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On any car, the A/C system is a complicated beast. This project is not intended to be a repair manual for your A/C system, but should serve more as a guide to how the system works and offer key points about the maintenance involved with its upkeep.
Air conditioning systems work using the theory of thermodynamics, whereby heat flows from a warmer surface to a colder one. Heat from inside the car is transferred to the cold metal fins of the evaporator. The refrigerant in the system picks up the heat from the evaporator and takes it to the compressor. The gas is then pressurized, which concentrates the heat by raising the temperature of the refrigerant gas. The gas is then sent to the condenser. The condenser cools the refrigerant and turns the gas back into a liquid. The liquid is then sent to the receiver-dryer, where any water vapor that may have formed in the system is removed. The receiver-dryer also acts as a storage container for unused fluid.From the receiver-dryer, the liquid flows into the expansion valve, which meters it into the evaporator located inside the car. Here the liquid absorbs heat and becomes a low-pressure gas. This evaporation, or boiling of the refrigerant, absorbs heat as a boiling pot of water absorbs heat from the stove. As heat is absorbed, the evaporator is cooled. A fan blows air through the evaporator and into the cockpit of the car, providing the cooling effect. The compressor then pumps the refrigerant through the entire system. An electromagnetic clutch on the compressor turns the A/C system on and off. In addition to cooling the car, the system also removes water vapor from the ambient air. It is not uncommon to find a small puddle of water underneath your car from the condensation of the air conditioning system. A thermostat control on the evaporator keeps the condensation in the evaporator from freezing and damaging the unit.
So what can you do to maintain and protect the system from deterioration? First and foremost, operate the air conditioning system at least once a week if the outside temperature is above 50 degrees Fahrenheit. Doing this will circulate the refrigerant in the system and help keep the seals from drying out. Most failures are caused by refrigerant leaking out of the system, which you can prevent simply by making sure that you run the system more frequently.
A belt that runs off the main crankshaft operates the A/C compressor. On cars with a manual adjustment, make sure you don’t tighten this belt too tightly, or you may place undue pressure on the bearings inside the compressor. If you suspect you might be having problems with the compressor, check the belt first. Turn on the system, and verify that the electromagnetic clutch is engaging. If it is not, you may need to replace it. Check the power connection to make sure it is live before replacing the clutch.
The original Freon used in the older-style R12 air-conditioned cars. In the early 1990s, auto manufacturers started phasing out Freon-based A/C systems and started implementing the newer R134a systems (BMW appears to have started installing R134a systems with the 1993 models). The cost of the replacement R12 Freon is skyrocketing as the current supplies disappear. This Freon, which was once sold to the public in do-it-yourself kits, can only be legally purchased by dealers now, who are trained in recharging these systems.
If your A/C system needs a major overhaul, it’s wise to upgrade your system to R134a, although the R134a refrigerant is not as efficient, is slightly more prone to leaks, and cools slightly less than the original R12. You can purchase R134a inexpensively at your local auto parts stores, and retrofit kits are easy to install (as I will explain a little later). You can determine which type of refrigerant you have in your car from the shape of the connectors and/or the sticker on the front right side of the engine compartment.
Refilling A/C systems
The biggest problem with A/C systems is a loss of refrigerant. Luckily, replacing and topping off refrigerant is a relatively easy process, particularly if you have an R134a system already installed. Interdynamics manufactures the kit I used to refill the car in this project (see Photo 1).Outside your garage, start the car, turn on the A/C system and fan to full blast, and let the car run for about three minutes. Following the instructions included with the kit, connect a new can of refrigerant to the hose/gauge assembly. Be sure you shake the can for about 30 seconds and turn it upside down when you connect it to the gauge assembly. Connect the gauge assembly to the low-side port of your A/C system (see Photo 2). Be sure to wear eye protection and heavy leather gloves when handling the coolant and gauge assembly! If coolant leaks out at any time, it can literally freeze a small patch of skin quite easily and give you frostbite.
With the car running and the A/C system turned on full blast, take a reading on the pressure gauge. A properly charged system should read between 25 and 45 psi. If the pressure is low, turn the valve on the can to release more refrigerant into the system. Be aware that the pressure gauge reading will automatically elevate as you are adding more coolant, so periodically close the valve on the can to check if the pressure is rising in the system. If the pressure doesn’t increase after adding one complete can, you most likely have a major leak in your system and should seek the help of a professional A/C system mechanic.
