We’re rounding third and heading for home with the a/c retrofit into the ’79 Euro 635CSi. Last week I used nitrogen to pressure-test the nearly-assembled a/c system, and after a few tweaks, I pronounced it leak-free. The only thing preventing me from charging it up was having the wiring in place to switch everything on.

The electrical needs of a vintage automotive air-conditioning system are pretty basic: The compressor is powered by a single wire and grounded through the engine. The condenser fan in front of the radiator and the blower motor in the evaporator housing both need power and ground. That’s it. If you need to troubleshoot the individual components by bypassing the wiring and powering them directly off the battery, those are the things you’d do.

Of course, to be useful, the system has to be able to be switched on from inside the car, not just hard-wired for testing. In a 2002, an E9, or a Bavaria, there are two knobs in the middle of the console. One controls the blower fan speed; turning it from “off” to any fan position switches on the a/c, in addition to powering the blower fan, by sending 12 volts to two places. The first is the temperature-control switch (the other knob). If the evaporator temperature is warmer than the setting on the knob, the switch sends 12V to the compressor to engage the electromagnetic clutch which starts the compressor doing its thing.

In a vintage BMW, the rotary temperature knob is a relative setting, it’s not calibrated in degrees. There’s no in-cabin temperature sensor. It doesn’t really matter; you usually peg the knob hard right to set it to maximum cold and leave it there.

The condenser fan in front of the radiator is the other device to which voltage is sent. This is a big, honking electric motor that can easily draw 10 to 20 amps of current, so in order to avoid having high current running from the fan to the dashboard switch and back, the condenser fan is controlled by a relay. As with any relay, there’s a low-current control side (the switcher) and a high-current load side (the switchee). On the low-current control side, Terminal 86 is connected to the on switch, and Terminal 85 is grounded. On the high-current load side, Terminal 30 is connected to the battery, and Terminal 87 is connected to the thing you want to switch on—in this case the hot lead to the condenser fan. The other leg of the fan wire is grounded. Thus, switching on the a/c sends voltage to 86, energizing the little electromagnet inside the coil, which closes the internal switch, connecting 87 and 30 together, sending high current to the fan without having to rout thick wires inside the passenger compartment. (I love relays.)

However, as I described a few weeks ago, the Shark is one generation newer than a 2002 or E9 or Bavaria, and as such, its electrical connections are more complex. It doesn’t have the two knobs on the console’s center panel; instead, the a/c has its hooks into the climate-control panel. It’s turned on by setting the temperature control—the same one that’s used for heat—to the right of zero, which also makes the heater-blower-fan control change its function to instead control the evaporator-blower fan. Fortunately, this fan-switching is handled flawlessly by two relays that are built into the air-conditioned version of the climate-control panel, and when I swapped panels and installed the air-conditioned version, that part of the wiring worked flawlessly.

But I couldn’t see how the compressor and the condenser fan are turned on.

Fortunately, when I bought the evaporator assembly and other related interior a/c parts for the car, the seller included the a/c wiring harness. I’d asked him to include every wire and connector—without really knowing what that meant. To his immense credit, he did just that, extricating the harness from where it goes from the fuse box, wraps clear around the nose of the car, and runs along the right inner fender, through the firewall, and into the passenger compartment.

Unfortunately, none of the connectors on it were labeled.

The typical procedure is to find a wiring diagram, but that’s less helpful than you’d think. Part of my tirade against wiring diagrams—yes, I, who wrote an electrical book, actively despise them—is typified by the fact that I located a wiring diagram for a 1980 633CSi, which is probably close enough to my Euro ’79 635CSi to be useful, and while it shows the surprisingly tortuous path that the voltage takes to the compressor, and while it shows four relays being part of the heat and a/c system, it doesn’t list where the connectors or relays are located in the car, or what color or size the connectors are. Initially, looking at the a/c wiring harness, it wasn’t even clear to me which end went inside the car and which went under the hood.

Eventually I noticed that there was a one-prong male spade connector inside a plastic sleeve—exactly the kind of connector that the compressor typically plugs into. I also noticed a small two-prong connector on the harness. I then looked under the dashboard, and to my delight, located a small two-prong connector on the wiring on the back of the climate-control panel that mated with the one on the harness. I turned the temperature knob past zero, checked for voltage on this connector, and… ta-DA! This was apparently how the compressor got switched on.

I used the voltmeter to ohm-out the harness, and determined that the male spade connector went to one wire on the little two-prong connector. I then plugged the connector on the wiring harness into it and repeated the test, checking for voltage at the spade lug inside the plastic sleeve, and… ta-DA a second time. I snaked that end of the wiring harness through the grommet in the firewall where one of the hoses went through. The compressor was now wired, although the bulk of the length of the new harness had no function and was just dangling over the side of the engine compartment.

