GENERAL SERVICING PROCEDURES
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flow even with the needle fully withdrawn.
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:
- Keep all tools as clean and dry as possible.
- Thoroughly purge the service gauges and hoses of air and moisture before connecting them to the system. Keep them capped when not in use.
- Thoroughly clean any refrigerant fitting before disconnecting it, in order to minimize the entrance of dirt into the system.
- 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.
- 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.
- 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.
- 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).
- 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.
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°F (-9°C) or lower. A reading of -15°F (-26°C) is ideal since this protection also supplies sufficient corrosion inhibitors for the protection of the engine cooling system.
For efficient operation of an air conditioned car'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 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.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.
There are two particular hazards associated with air conditioning systems and they both relate to the refrigerant gas.
First, the refrigerant gas is an extremely cold substance. When exposed to air, it will instantly freeze any surface it comes in contact with, including your eyes.
The second hazard relates to fire. Although normally non-toxic, refrigerant gas becomes highly poisonous in the presence of an open flame. One good whiff of the vapor formed by burning refrigerant can be fatal. Keep all forms of fire (including cigarettes) well clear of the air conditioning system.
Any repair work to an air conditioning system should be approached with caution. If there is any doubt concerning correct servicing, have it done professional. Do not, under any circumstances, attempt to loosen or tighten any fittings or perform any work other than that outlined here.
Because of the importance of the necessary safety precautions that must be exercised when working with air conditioning systems and R-12 refrigerant, a recap of the safety precautions are outlined.
- Avoid contact with a charged refrigeration system, even when working on another part of the air conditioning system or vehicle. If a heavy tool comes into contact with a section of copper tubing or a heat exchanger, it can easily cause the relatively soft material to rupture.
- When it is necessary to apply force to a fitting which contains refrigerant, as when checking that all system couplings are securely tightened, use a wrench on both parts of the fitting involved, if possible. This will avoid putting torque on the refrigerant tubing. (It is advisable, when possible, to use tube or line wrenches when tightening these flare nut fittings.)
- Do not attempt to discharge the system by merely loosening a fitting, or removing the service valve caps and cracking these valves. Precise control is possibly only when using the service gauges. Place a rag under the open end of the center charging hose while discharging the system to catch any drops of liquid that might escape. Wear protective gloves when connecting or disconnecting service gauge hoses.
- Discharge the system only into a container made for the recovery of used refrigerant. When leak testing or soldering this is particularly important, as toxic gas is formed when R-12 contacts any flame.
- Never start a system without first verifying that both service valves are backseated, if equipped, and that all fittings are throughout the system are snugly connected.
- Avoid applying heat to any refrigerant line or storage vessel. Charging may be aided by using water heated to less than 125°F (52°C) to warm the refrigerant container. Never allow a refrigerant storage container to sit out in the sun, or near any other source of heat, such as a radiator.
- Always wear goggles when working on a system to protect the eyes. If refrigerant contacts the eye, it is advisable in all cases to see a physician as soon as possible.
- Frostbite from liquid refrigerant should be treated by first gradually warming the area with cool water, and then gently applying petroleum jelly. A physician should be consulted.
- Always keep refrigerant can fittings capped when not in use. Avoid sudden shock to the can which might occur from dropping it, or from banging a heavy tool against it. Never carry a refrigerant can in the passenger compartment of a car.
- Always completely discharge the system before painting the vehicle (if the paint is to be baked on), or before welding anywhere near the refrigerant lines.
Refrigerant leaks show up as oil areas on the various components because the compressor oil is transported around the entire system along with the refrigerant. Look for oily spots on all the hoses and lines, and especially on the hose and tubing connections. If there are oily deposits, the system may have a leak, and you should have it checked by a qualified repairman.
A small area of oil on the front of the compressor is normal and no cause for alarm.
A lot of A/C problems can be avoided by simply running the air conditioner a least once a week, regardless of the season. Simply let the system run at least 5 minutes a week (even in the winter) and you'll keep the internal parts lubricated as well as preventing the hoses from hardening.
