134a system, the charging fittings are different for the low and high sides, so you can’t accidentally switch them, but on old R12 systems, the low- and high-side fittings may be the same.

The blue low-side fitting is on the suction line between the evaporator and the compressor. The red high-side fitting is on the discharge line that the receiver-drier is on, between the condenser and the evaporator. If you accidentally connect a can of refrigerant to the high side instead of the low side, it can blow up the can and injure or kill you, so if you don’t understand these terms and are at all confused about the fittings on your car, don’t try to charge it! Have a professional do it. In my case, I connected the manifold gauge hoses directly to the two small charging fittings on the back of the compressor.

Start the car and turn the a/c and fan on full. Put the thermometer in the main vent. If you have one of those big shop fans, aim it at the condenser in the nose of the car, as you want to simulate the air flow you’ll have while driving.

Then connect the yellow hose—the one that goes to the center of the manifold gauge set—to the can of refrigerant. Unless you have a can of R134a that has a hose and a valve built in, this requires using some sort of a can tap to pierce the can. Wear gloves and eye protection, and if you’re not comfortable with it, don’t do it at all.

Keep the can of refrigerant upright at all times. Don’t follow the things you might read online that say you can save time by shooting the first can in upside down! The can is full of liquid. You’re shooting refrigerant into the a/c system, which has a compressor, and liquid is not compressible. You need to be shooting in gaseous refrigerant, not liquid refrigerant. Keep the can upright so the gaseous refrigerant slowly wafts in from the top of the can.

Then slowly open the blue knob—and only the blue knob; never, under any circumstances, open the red knob while charging an a/c system. You only want to introduce the refrigerant on the low-pressure side of the system, always always always.

Watch the blue low-pressure gauge. The pressure should start to rise. Use the blue knob to control the rate at which the gas is drawn in so the low-side pressure stays at about 30 psi or less; it’ll want to go considerably higher as the refrigerant is initially drawn in. Eventually it’ll stabilize, and you’ll be able to open it all the way and not have the pressure rise above 30 psi.

The process is slow; it may take ten or twenty minutes for the liquid inside each can to completely vaporize and be drawn into the system.

If you see the low-pressure reading settle at about 30 psi or less, and the high-pressure reading gradually rise as the refrigerant goes in, then the system is working; the compressor is doing its compression thing, and there aren’t any obvious system blockages like a clogged expansion valve. You’ll likely hear the compressor cycle on and off, which will cause the high reading to fall and the low reading to rise, and then return to where they were. That’s normal. Watch the vent temperature, and watch for sweating on the liquid line.

If you need to shoot in more than one can (and you almost always do), fully close the blue knob before disconnecting the empty can and connecting the next one.

In my case, I shot in what seemed like a reasonable amount of The Refrigerant That Shall Not Be Named. I smiled as I felt the can get cold as the refrigerant was drawn into the system, and as I saw the low-side gauge move low, and the high-side gauge move high.

Regarding the can getting cold: As I explained months ago, the fundamental principle of air-conditioning and refrigeration is that a gas cools when it expands (or, more correctly, that when a liquid boils and changes its state to a gas and expands, it cools). This means that when you spray a can of deodorant or paint and feel it get cold, or feel a tank of propane on a barbecue grill grow cold—or, in our case, feel a can of refrigerant—you are, in fact, experiencing exactly what makes an a/c system work. The only difference is that, in an a/c system, the refrigerant is being re-compressed back into a liquid form so that it can expand and cool over and over again.

I looked at the refrigerator thermometer I’d placed in front of the vents. It was registering 34 degrees. Booya!

I stopped, unhooked the gauges, took the car for a brief drive, and experienced very cold a/c on that 65° day.

I’m calling it done for the season. And it’s done in the season, effectively making it my holiday present to myself.

So, was it worth it, all the evenings and the not-inconsiderable expense spent air-conditioning the Shark? I think so—I certainly hope so. But I really won’t know until the spring, until I’m on some road trip and both the outside air temperature and humidity are in the 90s. Perhaps we’ll have a Part XIV: How I Got It Wrong column then.

That was several weeks ago. Gee, my time now feels kind of empty, as if I lack focus and purpose. Of course, Louie is sitting in the garage, and he could use a/c. After all, as the Beatles said, it is a long and lonely winter, and I’ve got nothing but time.—Rob Siegel

Rob’s new book, Ran When Parked: How I Resurrected a Decade-Dead 1972 BMW 2002tii and Road-Tripped it a Thousand Miles Back Home, and How You Can, Too, is now available on Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.