With the system properly filled and measured with your gauge, head to the passenger compartment and check the temperature of the air exiting the vents. On a system that is operating really well, the temperature will be in the mid-30s Fahrenheit. For older systems or ones retrofitted to work with R134a, the temperature readings will most likely be higher. Also keep in mind that if your system is cooling air in the 30 degrees Fahrenheit range, the compressor will tend to turn itself on and off, causing the temperature to go up and down slightly. This is not a defect of the system; the compressor turns itself off as the evaporator nears the freezing temperature of water. This prevents the evaporator from becoming frozen and clogged with icy buildup.
Retrofitting R12 systems
What are your options if your system uses R12 and is currently not working properly? Many drop-in replacements for R12 are out there, but it’s unwise simply to place them into your system without performing a valid R134a retrofit. Why? When mechanics at an A/C service station work on your car, they will need to vacuum out and reclaim the refrigerant in your system. If the system contains R12 or R134a, they can combine it with their existing stock. However, if your system contains some aftermarket additive, most A/C service stations will refuse to work on your car (their sensors can determine whether the system is running R12, R134a, or something else). Needless to say, placing these additives into your system limits your options. The best thing to do is either stick with R12 (expensive) or perform a qualified upgrade to R134a.
What are the downsides to upgrading to R134a? The refrigerant doesn’t cool as efficiently as R12, meaning your system will perform marginally less than with the R12 Freon. In most cases, however, you won’t be able to tell the difference between the two. In addition, the R134a molecule is a little bit smaller than the R12 molecule, meaning an R134a system is more prone to leaks. However, if your system’s seals and O-rings are in good condition, this should not be a concern. Some of the Bosch compressors used on the older cars are not compatible with R134a, so you may need to replace your compressor to convert to R134a. All of the E36 cars should have R134a-compatible compressors. An E36 conversion kit is available for the early cars that ran R12 instead of R134a (P/N 82-31-9-067-403, about $200). This kit contains a new receiver drier, new O-rings, a capacity label, and a set of R134a valve adapters. You will also need PAG compressor oil (P/N 82-11-1-468-042).
The E30 cars are a little more difficult, as almost all of them were fitted with R12 systems. A detailed BMW Tech Bulletin covers the R12 to R134a conversion (BMW Document # 64 05 96). The E30 cars with compressors that are not compatible with R134a include the 318i (July 1985–August 1985); the 325e, 325i, and 325iC (July 1987–October 1988); the 325iX; and the M3. You should check the part number on your compressor prior to the upgrade to make sure that it is compatible with R134a.The following table shows a list of compressors that are not compatible with R134a. You should replace these compressors with part number 64-52-8-363-550.
64 52 1 377 944
64 52 1 377 947
64 52 1 385 416
64 52 1 385 930
64 52 1 386 411
64 52 1 377 940*
64 52 1 377 941*
64 52 1 377 943*
64 52 1 377 946*
*Requires clutch wiring adapter P/N 64-52-1-386-224
If your E30 has an R134a-compatible compressor, or if you purchase the upgraded one, all you need is the E30 R134a retrofit kit. Like the E36 kit, it contains a new receiver drier, new O-rings, a capacity label, and a set of R134a valve adapters (part number 82-31-9-067-394). If your compressor is low on oil, you will also need PAG compressor oil (part number 82-11-1-468-042).
The process of installing retrofit kits typically requires specialized equipment available only at an A/C service shop, but I’ll give an overview of the process. First, have a shop mechanic remove and recycle any old R12 you have left in your system (don’t vent it to the atmosphere). Then, swap out the compressor if you are replacing it. Be sure to use new O-rings on all the connections that are opened in the system when you are working on it. Install the new pressure switch (included in the kit) on the receiver/drier. Install the new receiver drier in the car, replacing all O-rings in the process (they are included in the retrofit kit as well). Splice the new switches into the chassis wire harness according to the instructions in the factory retrofit bulletin. Install the R134a adapters on both the high and low side of the system. Then, pull a vacuum on the system for a minimum of 40 minutes and fill with R134a according to the instructions detailed above. Remember to add compressor lubricant if you haven’t replaced your compressor. Finally, check the system for leaks using an R134a leak detector, and check the temperature of the air in the passenger cabin.
As you can tell, you need a serious selection of specialized equipment to perform a proper retrofit of your A/C system. The bottom line is giving your A/C system a major overhaul can be a difficult and time-consuming process. Clearly, the magnitude of repairing and replacing most A/C components is beyond the scope of the average weekend mechanic. Seek professional assistance if your system needs any major work beyond a simple refill.
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