Since we are now in the hottest part of the year, I thought I would create this thread so that the do it yourself people can fix their own AC problems, or at least give themselves a vague idea on what’s going on….
First off, here is some information about refrigerants.
All cars up until 1992 or 1993 used R12, which is considered the older style refrigerant. It is an ozone depleting substance which is why in 1994 they switched to R134a refrigerant. Currently R134a is the industry standard…
R12 naturally is a better refrigerant, which basically means that R12 does a better job at making your AC icy cold! So, with that said, if your system is low you may be best off finding R12 and topping it off…
Note: R12 and R134a are NOT compatible with each other, due to the differences in the oils. In an AC system, the oil is constantly moving with the refrigerant. Using R134a in an R12 system will cause the oil not to move with the refrigerant, causing a system failure and probably won’t even cool the system well, if at all.
As an alternative, there is a product out there called freeze 12. Freeze 12 is 80% R134a and 20% R142b. R142b is used to carry the mineral oil that is used in R12 systems. Freeze 12 is a drop in replacement for R12 so if your system is low, it can often be topped off with R12. NOTE that by doing this, most places will not recover your refrigerant, since mixing refrigerants is technically illegal….If you do your own AC work, this may not pose as an issue for you. Freeze 12 can be obtained without EPA certification and very inexpensively…
R12 can be obtained fairly easy. You can find virgin R12 on eBay very easily, however you must be EPA certified. The EPA 609 certification can be had by taking an online quiz, at a fairly nominal cost. From there, you are legally able to buy and handle ozone depleting substances. This could be useful if you have multiple cars with R12… For more information, visit http://www.epatest.com/609/.
Note: There are other drop-in refrigerants for R12, but often they contain butane which is of course very flammable. Many people have successfully charged their AC system with propane or butane, as this works very well as a refrigerant, but obviously using a flammable substance in an automobile takes its own risks. Freeze 12 is not flammable and does not pose any more of a safety hazard than R12 or R134a does.
How an A/C system works
An air conditioner technically does not cool air. It removes moisture from the air, and by removing moisture from the air thus removes energy from the air, therefore making it cooler .This is why on a hot day you see water dripping out of your air conditioner.
Air conditioners work by compressing a gas into a liquid and then evaporating it back into a gas. Think about whenever you shake an aerosol can, it gets colder. Remember that thermodynamically speaking, you cannot remove energy (or heat), but rather you transfer it from one place to another.
There are 7 major parts to an A/C system.
1. Compressor– This is the central part of an A/C system. It keeps the refrigerant flowing, and also compresses the low pressure gas into a high pressure gas. This is located on the motor and is driven off the engines belts.
2. Condenser– Works very similar to an intercooler. The condenser takes this high pressure gas and by using the outside air, cools it down so that it condenses into a liquid.
3. Receiver drier– Works as a filter and also a moisture remover. The system is obviously not impervious 100% to leaks, and the refrigerant (mainly its oil) tends to absorb moisture. The drier removes this moisture right before it heads to the expansion valve.
4. Expansion valve– Regulates the amount of refrigerant that enters the evaporator. Too little refrigerant and you have insufficient cooling, too much refrigerant and the evaporator freezes, causing ice to form on the evaporator which leads to decreased air flow.
5. Evaporator– Takes this high pressure liquid and turns it into a high pressure gas, by evaporation. This is where all of the cooling occurs.
6. Blower Motor– Brings in air, either from the cabin or the outside and moves it across the evaporator into the cabin.
7. Pressure switches– When the pressures in the AC system become abnormally high or low, these switches cause the compressor to turn off and thus preventing damage to the rest of the system.
The following diagram illustrates the flow of refrigerant.
Repairing A/C: Tools, tricks and tips
One thing that is nice about A/C systems is that in essence, every A/C system is exactly the same. They all work on basically the same principle, and thus something that applies to one car most likely applies to another.
The main and most important tool to any A/C work is the manifold gauge, pictured below.
What this tool does is lets you read the pressure inside the AC system. It also incorporates valves so that the system can be charged or discharged right from the gauge, so you can monitor pressure while you work.
The next major tool is the vacuum pump, shown here.
What this tool is used for is removing moisture from the system whenever the system is opened for a major repair (a component replacement).
Now that you have you gauge hooked up, you need to know what these readings mean.
The high side measures the discharge side of the compressor, or what the compressor is actually compressing. The low side measures the suction of the compressor. Both the high and low side must be within a certain range to maximize cooling.
As a general rule, most R12 systems are going to be between 140-170 PSI high and 27-33 PSI low. Likewise, most R134a systems are going to be between 190-230psi high and 27-33 PSI low (this is at about 86F ambient temperature). This is with the engine RPM’s around 1500 RPM and a running time of more than 10 minutes (for the system to equalize). Please note that this is just a reading to have in the back of your mind, to get more accurate pressure readings for your specific car you can turn to the HA section in your Nissan or Infiniti service manual. It will give the pressure readings at various ambient temps and the setting that the AC should be on to do a test.
The most common situation is when the low side is too high and the high side is too low, which almost always means the compressor is worn. Another common situation is when low and high side is too low, meaning that you may just need to add a little refrigerant to the system. Nissan AC systems are fairly reliable and not much outside these things usually goes wrong. Note that if your compressor starts to make noise while operating it is much better to replace it soon. If you do not replace it and the compressor fails, it can throw contamination into your system, which means that one has to flush all the components to get the compressor pieces out of the system before it can be used again. Even if the compressor still works but makes a bunch of noise, you should get it replaced.
The following will help you diagnose your A/C system
HIGH and LOW sides are too HIGH
•Pressure is reduced by splashing water on condenser
-Excessive refrigerant in system.
•Air flow across condenser is not sufficient
oClean condenser fins
•Low pressure service port is much colder than evaporator or expansion valve outlet (often covered in frost
oExpansion valve is not installed properly or adjusted, replace expansion valve
HIGH is too LOW and LOW is too HIGH
HIGH and LOW sides are too LOW
•Large temperature difference between receiver drier inlet and outlet
oReplace receiver drier (receiver drier is clogged)
•Large temperature difference between expansion valve inlet and outlet
oReplace expansion valve
•Air flow is diminished (evaporator is frozen)
As a general rule, the condenser and evaporator should not need replacement unless they are leaking. Leak detection kits can be bought in many stores. This involves injecting a dye into the system that includes a UV light and a special pair of glasses. You shine the UV light at various components and that can help determine where the leak is coming from. Usually large leaks should be visibly obvious, but slow leaks that cause the system to lose some charge over a longer period of time may take longer to find.
Proper A/C service tips
Whenever the A/C system is opened for repair (IE a component is removed for replacement), it should be done in as quickly of a fashion as possible. While the system is open it will absorb moisture and minimizing this is always a good idea.
After the system has been fixed, you must put the system under vacuum for a minimum of 30 minutes. The longer the better, but 30 minutes is the generally regarded minimum. Also, anytime the system has been opened, the receiver drier must be replaced as opening the system saturates it with moisture. After the system is ready to be taken off the vacuum pump, you must let the system sit for 30 minutes to make sure that it does hold the vacuum for this. This ensures there are no leaks.
Anytime a component is replaced, its corresponding O-Rings must be replaced. At most auto parts stores, you can get O ring kits fairly cheap. You would hate to find a leak in the system at an O-Ring you didn’t replace when the system was open.
In your service manual, there is a chart that tells how much oil each component holds. So for instance, when you replace an evaporator you may only need to replace 3 fluid ounces. Too much oil and you will have insufficient cooling, too little oil and you will destroy the compressor.