BMW Coupes and Sedans 1970-1988 Repair Guide

Air Conditioner System



R-12 refrigerant is a chlorofluorocarbon which, when released into the atmosphere, contributes to the depletion of the ozone layer in the upper atmosphere. Ozone filters out harmful radiation from the sun. Consult the laws in your area before attempting to service the air conditioning system. In some states it is illegal to perform repairs involving refrigerant unless the work is done by a certified technician.

R-12 and other chlorofluorocarbon refrigerants, when released into the atmosphere, may contribute to the depletion of the ozone layer of the upper atmosphere. Ozone filters out damaging ultraviolet radiation from the sun.

Consult the laws in your area before attempting service on the air conditioning system in your vehicle. In some states it is illegal to perform repairs involving refrigerant unless the work is done by a certified technician.

The air conditioner is filled with refrigerant R-12, which produces very high pressure even when the system is not operating. Not only can a component broken by mishandling, crack or break explosively, but the escaping refrigerant will immediately drop to -27°F (-33°C), causing severe frostbite to any part of the body exposed nearby. The problem is worsened by the fact that refrigerant systems employ thin sections of light alloys to transfer heat efficiently, thus components are readily damaged by inexperienced mechanics.

As a result, we recommend that you make no attempt whatever to repair any component on the air conditioning system. If a refrigerant component must be moved to gain access to another part, we recommend you leave at least the part of the job involving the refrigerant component to someone with the specialized training, tools, and experience to handle the job safely. However, if you are going to work on the system yourself, you must be very careful and use common sense before making any repairs.


See Figures 1 and 2

Once a year, before hot weather sets in, it is advisable to check the refrigerant charge in the air conditioner system. This may be accomplished by looking at the sight glass located in the engine compartment. First, wipe the sight glass clean with a cloth wrapped around the eraser end of a pencil. Have a friend operate the air conditioner controls while you look at the sight glass. Have your friend set the dash panel control to MAX cooling. Start the engine and idle at 1500 rpm. While looking at the sight glass, signal your friend to turn the blower switch to the HIGH position. If a few bubbles appear immediately after the blower is turned on and then disappear, the system is sufficiently charged with refrigerant. If, on the other hand, there are a large amount of bubbles after the blower has operated for a few seconds, then the system is in need of additional refrigerant. If no bubbles appear at all, then there is either sufficient refrigerant in the system, or it is bone dry. The way to clear this question up is to have your friend turn the unit OFF and ON (engine running at 1500 rpm) about every 10 seconds or so while you look at the sight glass. This will cycle the magnetic clutch. If the system is properly charged, bubbles will appear in the sight glass a few seconds after the unit is turned off and disappear when it is turned on although they may linger awhile in extremely hot weather. If no bubbles appear when the unit is in the OFF position, then the system should be serviced by an authorized dealer and checked for leaks. Do not operate the unit if you suspect that the refrigerant has leaked out.

Check the radiator and condenser for clogging by bugs and road debris. If the fins of the radiator and condenser are blocked, heat cannot be transferred from the fins to the airstream. Clean the debris from the fins to restore the airflow.

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Fig. Fig. 1: Air conditioning sight glass-5 Series models

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Fig. Fig. 2: Air conditioning sight glass-3 Series models



In order to prevent heater core freeze-up during A/C operation, it is important to maintain a permanent type antifreeze protection of +15°F (9°C) or lower. A reading of 15°F (9°C) is ideal because this protection rating also supplies sufficient corrosion inhibitors for the engine cooling system.

Do not use antifreeze longer than specified by the manufacturer.

Radiator Cap

For efficient operation of an air conditioned car's cooling system, the radiator cap should have a holding pressure which meets the manufacturer's specifications. A cap which fails to hold this pressure level should be replaced.


Any obstruction of or damage to the condenser configuration will restrict the air flow which is essential to its efficient operation. It is therefore, a good rule to keep this unit clean and in proper physical shape.

Bug screens are regarded as obstructions. Therefore their use is not recommended.

Condensation Drain Tube

This single molded drain tube expels the condensation, which accumulates on the bottom of the evaporator housing, into the engine compartment.

If this tube is obstructed, the air conditioning performance can be restricted and condensation buildup can spill over onto the vehicle's floor.


See Figure 3

Most of the service work performed in air conditioning requires the use of a set of two gauges, one for the high pressure side of the system and the other for the low pressure side of the system.