Next came the chore of figuring out the condenser-fan wiring. I began by trying to figure out what the other eight or so connectors on the harness were; I cross-checked them with the wiring diagram, and figured most of them out by process of elimination. But some things were baffling: The wiring diagram showed four relays, and on my car I could only find three. In the diagram, the missing relay appeared to have its hooks into the wiring for the temperature switch at the bottom of the radiator, but on my car, I saw the switch but no evidence of a relay. And unfortunately, it looked like the wires on the new a/c harness that connected it to the fusebox had been cut in order to get it out of the car from which it was taken.

I slowly realized that I was in uncharged territory. Unlike any other car into which I’d ever retrofitted air, the Shark already had an auxiliary cooling fan that was turned on via a temperature switch connected to the radiator—or at least it was supposed to be; the original fan was dead. I’d thrown it away and installed a new Spal fan on the condenser, but I hadn’t actually thought about how to get the Spal working both as an a/c-condenser fan and as an auxiliary cooling fan.

I could see the big, flat, rectangular connector on the old harness that mated to the old fan, but on the new harness, there were two of these connectors. It dawned on me that it was possible that there was a non-a/c and an a/c version of the fan-wiring harness, and that I might need to remove the one in the car and substitute the a/c version that I’d bought. But to do that, I first needed to understand how the aux cooling fan was supposed to be wired.

The wiring diagram I had was for a U.S.-spec 1980 633CSi, all of which had a/c. I didn’t know what the differences were for a non-a/c car—or if there were differences. In fact, the original harness had exactly the same male spade connector in the same rectangular plastic housing as the new a/c harness, and it was located exactly where it needed to be to reach the compressor. Why would it have that unless it was for the compressor?

And yet I couldn’t find anything to connect it to the climate-control panel. I posted these questions to an E24 forum, but I appeared to be on my own.

I began by simply wiring the Spal fan to the connector that went to the original aux fan. I read that you can get the aux fan to turn on by removing the two connectors to the temperature switch on the bottom of the radiator and connecting them together to bypass the switch. I did that, and nothing happened. It seemed that in order to get the condenser fan working, I first needed to trouble-shoot the original wiring to the auxiliary fan.

It didn’t take long to realize that there were numerous problems.

I found references online to a small fuse box that was supposed to be located above the battery and hold two fuses, one of which was for the aux fan. This was missing on my car, but I saw the holes where it should’ve been, and found—dangling under the battery—the connector that presumably originally fed it. I could not locate the output connector.

I was aware that my shark had been hit in the front at some point in the distant past. It was looking increasingly likely that the wiring and the aux fan were never fully reconnected after the accident and the bodywork.

So I had three choices: I could ignore the a/c-condenser-fan issue for now, and keep trying to get the Spal working as an aux cooling fan. Or I could jump straight to trying to integrate the a/c wiring harness, even though the portion to the fuse box appeared to have been cut, and a relay appeared to be missing.

Or I could go full Hack Mechanic on its ass, ignore every connector on the a/c wiring harness except the one I had successfully powering the compressor, and simply wire the Spal fan, via a relay, to a switch on the dashboard, allowing me to manually turn it on for a/c, or auxiliary cooling, or both.

Any guesses which option I selected?

I looked at the dashboard to see where I might mount a switch, saw that there was a factory pull switch for non-existent fog lights, found that there were even wires running from there into the engine compartment, and totally took it as a sign.

In addition to having bought the Spal fan, I’d bought the proper two-pin Molex connector and a Spal relay kit. I usually make these connectors myself and use generic relays, but I’d found an open-box Spal kit on eBay for a good price.

I mounted the relay inside the nose, connected the high-current wire to the battery via a fuse, and ran the thin low-current wires to the wires connected to the switch, all in about an hour. It was a great rush pulling the factory fog-light switch and hearing the Spal fan gloriously go whooooooosh.

Then I simply wrapped up the unused length of the a/c wiring harness—which was most of it—into a coil and zip-tied it in place beneath the washer bottle at the lower right corner of the engine compartment. At some point, if I find someone with an early E12-based shark that I can poke around in (which is a distinct possibility this spring at Southeast Sharkfest) and have a slow peek, I may resurrect the idea of using the a/c harness. But for now it’s all wired up.

Usually, when I do a/c work that’s had the evaporator assembly out of the car, I don’t want to button up the interior—reinstall the console and glovebox—until I’m absolutely certain that the system is working. This is a reaction to twenty years ago when I had to pull the evaporator assembly out of the 3.0CSi and reinstall it twice before I had it leak-free. But having pressure-tested the evap assembly à la carte, and then having pressure-tested the whole system for several days, I was starting to feel lucky.

Plus, the interior of the Shark had been apart for so long that it was really starting to drive me nuts. But in the course of an afternoon, it went from this:

To this: (And yes, the Blaupunkt Lexington SQR-46 radio, likely from a Porsche 928, sticks out too far. It was that way when I bought the car. I’ll get to it.)

And now there’s nothing left to do but evacuate and charge the system. 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… during the week of Thanksgiving. In New England.

Timing was never my strong suit.—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.