In this technical article I will be focusing on the steps involved in diagnosing, repairing and recharging your air conditioning system on the 1984-92 BMW E30 3 Series. Keep in mind that this technical article is written with my own BMW 325is in mind, however the procedures and information herein can be applied to nearly every modern car with air conditioning out there on the road today.
First off, lets begin by first explaining in layman's terms how air conditioning works.
The main function of the compressor is to compress and pressurize the refrigerant in the system, and also to keep the refrigerant moving through the system when the A/C is turned on. The compressor is belt driven off the engine. The compressor takes in cold, low-pressure gaseous refrigerant and compresses it. As it compresses, it builds up heat. This now-pressurized hot gas is sent to the condenser.
The condenser then receives the hot gaseous refrigerant from the compressor. Usually, condensers are placed in the front of a car, where it receives oncoming airflow from the radiator fan. The condenser then turns the hot gas into a liquid. This liquid (still under high pressure) is then sent to the receiver drier.
The receiver drier next receives the hot pressurized liquid refrigerant from the condenser. It is essentially nothing more than a desiccant tank. It removes moisture from inside the hot liquid refrigerant. Moisture in the lines is a potential problem, as refrigerant can eventually react with moisture and corrode the insides of the system. The process is completed when the hot liquid refrigerant then exits the receiver drier and passes through the expansion valve.
The expansion valve receives the hot liquid refrigerant. It allows the pressure of the liquid refrigerant to drop. The drop in pressure causes the liquid refrigerant to cool down. The result is cold liquid refrigerant, which is then sent to the evaporator.
The evaporator receives the cold liquid from the expansion valve. The evaporator is usually located inside the car under the dashboard.When you turn on the A/C fan, it channels air over the evaporator, cooling the air; this air is then blown through the vents of the car. As the cold liquid passes through, it evaporates from a liquid back into a gas, and is then routed back to the compressor to start the entire process again.
Additionally, there are two more components to the system that control the operation when you turn the compressor on and off.
The A/C Clutch is used to engage or disengage the compressor when you press the A/C button on the dashboard. When the A/C is turned off, the belt that drives the compressor spins freely on the A/C Clutch, which is mounted on the front of the compressor. Now when you push the A/C button, it sends current to the clutch, which locks it and allows the compressor to begin turning. High Pressure/Low Pressure Switch
These two switches are a built-in safety feature. When the system pressure is too low it interrupts the voltage going to the clutch on the compressor, which then disengages the drive belt from the compressor, and turns off the A/C. It works the same way when it senses there is too much pressure in the system.
So now that I have explained a little about how the system works, we can now begin to go over the steps involved with recharging your A/C system. The first thing to know is that there are two types of refrigerant used in most modern A/C systems, R-12 and R-134a.
Up until about 1991, all major automobile manufacturers used R-12 as a refrigerant in A/C systems. It was discovered that R-12 contains CFC, or chlorofluorocarbons, which destroy the ozone layer. Hence it was necessary that they come up with a replacement. This came in the form of R-134a. Nowadays every car uses R-134a. There has been much debate over the years as to how well R-134a works as an automobile refrigerant. When R-134a was first used in cars, it was not uncommon to notice that the new refrigerant simply did not cool as well as the older R-12. It was also pretty common for the A/C system to emit unpleasant odors after running for long periods of time. These days engineers have sorted out nearly all of the problems first encountered.
These two types of refrigerants cannot be used together. For instance, you cannot put R-134a directly into a system that originally used R-12. Thus we run into one of the major problems with recharging A/C systems. R-12 is no longer available to the general public. I have heard of certain vendors that do still make R-12, however it is unlikely that one could purchase it. These days, A technician has to have a special license to purchase and charge R-12 systems. What this means is that you can still have your old R-12 system charged, however the increasing costs of stockpiled R-12 is making this less and less of an option. If you choose to have you're A/C recharged at a shop that still services R-12 systems, expect to pay $50 a pound or more. As most cars use two pounds or more of refrigerant, you can quickly see why R-134a becomes a more attractive option. I also have heard of products on the market that are a direct replacement for R-12, such as Freeze 12 or Duracool, however I have no direct experience with using them. I have heard from various sources that Freeze 12 seems to work very well as a direct replacement.
You may be asking now what is involved in converting to 134a. Well, there are many "kits" on the market you can buy at your local auto parts store that say all you have to do is replace the fittings on the fill ports, and recharge using R-134a. I wish it were this easy. The simple fact is that a lot of preparation work needs to be done in order for the A/C to work correctly with the new refrigerant. To correctly convert to 134a, you need to replace all the seals in the A/C system. The original seals in the system were designed to work with R-12. The chemical composition of the R-134a will eventually degrade the seals, causing the refrigerant to leak out. You will also need to purchase a receiver drier specifically for R-134a. The last step is to also convert to a R-134a compatible compressor. I have heard that it is possible to use an R-12 compressor to do this, however it will require a total flush out of the compressor, as well as replacing all internal seals with R-134a compatible seals. When you get into this realm, it is easier to simply replace the compressor. I have also heard that depending on the climate, you may also need to replace the condenser to a unit with a larger surface area. If you were to take this to the shop to have it done, the cost and labor involved in doing this can easily exceed $1000 or more.