The low side gauge records both pressure and vacuum. Vacuum readings are calibrated from 0 to 30 inches Hg (0-99 kPa) and the pressure graduations read from 0 to no less than 60 psi (0-414 kPa). The high side gauge measures pressure from 0 to at least 600 psi (4140 kPa).

Both gauges are threaded into a manifold that contains two hand shut-off valves. The manifold valves are designed so they have no direct effect on gauge readings, but serve only to meter the flow of refrigerant through the manifold. During all testing and hook-up operations, the valves are kept in the closed position to avoid disturbing the refrigeration system.

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Fig. Fig. 3: Air conditioning manifold gauge set

Connect the manifold gauge set as follows:

  1. Turn both manifold valves fully to the right, to close the high and low pressure hoses to the center manifold and hose.
  3. Remove the caps from the high and low pressure service gauge port valves. The high pressure service gauge port valve is located between the compressor and the condenser on the high pressure discharge line. The low pressure service gauge port valve is located between the suction accumulator/drier and the compressor on the low pressure suction line, or is located directly on the suction accumulator/drier.
  5. Connect the high and low pressure hoses to the respective high and low pressure service gauge port valves.


See Figure 4

The most important aspect of air conditioning service is the maintenance of pure and adequate charge of refrigerant in the system. A refrigeration system cannot function properly if a significant percentage of the charge is lost. Leaks are common because the severe vibration encountered in an automobile can easily cause a sufficient cracking or loosening of the air conditioning fittings. As a result, the extreme operating pressures of the system force refrigerant out.

The problem can be understood by considering what happens to the system as it is operated with a continuous leak. Because the expansion valve regulates the flow of refrigerant to the evaporator, the level of refrigerant there is fairly constant. The receiver/drier stores any excess of refrigerant, and so a loss will first appear there as a reduction in the level of liquid. As this level nears the bottom of the vessel, some refrigerant vapor bubbles will begin to appear in the stream of liquid supplied to the expansion valve. This vapor decreases the capacity of the expansion valve very little as the valve opens to compensate for its presence. As the quantity of liquid in the condenser decreases, the operating pressure will drop there and throughout the high side of the system. As the R-12 continues to be expelled, the pressure available to force the liquid through the expansion valve will continue to decrease, and, eventually, the valve's orifice will prove to be too much of a restriction for adequate flow even with the needle fully withdrawn.

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Fig. Fig. 4: Air conditioning charge label

At this point, low side pressure will start to drop, and severe reduction in cooling capacity, marked by freeze-up of the evaporator coil, will result. Eventually, the operating pressure of the evaporator will be lower than the pressure of the atmosphere surrounding it, and air will be drawn into the system wherever there are leaks in the low side.

Because all atmospheric air contains at least some moisture, water will enter the system and mix with the R-12 and the oil. Trace amounts of moisture will cause sludging of the oil, and corrosion of the system. Saturation and clogging of the filter/drier, and freezing of the expansion valve orifice will eventually result. As air fills the system to a greater and greater extend, it will interfere more and more with the normal flows of refrigerant and heat.

A list of general precautions that should be observed while doing this follows:

  1. Keep all tools as clean and dry as possible.
  3. Thoroughly purge the service gauges and hoses of air and moisture before connecting them to the system. Keep them capped when not in use.
  5. Thoroughly clean any refrigerant fitting before disconnecting it, in order to minimize the entrance of dirt into the system.
  7. Plan any operation that requires opening the system beforehand in order to minimize the length of time it will be exposed to open air. Cap or seal the open ends to minimize the entrance of foreign material.
  9. When adding oil, pour it through an extremely clean and dry tube or funnel. Keep the oil capped whenever possible. Do not use oil that has not been kept tightly sealed.
  11. Use only refrigerant 12. Purchase refrigerant intended for use in only automotive air conditioning system. Avoid the use of refrigerant 12 that may be packaged for another use, such as cleaning, or powering a horn, as it is impure.
  13. Completely evacuate any system that has been opened to replace a component, other than when isolating the compressor, or that has leaked sufficiently to draw in moisture and air. This requires evacuating air and moisture with a good vacuum pump for at least one hour.

If a system has been open for a considerable length of time it may be advisable to evacuate the system for up to 12 hours (overnight).

  1. Use a wrench on both halves of a fitting that is to be disconnected, so as to avoid placing torque on any of the refrigerant lines.


BMW recommends the use of their special charging and recovery system for R-12 systems. The unit is manufactured by BEHR and is designed for use by a trained technician. If your vehicle requires the air conditioning system to be discharged and later charged, let a trained and certified technician perform the work.