Solving Automotive HVAC Problems
The phrase “can of worms” comes to mind when I think about the DIYer servicing an AC system. There’s so much to be considered. Technicians have to be certified to work on AC systems; that should tell you something. But even that’s not enough most times. You also need a good amount of experience working on AC systems. With that said, I strongly recommend you take your AC work to a professional. They have the proper equipment and experience to get the job done, or at least they should have it. Weigh your options carefully before you tear into your AC system. AC parts can be expensive; I’d hate to see you install a new expensive part that didn’t fix the problem, or end up damaging your system due to improper service. Ignorance can cost you big time here. Proceed with caution.
You need to know what you’re getting into when working on your vehicle’s AC system. Several laws govern the use of refrigerant and how it is handled in automotive systems. Be sure to check your local laws to confirm that you’re even allowed to service your AC system.
To be honest, I feel like I’m handing you a loaded gun with this information. I can’t tell you how many times I’ve seen even seasoned techs handle AC improperly. I take it a bit more seriously, and I suggest you do as well. After all, I had to pass a certification test before I was even able to service AC systems. Anyone who isn’t certified and is caught in a shop working on AC is subject to some pretty steep fines. Keep that in mind and take it seriously.
It doesn’t matter who makes it; all AC systems work on some rather simplistic principles. The main principle is the pressure-temperature relationship. This is a term used in physics to describe how a material reacts at different temperatures and pressures, and also what happens to it when it changes states. By changing states, I’m referring to when refrigerant changes from a liquid to a gas within the AC system. In the case of your automotive AC system, HFC R-134a is the most widely used refrigerant at this time. Before the 1994 – 1995 model years, CFC R-12 was used in systems; starting in 2013 – 2014, they will be using HFO-1234yf. At least they plan to use HFO1234yf; I’ve heard of some controversy surrounding that refrigerant that might give it a short life span. Honestly, unless you’re in the collision industry, I think it will be some time before you need to service one of these systems anyway. We’ll cross that bridge when we come to it.
Regardless of the type of refrigerant used, they all use the same basic principles. You take a high-pressure liquid, run it through a small opening, and turn it into a low-pressure gas. This is what causes the cooling effect in your AC system. This change of state and pressure is the pressure-temperature relationship at work. As the refrigerant goes from a high-pressure liquid to a low-pressure gas, it gives up a lot of heat. A common misconception is that the AC system cools. This is not necessarily true. What really happens is the AC system takes the warm air from the passenger compartment and moves it outside the passenger compartment in front of the radiator. It’s more of a heat exchanger than anything else. Your AC system moves heat; think of it that way. When the evaporator inside your dash gets cool, it absorbs heat. It then takes this heat and moves it out to the condenser in front of the radiator where the heat is dissipated into the atmosphere.
The refrigerant is moved through the system via the AC compressor. The compressor draws in low-pressure gas from the exit pipe of the evaporator, and then outputs it at a higher pressure and sends it towards the condenser. The compressor should never have liquid refrigerant in it. It could suffer a lot of damage if it did. Your compressor is meant to compress gas, not liquid.
While the refrigerant is in the condenser, it gains in temperature and pressure and turns back into a liquid. Once it leaves the condenser as a high-pressure liquid, it then moves it to the orifice just before the evaporator where it is turned back into a gas, the pressure is greatly reduced, heat is released, and the cycle starts again.
The AC system is split into two parts: the high-pressure side and the low-pressure side. The high-pressure side is from the output side of the compressor, through the condenser, to the back side of the variable opening before the evaporator. There are many configurations and terms for this opening. It’s sometimes called an expansion valve and sometimes an orifice tube; either way, they all do the same thing, which is to help change the high-pressure liquid refrigerant into a low-pressure gas. The low-pressure side is from that orifice near the evaporator (low-pressure side) all the way to the intake of the compressor.
In addition to the compressor, evaporator, and condenser, a couple of other components are usually found on the AC system. One of these components is a variable opening to the evaporator. There are many different configurations and names for this, but the idea is to regulate the size of the opening going into the evaporator for maximum efficiency.
Besides a variable opening, you’ll also see either a receiver dryer or an accumulator, depending on where it’s located in the system. Receiver dryers are normally located near the outlet of the condenser on the high side of the system. These are placed in the system to help remove moisture. Any moisture in the AC system will affect its performance, so you should take great pains to keep it out when working on your AC system. Here is a video describing in more detail how the AC system operates.
The first step you should take when checking your AC system is to run an AC performance test. This test will let you determine if your AC system is up to the task or not. It’s a simple test; all you need is a thermometer. They make special thermometers for this that you can get from your local auto-parts store.
Be sure to take your measurement from the center vent. This ensures the most accurate reading. Before you start your test, take a reading of the air temperature around you. This will be your baseline. Then:
- Start your vehicle and turn on the AC. Do not put it on recirculate.
- Put the fan on the lowest setting and insert your thermometer. In a few moments the temp should bottom out and you can take your reading.
- Compare that reading to the temperature reading you took before starting the test. It should be 30º to 40º F cooler than the outside air.
Humidity will greatly affect this reading and the ability of your AC system to cool. The higher the humidity, the harder your AC system needs to work, so be realistic with your assessment. If it’s a very humid day, your AC system will have to work pretty hard to do anything at all. If the AC isn’t cooling at all, open the hood and see if the compressor is turning. If it’s not, then your diagnosis should start there. If it is, your problem lies elsewhere and it’s time to break out the pressure gauges.
Here’s a video showing how to do the AC performance test.
