Vehicle air conditioning service you can perform

Jan. 1, 2020
In today's world of tightening environmental regulations, global warming alarmism and all the rest of it, everybody who lays a wrench on an A/C system should be certified to do so.

Learn what you can perform in your service bay to capitalize on A/C service.

Most of us need our vehicles every day, and when our traveling machine breaks down, a lot of other things in our lives break down. Desktop computers and cell phones might be just about as pervasive as cars, but they can't get us from Point A to Point B the way a vehicle can.

Everybody likes an engine that starts easily and idles smoothly, and when we're behind the wheel and in the slipstream, nice responsive acceleration into and around traffic is only a part of a pleasant (and safe) driving experience. We all enjoy a tight comfortable hug-the-road steering, suspension and tire-tuning package, and we appreciate our transmission when it properly multiplies the engine's torque and moves the car while making it through all gears without a hitch. And who could survive without brakes that can safely bring it all to a halt? But let a person's A/C blow hot air when the asphalt is egg-frying hot, and that customer will waste no time finding somebody to reverse the situation. If you're the wrencher that gets that customer's A/C frosty cool again (provided it stays that way), well, you're the hero, and the repair will be remembered for a long time.

First Things First

In today's world of tightening environmental regulations, global warming alarmism and all the rest of it, everybody that lays a wrench on an A/C system should be certified to do so. Why? Well, one good reason is that it's a $32,000 fine if a government audit reveals that a technician in a shop wasn't certified and had done A/C work, even if the shop has another tech who IS certified.

There are caveats, according to Ward Atkinson of Suntest Engineering. If, for example, you're fixing your neighbor's A/C for free (that means nothing at all can change hands in the way of payment, not even favors), you still can't vent refrigerant to the atmosphere, but you can evacuate and charge your friend's A/C without certification and not break the law. If the neighbor gives you anything at all in return for your service, well, you've been paid and you're on the wrong side of the line.

My own certification is with ESCO ( ), and it really isn't that difficult to become certified. There's a downloadable study guide, a small fee and an online 50-question test. The Mobile Air Conditioning Society (MACS) is an international organization that also offers excellent certification and quite literally has some of the best training in the world. The MACS Web site is
One way or another, more activity is coming down the pike regarding certification requirements, and technician certification is mandated in a lot of states. But the states in question typically don't have a budget to enforce the mandate. Further, OEs don't typically alert their dealers concerning mandated requirements for technician certification. Many dealers just don't know about it because they haven't been told, and we all know that in the eyes of John Law, ignorance is no excuse. If you're reading this article, well, now you've been told.


It's astounding how many shops don't even use a refrigerant identifier. Subsequently, we sometimes find cars with a full charge of 134a in a system with R12 fittings, and there are a lot of cars with blends of R134a and R12. Get an identifier and use a decent older A/C machine to draw contaminated refrigerant out of contaminated systems so as not to spread nasty juice from one car to another, and make sure you dispose of the blended juice properly. It should go in a gray tank with a yellow top. Getting rid of contaminated refrigerant requires sending it to an approved processor for recycling or destruction.
The new SAE J2788 machines are designed to draw just about all the refrigerant out in about 30 minutes and are reprogrammable via a Secure Digital card slot. But if you're savvy, you can get the refrigerant out of there with a well-maintained older machine. If the weather is cool, even the best machine is subject to leave quite a bit of refrigerant trapped in the oil, so fire up the engine, heat all the underhood A/C system components to a toasty temp with engine's heat and you'll be more likely to get all the gas out.
My 2001 Cherokee didn't have any leaks, but it had been drawn down and recharged a couple of times over the years for good measure (it only holds 1.2 pounds). When I recovered the refrigerant from that system to replace a noisy compressor, I got busy doing something else and came back to notice that the system pressure had increased to about 30 psi. When everything was said and done my system gave up more than 2.0 pounds of refrigerant! No wonder it wasn't cooling well!

Down to Business

With the A/C engaged and the blower operating, the air should obviously be cold. If it isn't, the compressor should be running and the pressures should be commensurate with the ambient temperature. On warm day, low side pressures generally run around 30 and high side pressures about 250. These pressures will be higher in hot ambients. The refrigerant is supposed to evaporate in the evaporator. If it doesn't, some liquid might make it back to the compressor, and we all know liquid won't compress, so the compressor can take that liquid it on the chin to the point of being down for the count.

So what can cause liquid refrigerant slugging? Try heavy leaf or pine needle clogging in the evaporator housing, or for a more contemporary concern, clogged cabin air filters. Cabin air filters are often overlooked, and when they do get changed, they're nasty enough to prevent airflow in a big way, thus interfering with the evaporation of refrigerant and increasing the possibility of unwanted liquid in the suction line. Refrigerant isn't the only liquid that can damage the compressor – there's oil in the system, too. Incidentally, the oil is measured by volume and refrigerant is measured by weight. And when the compressor is mounted low in the system, oil can gather down there and potentially cause damage when the compressor starts gins up and starts pumping.
Ford calls their way of dealing with this Compressor Anti Slugging Strategy (C.A.S.S.) on platforms where the A/C compressor is mounted low enough to collect oil during shutdown. These units engage the compressor clutch while the starter is spinning so as to gently shove any oil out of the way that might damage the compressor. Research the system you're working on and make sure you understand how it operates, or else you may chase rabbits and waste cabbage on fruitless repairs.

