GENERAL SERVICING PROCEDURES
To properly discharge and charge the A/C system, a special charging system with quick connectors, as well as complete training in refrigerant recycling and service procedures, and a certification license are absolutely necessary. UNDER PENALTY OF LAW, DO NOT VENT ANY REFRIGERANT (EVEN SMALL AMOUNTS) INTO THE ATMOSPHERE!
The most important aspect of air conditioning service is the maintenance of a 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 refrigerant, 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 or R-134a 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the valve's orifice will prove to be too much of a restriction for adequate flow, even with the needle fully withdrawn.
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 or R-134a 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, as well as freezing of the expansion valve orifice, will eventually result. As air fills the system to a greater and greater extent, it will interfere more and more with the normal flows of refrigerant and heat.
A list of general precautions that should be observed while servicing the system include:
If a system has been open for a considerable length of time, it may be advisable to evacuate the system for up to 3 hours.
ADDITIONAL PREVENTIVE MAINTENANCE CHECKS
In order to prevent heater core freeze-up during A/C operation, it is necessary to maintain permanent-type antifreeze protection of +15°deg;F (-9°deg;C) or lower. A reading of -15°deg;F (-26°deg;C) is ideal since this protection also supplies sufficient corrosion inhibitors for the protection of the engine cooling system.
The same antifreeze should not be used longer than the manufacturer specifies.Radiator Cap
For efficient operation of an air conditioned truck's cooling system, the radiator cap should have a holding pressure which meets manufacturer's specifications. A cap which fails to hold these pressure should be replaced.Condenser
Any obstruction of, or damage to, the condenser configuration will restrict 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 and should, therefore, be avoided.Condensation Drain Tube
This is normally a single, molded drain tube that 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.
Because of the importance of the safety precautions which must be exercised when working with air conditioning systems and R-12 or R-134a refrigerant, a recap of the safety precautions follows:
Check with your local authorities before attempting to service you vehicle's A/C system. In many areas, it may be illegal to purchase R-12 or R-134a and service the system, unless you are a certified technician.
See Figure 1
Most air conditioning service work requires the use of a set of two gauges, one for the high (head) pressure side of the system, the other for the low (suction) side.
The low side gauge records both pressure and vacuum. Vacuum readings are calibrated from 0-30 in. Hg (0-101 kPa), and the pressure graduations read from 0 to no less than 60 psi (414 kPa).
The high side gauge measures pressure from 0 to at least 600 psi (4137 kPa).
Both gauges are threaded into a manifold that contains two hand shut-off valves. Proper manipulation of these valves and the use of the attached test hoses allow the user to test high and low side pressures.
The manifold valves are designed so that they have no direct effect on gauge readings, but serve only to provide for (or cut off) the flow of refrigerant through the manifold. During all testing and hook-up operations, the valves are kept in a closed position to avoid disturbing the refrigeration system.
Sight Glass Check
See Figure 2
You can safely make a few simple checks to determine if your air conditioning system needs service. The tests work best when the temperature is warm (about 70°deg;F/21°deg;C).
If your vehicle is equipped with an aftermarket air conditioner, the following system check may not apply. You should contact the manufacturer of the unit for instructions on system checks.
- Place the automatic transmission in Park or the manual transmission in Neutral. Set the parking brake.
- Run the engine at a fast idle (about 1500 rpm) for a few seconds, either with the help of an assistant or by temporarily readjusting the idle speed screw.
- Set the controls for maximum cold with the blower on High.
- Locate the sight glass, if so equipped, in one of the system lines. Usually, it is on the left alongside the top of the radiator.
- If you see bubbles, the system must be recharged. Very likely, there is a leak at some point.
- If there are no bubbles, there is either no refrigerant at all or the system is fully charged. Feel the two hoses going to the belt-driven compressor. If they are both at the same temperature, the system is empty and must be recharged.
- If one hose (high pressure) is warm and the other (low pressure) is cold, the system may be all right. However, you are probably making these tests because you think there is something wrong, so proceed to the next step.
- Have an assistant in the truck turn the fan control or A/C switch on and off to operate the compressor clutch. Watch the sight glass.
- If bubbles appear when the clutch is disengaged and disappear when it is engaged, the system is properly charged.
- If the refrigerant takes more than 45 seconds to bubble when the clutch is disengaged, the system is overcharged. This usually causes poor cooling at low speeds.
Operate the air conditioner for a few minutes, every month or so, during the cold months. This avoids the possibility of the compressor seals drying out from lack of lubrication.
TESTING THE SYSTEM
- Park the vehicle in the shade.
- Connect a gauge set.
- Close (turn clockwise) both gauge set valves.
- Mid-position both service valves.
- Start the engine, set the parking brake, place the transmission in NEUTRAL and establish an idle of 1500 rpm.
- Run the air conditioning system.
- Insert a thermometer into the center air outlet. Evaluate the temperature and pressure readings of the A/C system.
DISCHARGING AND CHARGING THE SYSTEM
To properly discharge and charge the A/C system, a special charging system and quick connectors, as well as complete training in refrigerant recycling/special service procedures, and a certification license are absolutely necessary. DO NOT VENT ANY REFRIGERANT (EVEN SMALL AMOUNTS) INTO THE ATMOSPHERE. THIS PRACTICE IS PROHIBITED UNDER PENALTY OF LAW!
If you do not have the certification and equipment necessary to service the A/C system, take your truck to a local repair facility and have the system recovered. Once service is finished, you can return the truck to the facility for proper evacuation and charging.
EVACUATING THE SYSTEM
If you do not have the proper equipment and training, take the truck to a certified technician.
Some leak tests can be performed with a soapy water solution. There must be at least a 1 / 2 lb. charge in the system for a leak to be detected. The most extensive leak tests are performed with either a Halide flame-type leak tester or the more preferable electronic leak tester.
In either case, the equipment is expensive, and the use of a Halide detector can be extremely hazardous!