Trying to Get My A/C System Through the Summer (Part II)

A few weeks ago, I wrote a piece about “the good kluge.”

Just so we’re clear, what I describe below is not one of them.

Last week, I talked about the A/C in my 2008 Nissan Armada having a leak that was small enough that I could keep topping it off with a can of R134a about once a month. The leak, discovered by using dye and a U/V light, was in the condenser, a part that costs about a hundred dollars but is a non-trivial amount of work to replace due to the way that the radiator, condenser, transmission cooler, and fans are packed in the nose. Unfortunately, the A/C also would mysteriously switch off. It was unclear to me whether or not the two problems were related. I wanted to defer the condenser replacement until fall, but I also wanted working A/C.

When the shut-off problem initially cropped up, it was a mid-level inconvenience. I was barely using the truck anyway. It had been mostly sitting at the end of the driveway waiting for me to use it to snag and tow home some cool car, which has more to do with my self-image than actual reality or need. But then a good friend of mine had some personal issues that resulted in my loaning him my daily-driver 2003 BMW 530i. While I have several air-conditioned vintage cars, keeping them out of traffic and rain and in compliance with the terms of my Hagerty policy has meant that the Armada and its wonky A/C system has been pressed into service.

I ran the system multiple times and verified that, when the A/C worked, the reading on the red high-side gauge increased and that on the blue low-side gauge decreased, validating that the compressor was doing its thing. The high-side reading, though, appeared to be slightly lower than what the A/C section of the Nissan factory service manual (FSM) listed it should be for 90° temps, likely indicating that the system was slightly undercharged. Unfortunately, it’s not really feasible to drive with the manifold gauges attached and placed where you can safely see them. You need to instead infer the operation of the compressor by the vent temperature. This seat-of-the-sweat-glands method told me that, when it got toasty in the car, the compressor wasn’t, uh, compressing. This was confirmed by stopping, popping open the hood, looking down at the compressor, and seeing that the magnetic clutch wasn’t engaged (the pulley was spinning but the nose that pokes through it was stationary).

Usually, as part of this test, you’d verify that voltage is reaching the compressor, and maybe hot-wire it to the battery to make sure that the clutch engages and it comes on. Unfortunately, the connector on the Armada’s compressor is very difficult to reach, and is one of those little Japanese puzzles where you need to depress a tiny tab with a fingernail while you pull the halves apart. I eventually managed to separate it, but instead of finding a standard flat quick-connect terminal, there was a tiny pin with no easy way to connect to it. So instead, I used one of those wire-insulation-piercing splice adapters that are often used to tap into key-on accessory wires to install a radio. Really, I recommend that you not do this, and I strongly recommend that you not drive a vehicle this way, as you’re defeating sensors that are in place to protect the compressor and the A/C system as a whole. But I found out what I needed to, which was that with the compressor running, the system began blowing cold again.

So, something was shutting off the compressor. But what? Last week I showed a couple of diagrams from the factory service manual and quoted the description of the complicated relationship between several control modules and half a dozen sensors that results in compressor turn-on or its failure. I read a cluster of posts on the Armada forum implicating the evaporator “intake sensor” and listing symptoms identical to mine that went away with its replacement, but swapping it out ($60 and ten minutes) made no difference.

The next-most suspect sensor was the pressure sensor screwed into the receiver-dryer, which on this vehicle is an integral part of the condenser. On older vehicles with A/C, these are simple, easily-bypassed switches that cut off voltage to the compressor if system pressure is too low, indicating a big leak, or too high, indicating a blockage. But on newer vehicles like the Armada where the compressor is managed by a control module, they’re sensors, not switches—that is, they report actual system pressure back to the control module which then makes the decision regarding whether or not to send voltage to the compressor’s magnetic clutch. The one on the Armada is a three-wire sensor with the wires being ground, a 5V reference voltage, and the signal from the sensor. You can’t simply bypass the sensor with a jumper wire, as either doubling back the 5V pin into the signal pin or grounding it at best sends the control module an out-of-range pressure reading, and at worst risks damaging the module that’s reading it.