REFRIGERANT LEVEL CHECKS
You can safely make a few simple checks to determine if your air conditioning system needs service. The tests work best if the temperature is warm (about 70 F).
- Place the automatic transmission in Park or the manual transmission in Neutral. Set the parking brake.
- Run the engine at a fast idle (about 1,500 rpm) either with the help of a friend, or by temporarily readjusting the idle speed screw.
- Set the controls for maximum cold with the blower on high.
- Locate the sight glass in the head of the receiver/drier. 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 alright. 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 car turn the fan control 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.
See Figures 1 through 5
Servicing, such as discharging and charging the system, and all other work that requires opening the sealed system requires the use of a set of two gauges. The required set consists of a high (head) pressure gauge, for the pressure side of the system; and a low (suction) pressure gauge 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 and the pressure graduations read from 0 to no less the 150 psi.
The high side gauge measures pressure from 0 to at least 300 psi.
Both gauges are threaded into a manifold that contains two hand shut-off valves. Proper manipulation of the valves, and the use of the attached hoses allow the user to perform the following services. Test high and low side pressures. Remove air, moisture, and contaminated refrigerant. Purge the system (of refrigerant). Charge the system (with refrigerant).
The manifold gauges are designed so they have no direct effect on gauge readings, but serve only to provide for, or cut off, 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. The valves are opened only to purge the system or to charge it.
When purging the system, the center hose is connected to a recovery tank at the lower end, and both valves are cracked open slightly. This allows refrigerant pressure to force the entire contents of the system out through the center hose. During charging, the valve on the high side is closed, and the valve on the low side is cracked open. Under these conditions, the low pressure in the evaporator will draw refrigerant from the relatively warm refrigerant storage container into the system.
DISCHARGING THE SYSTEM
- Operate the A/C system for at least 10 minutes.
- Shut off the engine and attach the gauge set.
- Connect a recovery container to the outlet of the center manifold hose. The discharging refrigerant and system lubricating oil will be discharged through the hose into the container.
- Open the low side manifold control slightly. Open the high side manifold control slightly.
Too rapid a discharge process is identified by the appearance of oily foam, close both valves slightly until the foaming stops.
- Close both valves on the gauge set when the pressures read 0, all of the refrigerant should be discharged from the system.
CHARGING THE SYSTEM
One pound cans of R12 refrigerant are available from auto parts and various retail stores. Always follow the manufacturer's instructions on the can when charging the systems.
- Attach the gauge set to the proper service port valves.
- Install a R12 can dispensing valve to the center manifold hose (be sure the can puncturing needle is fully raised). Carefully attach a one pound can of refrigerant to the dispensing valve. The can of R12 MUST remain in the upright position so that gas and NOT liquid refrigerant enters the low side of the system.
- Screw in the dispensing valve to puncture the can and then open the valve fully to permit the R12 to enter the center hose.
- Loosen the center hose to gauge manifold slightly to purge the hose. Tighten the connector. Open the low side control valve slightly and loosen the low side connector at the service port slightly to purge the hose. Tighten the connector and close the manifold control valve.
- Roll down the car windows, start the engine and place the A/C controls to the full maximum position(s).
- On models that are equipped with a low pressure cut-off switch mounted on the receiver-dryer, connect a jumper wire to the switch terminals so that the compressor clutch will remain engaged.
- Open the low side manifold control valve to start charging the system. Adjust the valve so that the charging pressure does not exceed 40 psi. Too sudden a surge of refrigerant may permit unwanted liquid to enter the system and freeze block it.
- Adjust the engine speed to a fast idle of about 1200 to 1500 rpm to help charge the system.
Placing the refrigerant can into a container of hot water of no more than 125°F will speed charging.
- The A/C system hold approximately 32 ounces of refrigerant, or about two and one-half small cans of R12. When changing cans, close the low side valve. After the system is completely charged, close the manifold gauge set valve, shut off the engine and remove the hoses and jumper wire.