Every vehicle is typically engineered for some measure of passenger comfort. The seats are contoured for the human body, and the instruments and controls range from merely functional to super-sexy dashboard eye candy, replete with 21st century gadgets and widgets that range from really handy to silly and superfluous. Oh, and we like our entertainment systems, too. But even the most comfortable vehicle with the richest concerto can morph into an odiously uncomfortable ride if the weather outside is steamy and the driver can’t control the cabin’s climate. And then there is the problem of a heatless vehicle in the winter. In the subtropics where I live, we don’t have a lot of dreadfully cold weather, but on days when it’s cold enough to freeze, driving a car without heat can be pretty miserable. On those days, we find ourselves digging for heat. But even if the heat works, yet the A/C is out of commission, the windshield doesn’t de-fog properly because the A/C is as much a dehumidifier as it is a refrigerator.
One thing is certain; a vehicle owner who comes to your door in hot weather with hot air blowing will certainly be a lot happier if they leave having found cold at your shop.
In this article, we’ll do a quick run down on where to begin and what to look for when a sweaty and uncomfortable driver comes digging for cold. And if you can surgically get a person’s A/C back online when they’ve been suffering in the heat for a few days or weeks, it’s a guarantee that you’ll come away a hero.
In the driver’s seat
To begin with, make sure you know how to turn the A/C on. That statement might sound silly, but once back in the early 2000s, we were digging for cold on an ’85 Buick Riviera and so I told a 44-year-old student to turn on the A/C. After sitting in the seat staring at the dash and rubbing his chin for a couple of minutes, he returned red-faced and embarrassed to where I was waiting and watching the compressor, reporting that he couldn’t figure out how to turn the A/C on. Rolling my eyes and groaning at his incompetence, I sat down in the car and spent some time rubbing my chin and staring at that crazy CRT screen in the middle of the dash, just like he did before I could figure out the combination. I got it turned on, but it was most humbling.
And I’ll never forget the five-year-old Geo Tracker that came in late one spring with the complaint that the A/C had worked fine the previous fall, only to find that there was no A/C button on the control head to engage the A/C, just a blind plastic plug and a stiff harness connector behind it that had never been plugged into anything. Not being familiar with the Tracker, I didn’t at first notice that the A/C button wasn’t there. I could jump the wires at the missing button’s connector and get good A/C, but there had never been a button there, so how did the A/C work the previous fall? How did it ever work? I’ll never know, and the customer didn’t either.
The point is that it’s important to understand the dashboard operation of the A/C before plowing ahead with another diagnosis.
Moving past turning on the A/C, the blower should be capable of moving air, as elementary as that sounds. When a customer says, “My A/C doesn’t work,” that statement carries about as much information as, “My engine runs rough.” And since I’ve seen quite a few A/C concerns that were blower related but weren’t recognized as such by the customer, I look for appropriate airflow at all speeds, and if I don’t have it, I go after cabin air filters first, if the vehicle has them. Most folks (and many technicians) forget about the cabin filters and by the time they’re checked, they’re pretty loaded. If the filters aren’t a part of the picture, leaves and trash in the evaporator case may be to blame if the car is regularly parked under trees. If the fan vibrates the dash when it spins, remove it and look for something in the squirrel cage. I once pulled a fan on a very clean Camry and found what looked like the bleach-tattered remains of an old dishrag in the cage, and the customer had not a clue how that stuff got in there.
If the airflow and cooling are normal early in the drive but then they taper off to nothing as the miles and minutes go by, look for an evaporator that is freezing up. I saw that concern on three different vehicles last summer. The first one was a Mazda pickup I mentioned in a previous Motor Age Garage article. A welded-together A/C clutch relay was keeping the clutch engaged, freezing the evaporator solid and killing the battery overnight to boot; the driver had noticed a loss of cooling and airflow after a few minutes of driving. The compressor was always engaged, thus causing the freeze-up as well as the parasitic drain.
Secondly was a Dodge pickup we checked a few days later that had the same A/C concern – lost airflow after driving. With a thermometer in the register on low blow/recirc (MAX), both doors closed and the windows up, we saw the register temperature drop south of 30 degrees due to a stuck-closed low-pressure cycling switch. Max/recirc, windows up, and low blow is where the lowest temperatures can be measured.
A third icemaker we ran into in as many weeks was the 2010 Ford Fusion on which we initially replaced the evaporator thermistor only to find that the A/C control head was failing to respond to the thermistor’s signal. After I explained what was going on, the owner opted to manually cycle the A/C to prevent the freeze rather than spending the money for a new control head. In her case, knowledge was power, I suppose. Anything that prevents the clutch from cycling off will freeze up the inner heat exchanger.
Make sure you have a fan blowing through the condenser — we didn't on this one.
Speaking of evaporator thermistor issues, we tackled a Hyundai Santa Fe that had been repeatedly taken to a dealer because the A/C wouldn’t cool, and lots of money had been spent to no avail, but a simple button-dance on the control head revealed a code that pointed to a faulty thermistor. The problem was that the dash had to be removed and it was quite a large project, but when we were done, we had found cold, and the owners decided to keep that vehicle and have it repainted instead of trading it in.
Under the hood
If the register is blowing warm or even hot and there is no control head diagnostic button combination, I like to pop the hood and check for compressor operation if it has a magnetic clutch, and if the compressor is spinning, I want to feel a cold suction line and a hot liquid line. If the compressor isn’t spinning, it’s time to sniff the juice with a Neutronics identifier and then connect the gauges or the machine to check for pressures.