But what you can do is fool the system by sending it a reading that the vehicle’s electronics interprets as a valid A/C system pressure between 17 psi and 292 psi. The trick is to use two 1.5-volt batteries in series, as a 3-volt reading corresponds to a pressure within the acceptable range. I don’t recommend that you do this because if you get the wiring wrong, you can blow up the a/c control module, but I was willing to take the risk. So after spending $5 for a battery holder, carefully using a multimeter to verify which of the three pins on the connector were 5V, signal, and ground, tinning the ends of the leads with solder, carefully inserting them into the signal and ground sockets on the connector, and hitting the switch on the battery box, the compressor engaged and the condenser fan turned on. I used a sloppy wrapping of electrical tape to temporarily hold the leads in place while I test-drove the car.

Let me interject here that there’s a distinction between the compressor short-cycling, which is the pressure switch turning the compressor off when the refrigerant level is low because the low-side pressure drops too low and then turning it back on when it rises, and the A/C turn-off that my Armada was experiencing. And, yes, I admit that the refrigerant was, or could have been low, as the system definitely did have a slow leak. Short-cycling, though, tends to repeat at intervals more like several times a minute, whereas what I was experiencing was the A/C shutting off and not coming back on again until I did something like parking and restarting the car. Do-it-yourselfers like me often bypass the pressure sensor, or hot-wire the compressor when filling an empty system to speed up the delay caused by short-cycling.

But as far as fixing my shut-off problem, my AA-battery-box pressure sensor bypass made no difference. After driving for maybe ten minutes, the vent temperature gauge rose, and stopping the truck and popping the hood verified that the compressor wasn’t engaging. The same thing happened on repeated tests. So it appeared that the shut-off was neither due to short-cycling due to the system being low on refrigerant from the leak, nor from some other pressure issue being flagged.

I wondered if perhaps it was as simple as the relay controlling the compressor clutch having some heat-related problem. The relay is drawn in the A/C system layout as if it’s a separate part. Unfortunately, when I searched on the Armada forum to determine the relay’s location, I learned that it’s an integral component of the Intelligent Power Distribution Module (IPDM). Strangely, whereas the IPDM is a collection of fuses and relays, the compressor relay is a non-removable part of the module and is not independently replaceable. If the relay is bad—and I didn’t know if it was—you need to replace the entire module. New, it’s about $500. Used ones are available on eBay for about $100, with no guarantee that they don’t have the same issue. The same was true of the A/C control module (the oddly-named “A/C amp”), which was also widely postulated as “maybe it’s this” on the Armada forum.

I delved back into the forum and saw numerous posts where others experienced similar A/C-shuts-off problems similar to mine (absent the slow leak), analyzed it with scan tools, replaced the IPDM and/or the “amp” or had the dealer do it, and it made no difference—some sensor was still causing the amp to shut down the compressor, but they could never figure out which one.

I thought about it carefully, as I do most things (I’m not the unhinged whack job that I sometimes appear to be).

While I’ve run my vintage cars with the compressor hot-wired, I really didn’t want to do this to the Armada, as it’s a complicated system and bypassing everything felt like disabling all the safety interlocks on a train. The risk in driving a car with the compressor hot-wired is that you run it low on refrigerant and/or oil and seize the compressor, which typically causes it to throw metal and rubber debris into the system. To resurrect a system after this happens, the compressor, parallel-flow condenser, and receiver-dryer all need to be replaced, and the remaining components (hoses and evaporator) need to be flushed out. But since there’s an expansion valve in line with the evaporator that you can’t flush through, it means that the evaporator core needs to be removed to be flushed. And this vehicle has rear air, so there are two evaporator cores. So if I was keeping the Armada long-term and needed the A/C to work, risking compressor seizure for short-term usage was foolish.