If your AC is not cooling like it should, the next step is to find out why. This is where things can get tricky. In order to properly diagnose an AC system problem, you need at the very least a set of AC pressure gauges. At present there are two types: one for R-12 and one for R-134a. Be sure to use the correct gauges for the system you’re working on. You don’t need to hook up the gauges right away, however, as a good visual inspection can get you started. As stated in the previous section, be sure to check the operation of the AC compressor. The compressor is divided into two parts. One part is the actual compressor and the other part is the compressor clutch assembly.
The compressor clutch assembly engages the compressor when activated. It’s an electromagnetic device that, when active, connects the compressor input shaft to the drive belt on the engine. When it’s not active, it allows the compressor pulley to just freewheel. You usually hear a click when the AC compressor clutch engages, and then the front of the pulley starts to spin (or it should start to spin). If this happens, it means your compressor and compressor clutch are capable of working. You might also notice a slight change in the engine idle when the AC compressor clutch engages, this is due to the extra load the AC compressor puts on the engine when it’s active and is perfectly normal. If your engine stalls out when the AC is active there could be an issue with the idle circuit or there could be a problem with the AC system. See the Idle Issues Article for more information on how to solve idle issues.
If you hear a lot of noise when the compressor engages, first check the belt tension. If the belt is loose, it might slip and cause noise. It could also be the compressor clutch or the compressor itself that’s making the noise, so be sure to try to nail down the source of the noise if you have one. You can use the information in the Noises article to help you with that diagnosis.
After verifying that the compressor works, the next thing to check for is cooling fan operation. If you have a FWD vehicle, it normally uses electric fans to cool the radiator and condenser. Make sure these are working when the compressor is on. If not, find out what the issue is and go from there. You can get more info in the Electrical article to help you figure out what’s going on with your fans.
In addition to making sure the compressor, compressor clutch, and fans work, it’s a good idea to look over all the hoses and fittings for the AC system. What you’re looking for is any oil residue that might indicate a leak. AC leaks are probably the number-one cause of AC system issues, and a good visual inspection is one way to
find them. Also, look at the front of the condenser. This is the part of the AC system that’s placed in front of the radiator. If it’s clogged up and blocked with debris, remove it and recheck your AC system performance. Remember, the condenser needs to get rid of heat. If it’s blocked off, it can’t do that very well and AC performance will be compromised.
There are a lot of things that can cause AC problems. That’s really the thing about AC. You can have a problem with leaks, electronic issues, or pressure problems created by a whole host of possible causes. It takes years to develop the skills to properly diagnose an AC system. Honestly, there really isn’t enough space for me to get into a complete AC diagnosis, as there is so much to cover.
I get asked about AC problems a lot. If I’m going to make any recommendations here, it’s to take your AC issues to a professional. They have the equipment and the know-how to do it correctly. If you’re just topping off your system until it gets warm again, you’re not addressing the root problem and you’re also causing damage to the environment. I consider this irresponsible behavior.
I won’t lie; AC work can be expensive, in fact very expensive, but it’s a luxury many people can’t do without. If you find yourself in this situation, you might consider taking your vehicle to a repair facility to be diagnosed and have the refrigerant recovered. You can then do the work yourself and take your vehicle back to the shop to have them evacuate and recharge the system when you’re done. The choice is yours where the law allows. I just urge you to be responsible when working on your AC system.
As I stated before, AC leaks are probably the number-one cause of poor AC system performance. Finding those leaks can be tricky, but there are tools out there to help you with that. There are two main tools used to find AC leaks. The first is a fluorescent dye you put into the system. When the refrigerant and the dye leak out, you can find the source with an ultraviolet light.
The second type is called a sniffer. This tool is placed near the suspected source of the leak. When it detects a refrigerant leak, it emits a beep or warning to the user. The faster the beep, the larger the leak.
Refrigerant dye is the most popular way to find AC leaks. It’s cheap and definitive. A sniffer can be used in situations where you’re not able to put your eyes on the potential source of the leak. Say you have a leaking evaporator located deep inside the dashboard; you can use the sniffer at the vent outlets to see if it detects a leak.
You can also use dye to find these leaks, but it’s a little more difficult. If you suspect an evaporator leak and you want to check for it with dye, check the AC drain tube located under the vehicle. This is the tube that allows the condensation from the evaporator to leak out. When dye is placed in the system, it sometimes comes out with this condensation.
One last method to find leaks I almost forgot about, is with soapy water. You can also spray the suspected area with a soap and water solution, then observe the area for bubbles. If there’s a leak, you’ll see bubbles accumulate in the effected area. It may seem crude but it’s very effective for the exposed parts of the AC system. Here is a video showing the AC dye and sniffer methods.
Repairing AC leaks usually means replacing the leaking part or seal. This could be the evaporator, the condenser, the compressor, or the lines, or perhaps an O-ring has gone bad. The majority of small AC leaks are at the service fittings. Especially after service, these fittings tend to leak. If you have a slow leak that’s difficult to find, I’d suggest taking a good look at the service fittings. You can sometimes
replace just the valve, but other times you might need to purchase the entire AC line in order to repair a leak at one of these fittings.
When repairing one of these leaks, it’s very important you do it properly. DO NOT VENT REFRIGERANT INTO THE ATMOSPHERE. This is very harmful to the environment and illegal in many locations. It’s one thing if it leaks out; it’s another thing altogether if you intentionally cause a leak to happen during AC service. Before servicing an AC system, you need to recover any residual refrigerant remaining inside the AC system before opening it up. There are special recovery machines that do this work.
As stated above, if you’re going DIY, you might consider taking your vehicle to a shop to have the old refrigerant evacuated before you start your repair. After you’ve completed your repair, you can then take the vehicle back to have the system vacuumed down and recharged.