When you first connect the gauges, you can make note of the static pressures, but there’s only so much those pressures can tell you. Ambient temps have a tremendous impact on system pressure, and on a hot day, a half-charged system might show 80 psi on both gauges. If, however, the static pressures look that decent, the low-pressure cycling switch should engage the compressor, unless there’s an electrical or mechanical problem, i.e., the compressor’s clutch has worn to the point that the air gap is too wide. More about the electrical stuff in a minute.
If the compressor is spinning and the suction line is cold all the way to the compressor on a hot day, the refrigerant charge isn’t likely to be a problem, but the blend door may be. And some platforms have a blend door and a heater control valve that bypasses the heater core when heat isn’t called for, so pay attention to the valve if it has one of those.
If the suction line is cold right out of the evaporator but grows warmer along its length as it travels through the engine room, suspect low charge, particularly if the compressor is short cycling. But realize that not all systems will cycle the compressor exactly the same way.
If the A/C charge is good and there is power making it to the clutch coil, gently bump the clutch with something (be careful!) to see if it kicks in with help. If it does, then doesn’t kick in again after cycling off the next time, the clutch air gap should be checked and adjusted, a procedure that differs from unit to unit. Some clutches have shims – others require special tools to press the clutch hub a bit closer to the pulley.
Pressures and juices
When the compressor kicks in, the pressures should dive in different directions, but how far they dive will be directly tied to ambient temperatures. On a cool day, you may see the low side on a fully charged system drop into the mid-20s with high-side pressures bouncing between 150 and 200, accompanied by short-cycling clutch times. On a hot day, you might see low-side pressure between 40 and 50 and high-side pressure up around 300. Make sure you have good airflow through the condenser and a healthy fan that works every time it receives power.
Low pressure on both sides with a normal refrigerant charge typically means the expansion valve needs replacing on systems that have those, but a clogged evaporator can have the same effect.
Too much oil in the system can be a problem, too; oil-saturated heat exchangers are inefficient at best, and flushing may be necessary if too much of the slippery stuff has been added. And if you aren’t sure whether the charge is right or not, the best way I know to determine if the right amount of refrigerant is present is to sniff it first with the analyzer, then recover it completely (which may require heating the engine compartment by running the engine with the hood closed if your shop is cold), and then recharge the system with the proper poundage. Recognize that refrigerant gets trapped in system oil and that when loose refrigerant is recovered, oil-trapped gas will bubble into the system and will need to be recovered as well, or else you may overcharge. Watch for that!
If the underhood info sticker is gone or ruined beyond reading, look the vehicle up to find out how much refrigerant is required. Most modern systems on cars take only a pound or so. Vans and SUVS take a bit more. But if you’ve been recovering and recharging the same one every few months, pay attention to how much oil is extracted during the recovery process and re-add the same amount; we’ve replaced compressors that were destroyed from lack of oil when the same shop recovered and recharged the same A/C several times rather than fixing the leak.
Speaking of oil, when replacing components, always check the shop manual for the proper amount of oil that needs to be added for each one. The compressor may or may not come charged with oil, so never assume anything. Read the papers that come in the compressor’s box to find out if oil is in there, and act accordingly. Oh, and by the way, it’s a good practice to stand the compressor on its nose for a few minutes before installing it. That allows the oil to move toward the front and lube the shaft seal, and we all know we should replace all O-rings and seals between lines and components.
If the threads gall during the wrenching process, you’ll need to replace both components, and when you’re performing pressure-and-juice service, don’t ever fail to check and then replace the orifice tube with one that’s the right color, because that’s how they’re coded. Trash on the orifice can be quite revealing, too, so be prepared to do a bit of service bay forensics to see if you can determine what happened. Rear A/C units typically have expansion valves even if the front unit is an orifice-accumulator system, and when you’re dealing with a tubed one, make sure the TXV’s sensor tube is firmly in contact with the suction line and wrapped with press tape.
The A/C won’t work without electrical components, and while the PCM is typically the decision maker where clutch and condenser fan are concerned, some systems have an additional box that might be called an A/C amplifier. GM likes to call their box a “programmer” and on Automatic Climate Control systems, GM has called it a programmer ever since the late ’60s. Earlier Asian makes love to use the A/C control boxes, even if they don’t have Automatic Temp Control. The amplifier or programmer can be an issue, but pay attention to other modules that may be in the loop, because anomalies arise. Some might remember that I wrote about a 1998 Buick awhile back that had a faulty body control module (BCM). That BCM would talk to the scan tool but it was preventing the A/C from engaging, and for some reason it wouldn’t even let the PCM turn on the condenser fan – not ever, even with a blistering hot engine. Check the wire harness map for how the pressure cutout switches are wired, because you need to know.
If you suspect a burned out compressor clutch, simply remove the clutch relay, find the terminal that leads to the clutch coil, and use a low impedance test light (one that will draw some current) to see if you have a good path to ground through the clutch. If the wire is good to the compressor and the coil ground is solid, but you don’t see a light at the compressor clutch terminal, the coil is open, and it’s surprising how many times that scenario happens. If you DO see a light, feed some power into that terminal and see if you hear the telltale click of the clutch.
You can replace the clutch and coil if the coil is burned out or the clutch is coming apart (and that happens), but that usually costs about 70 percent of what it costs to replace the compressor, so I always swap the whole thing when a coil dies, along with the accumulator or drier and orifice or expansion valve. Go find some cold.