So what can cause liquid refrigerant slugging? Try heavy leaf buildup or pine needles clogging the evaporator housing, or for a more contemporary concern, clogged cabin air filters.

A/C Control heads do fail, and they're pretty pricey, but make sure you check everything else before you spend $800 on that black box with the green display.

If the refrigerant is evaporating the way it should, the big suction line should be cold all the way to the compressor. But that can't tell you if the evaporator is partially clogged, and with that crucial aluminum coil buried in a plastic housing, it's not always easy to determine where the coldest areas are if a restriction is mitigating refrigerant flow. The evaporator is simply not that accessible on most vehicles. I knew one A/C guy years ago who told me that as a last resort he always replaced the evaporator if nothing else he found and fixed would make the A/C cool the way it should.

The condenser should be getting nice and hot as the refrigerant dumps the heat it picked up in the evaporator, and you can use a temperature gun or some sprayed water to find cool spots on the condenser that might indicate internal condenser clogging. Cool spots will stay wet.

Don't forget my trick of checking the cooling fan for a dead spot. Wire a test light in series with the motor (you can perform this test quite easily at the fan relay if it's in the Power Distribution Center) and turn the motor through slowly to see if the light goes out. If it does, you've accurately condemned the condenser fan for an intermittent failure. The beautiful thing about this test is that it's 100 percent reliable if the light goes out during a turn-through.

Got normal high and low side pressures but warm air? Well, you can pinch the heater hose inlet (don't foul up and pinch the outlet or you could burst the core) and see if the air gets cold. If it does, suspect a blend door problem. It's easy if the pipes are two different sizes – the smaller one is the inlet.

The air-handler's blend door is so named because it allows warm air from the heater core to blend with cool air that has been de-humidified by the evaporator, and it's one of many doors in the dash panel that control air flow.

Actuators can fail and blend doors can break or become fouled by a pencil, cigarette lighter or some other flotsam that makes its way from the dash panel down through the defroster duct. If you measure the outlet temperature while cycling the blend door back and forth, you might smoke out an intermittent problem with that component. Remove the actuator (it can be a pain on some systems, I know), and operate the control head with the wires plugged in while watching the control head to see if the actuator moves as it should. Make sure you remove the right actuator! Some high-end vehicles have as many as 10 electric actuators CAN-bussed together, and you probably won't have a scan tool that will work on every car.

A run-down on gauge readings and electronic control head diagnostics is too lengthy for one article, but if the low side is showing a deep vacuum and the high side is showing not a whole lot of pressure, suspect a stuck closed expansion valve (variable orifice) or clogged fixed orifice.

Concluding Tips

Smoking out an intermittent might be as simple as putting the A/C on Max Cool and low blow and letting it cycle until it quits cooling. Relays and switches click when they're trying to work and you can feel the action, but that doesn't mean they're carrying the load, so have your jumper wire ready.

If bumping the compressor clutch clicks the compressor on line, an air gap adjustment will be next, but be aware that some compressors require special tools for that job.


The refrigerant is obviously supposed to evaporate in the evaporator and absorb heat in the process. Further, it's supposed to condense in the condenser to give up the heat that it absorbed in the evap coil.

Both of these coils have fans blowing across them for obvious reasons. Refrigerant enters the compressor as a low pressure gas and exits the compressor as a high pressure gas on the way to the condenser (by way of the discharge line), where it becomes liquid and makes its way through an orifice (either fixed or variable) into the evaporator as a low pressure liquid where it once again becomes gas. Newer condensers have the drier built in and are designed to cool the liquid refrigerant after it has been returned to its liquid state. The benefits of that change are obvious as well.

The A/C compressor gets its lubrication from oil that is carried with the refrigerant from the accumulator (a big can with a desiccant bag that is mounted in the suction line at the evaporator on fixed orifice systems) or the drier (a smaller can with a desiccant bag mounted in the liquid line on Thermal Expansion Valve equipped systems).

When things go awry, the orifice clogs with material and so it should be checked for evidence as a matter of course any time the system is emptied of refrigerant and unzipped. And don't be bashful about selling a drier or accumulator. The low pressure cycling switch is in a hostile environment and works its little buns off, so a new one of those might be in order too if you're a belt and suspenders type.

There are some dandy little screens that can be pressed into the suction line where it connects to the compressor. These little screens are a great idea if you want to protect a new compressor from stuff that might find its way to the compressor inlet.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. He is an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail him at [email protected].

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