So lets begin going over your system. The first step is to verify that the compressor kicks on when you hit the switch. Start the engine, then turn the A/C fan on to maximum and press the A/C button on. Now open the hood and look down at the compressor on the passenger side down near the radiator. Look at the front of the compressor.. Is the clutch engaged? What you are looking for is the plate on the very front of the A/C compressor. When the clutch is engaged the plate spins at the same rate as the belt. When it is disengaged, the plate remains stationary. Have a friend toggle the button in the dash on and off to check for operation. If the clutch does not engage when you press the button, you will need to determine why. Unplug the electrical connector going to the compressor and check for 12volts. If no voltage is present out of the connector, the next step is to check the low-pressure switch on the top of the receiver drier.
The receiver drier is located on the passenger side of the car in front of the wheel well and directly behind the headlights. You will see two switches on the valve for the receiver drier as well as the sight glass. What you need to do is unplug the low-pressure switch electrical connection, and check the leads for voltage with the A/C button turned on. If you read 12 volts from the leads, then the low pressure switch has tripped and interrupted the circuit proving voltage to the compressor. If you jumper the low-pressure leads together, the A/C compressor clutch should engage. (CAUTION: when attempting this, be sure to only run the compressor for a few seconds to prevent possible damage to the unit.)
Now look at the top of the sight glass on the receiver drier. A steady stream of bubbles usually indicates that the system charge is low and needs to be topped up. Streaks on the inside of the glass usually indicate that there is no refrigerant left in the system. Once you have finished checking the sight glass, shut off the engine.
The next step is to determine if the system will hold a charge. On all R-12 systems it is normal for the system to leak refrigerant through the lines. This is to bleed off any excess pressure that builds up in the lines. If the system has not been used in quite some time, it is possible for all the refrigerant to bleed out of the lines. This is the reason that R-12 is no longer used; it leaks out of the lines, and the resulting CFC's can damage the ozone layer. The newer R-134a lines are called barrier lines, and do not leak.
To determine if the system will hold a charge, you first need to have the system evacuated. This is where a vacuum pump is hooked up to the system and using vacuum pressure, all the air and refrigerant left in the system is drawn out. Typically when a system is evacuated, it is left on the vacuum pump for a period of about 30-45 minutes. This will insure that you have pulled all moisture and air out of the system. Most people do not have access to a vacuum pump, however any decent garage will be able to do this for you at a minor fee. Evacuating the system will also determine if there are any leaks in the lines. If your system does not hold pressure, you will need to trace down the leak, and fix it accordingly. In cases where you remove A/C lines, it is important that you always replace the receiver drier. When you open up the system to outside air, you are essentially saturating the receiver drier with moisture and can render it useless. Evacuating the system will also tell you the condition of other system components as well, such as the expansion valve and condenser. If either of these parts are fault, you must replace them and have the system evacuated once again.
Once the system has been tested for leaks and has been evacuated, it's a good idea to fill the system with about 4 ounces of oil charge. You can get this any local Pep Boys, Kragen, etc. Be sure to check the label on the can if it is for R-12 or R-134a systems. Luckily, you can still purchase R-12 oil charge over the counter.
To add the oil charge, you will need to locate the low-pressure return line. This is the line that returns the cold gas back to the compressor. In the case of the 325is, it is the line that runs directly in front of the passenger side wheel well. Locate the schrader valve on the top of the line. Remove the black protective cap on the top, and you will see the valve underneath.
You will need to have a fill hose such as the one shown here. These too are available in any Pep Boys. This special hose allows you to transfer the contents of the can into the line. With the car's engine off, Connect the hose to the low "pressure valve on the car. You will want to do this first with the engine OFF. Next, on the other side of the hose, turn the screw on top counter clockwise until it stops. Load the can of oil charge into the clamp on the hose and lock it in place. When the can is secure in the clamp, turn the screw on top clockwise all the way until it stops. This will pierce the top of the can and allow the oil charge to flow in. Turn the car back on, turn on the A/C and put the fan to maximum.
It's a good idea to put the can in a pot of warm water while you charge the system. As you open the can, the pressure drops, causing the can to get cold, and it slows down how fast the charge can get into the car. Put the can in the warm water and rotate the screw on top counter clockwise all the way. This will open the valve and allow the oil charge to flow in. I must also stress at this time that the can of charge must be upright at all times during charging. Failure to do so could result in not all of the oil or refrigerant entering the system.
Keep the valve open on the fill hose until the cans of oil charge are empty and then turn off the engine. We are now ready to add the refrigerant to the system. I was fortunate to have a previous supply of R-12 lying around from back when it was still commercially available.
The BMW e30 specifies 2 1/2 lbs. of charge to completely fill the system. I grabbed three one-pound cans and repeated the process I used for the oil charge. I'm specifying using can type refrigerant here as it is the method most commonly available to the do it yourselfer. There is another process for charging the system using a tank, however this type of charging requires use of special gauges and hoses, so it's best left to the professionals. Cans of R-134a are available at your local auto parts store for about $13 a pound. The procedure for adding these cans is the same, although they use a different kind of fill hose and ports. Cans are available in different ounce measurements, so calculate how much you need and then buy accordingly.
As you charge the system, look down at the sight glass on the receiver drier. Keep an eye out for when the bubbles in the system stop flowing and you see a steady stream of liquid. When you see this, close the valve on the fill hose as the system is now fully charged.
Now that the system is fully charged, you should be able to feel you're A/C blowing cold once again.
Well, there you have it - it's really not too difficult at all. If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs. If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one. Your continued support directly affects the expansion and existence of this site and technical articles like this one. As always, if you have any questions or comments about this helpful article, please drop us a line.