On the other hand, there was no way I was going to bring the vehicle into a dealership and pay them for a full A/C diagnosis and repair, as the cost could easily approach the vehicle’s value. This is a 182,000-mile Armada with a non-trivial amount of undercarriage rust and a whole variety of quirks and needs. I’ve already admitted that buying it was a mistake. Even with all the work I’ve done on it since buying it last winter for $3250, I doubt I’d get three grand for it. I’ll probably quasi-justify its existence by using it to tow my 1973 BMW 3.0CSi to and from a paint touch-up, and dragging my barely-running ’75 2002 back from storage, then sell it in the fall. It’s difficult to imagine that, if I seize the compressor, that will be the single thing that torpedoes the vehicle’s value.

Decision: For now, hot-wire the damned compressor. Well, wire it to a switch so you can directly turn it on when the Armada’s internal climate-control Skynet fails. And check the system pressure before you use it.

The switch part of it turned out to be a bit tougher than I thought because passing wires through the firewall on a modern vehicle is a lot harder than doing it on a 50-year-old car with saggy grommets that you can see light through, and where the challenge is keeping engine smells out of the vehicle. Fortunately, there’s a whole class of remote-controlled electrical relays (which is ironic because relays are, my definition, remote-controlled switches). For $16, I found a little remote-controlled switch box which I could mount in the engine compartment, wire with input voltage from the battery and chassis ground, connect the output to my spliced-in wire to the compressor, and turn on from the inside of the truck with a little key-fob-type remote.

My mounting scheme is currently having the relay in a Ziplock bag taped to the lid of the fuse box. It’s just enough to be functional. And yes, it’s wired with a fuse.

I took a little break from the compressor problem to look at the “woodpecker under the dash” issue. Like many modern climate-controlled cars, actuators move the flaps on the Armada that control the blend of hot and cold air and its distribution through the front-facing, defrost, and footwell vents. It’s common for something—a gear inside the actuator, or a piece in the linkage between it and the flap—to break. The result is often the actuator motor continuing to spin when it shouldn’t, causing the teeth on plastic gears inside it to skip and make a sharp clicking sound. The Armada has three of these actuators on its climate control box. If the clicking is coming from the one in the center, the FSM directs you to remove the entire dash, with YouTube showing a variety of workarounds to avoid such an invasive job. Fortunately, the one that was clicking on mine was the easy-to-reach one for the passenger-side blend door. The fasteners are accessible if you have a 5.5mm ratcheting wrench. I yanked it out and found that the problem was that the little triangular plastic stop tab that sits on the cam-shaped plastic plate between the actuator and the lever had broken off. Multiple posters said they tried to epoxy the broken-off stop, only to have it immediately snap off again, so went to order the part. Oddly, the actuators are fairly inexpensive and available from dozens of manufacturers and vendors, but this particular intermediate lever is a Nissan-only item that had gone out of production, meaning that I had to hunt around for a dealership that had one in stock. $40 and a one-week wait from California got it here. It didn’t help main A/C problem, but at least it got the woodpecker to shut up.

I learned that there was a fourth actuator on the heater control valve against the firewall under the hood. This is supposed to shut off the flow of hot coolant to the climate control box when the a/c is on high. I drove the truck and then laid my hand on both the box and the hose exiting the valve, and could tell that it they were staying hot, so I pulled the cover off the actuator. It’s designed so that you can remove the rod connecting the motor to the valve and rotate the valve by hand. The valve felt sticky. For now, I simply rotated it closed and left the control rod out. I verified that the climate control box no longer had a little furnace running inside it. When the a/c was running, this may have made the vent temps a degree or so colder, but it had no effect on the compressor shutting off.

So, one unforgivable kluge (the remote-controlled compressor switch), two pretty decent ones (the battery-powered pressure sensor bypass and the manual closing of the heater valve), and one absolutely correct repair (replacement of the intermediate actuator plate). It averages out to be almost defensible. But I feel that what I’m doing with the remote compressor switch is taking data. I can continue to drive the Armada with the normal climate control, note what’s going on when the cabin temperature rises even though the A/C light is on, shut the A/C off so I don’t risk back-feeding the relay with voltage, then remotely turn the compressor back on.

But seriously, on a car that you value, don’t do what I did with the compressor.

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Rob’s latest book, The Best Of The Hack Mechanic™: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon here. His other seven books are available here on Amazon, or you can order personally inscribed copies from Rob’s website, www.robsiegel.com.