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THE RIGHT AMOUNT OF REFRIGERANT
We no longer use a sight glass in an A/C system to tell us when the system has the right amount of refrigerant. With today’s refrigerants using 134a, if a system is over or undercharged, you will, in most cases, have poor louver temperature and premature compressor failures. If you don’t know how much liquid Freon is in an air conditioning system, you should recover the Freon, evacuate, and add the correct charge using the pressure temperature chart or the OEM specs for the correct charge.
CAUSES FOR COMPRESSOR FAILURES EXCESSIVE HEAD PRESSURE
An A/C system that has been overcharged will have high head pressure. If an electric condenser is being used, it will have electric motors and fan blades that bring fresh air across the condenser fins to remove heat from the refrigerant. When you have worn or burnt motors or missing fan blades, you will stop the air intake and cause high head pressures. Remove and replace broken fan blades or burnt-out motor(s). You can use a clamp-on DC amp meter to see if the motor(s) is drawing too many amps. Also, make sure that the condenser fins are not bent and they are clean. Even a small amount of dirt on them acts like an insulator, and you will not get the proper heat transfer.
If the system is equipped with a radiator-mounted condenser, be sure that the condenser fins are straight, not bent, also free of debris. Check to see if the engine fan blades are not damaged, and if equipped with a fan clutch, be sure it’s working properly. Check the engine fan drive belt to see if it is tensioned properly. Do a visual check for cracks, a glazed belt, or the wrong width.
LOW HEAD PRESSURE
An undercharged A/C unit, faulty expansion valve, or evaporator coil freezing up due to a plugged or dirty air filter will be some of the common things to look for. All of the above will cause premature compressor failure.
Be sure you have the proper refrigerant manifold gauges for the specific refrigerant being used.
Before connecting refrigerant manifold gauges to an A/C system, do a visual check to see that they are in good condition and that the refrigerant hoses are not frayed or damaged. Be sure that both High and Low manifold gauges are calibrated to zero.
FALSE PRESSURE READINGS
- Valve core depressors and hose gasket may be damaged.
- Are the service hoses on the manifold set connected to the correct service ports?
- The system may have more than one type of refrigerant in the system (example: 134a mixed with R12, so check the system with a refrigerant identifier.
WHY CHARGE WITH VAPOR, NOT LIQUID?
When charging with a liquid, you can cause severe damage to the compressor valves and have premature compressor failure. When charging an A/C system, for example, using a 30 lb. keg of 134a refrigerant, place it in the upright position. Do not turn the cylinder upside down. You can purchase a heating blanket to wrap around the keg of the refrigerant to help change the liquid to gas. This helps speed up the charging times.
COMPRESSOR CLUTCH FAILURE
Common causes of compressor clutch failure
- The wrong voltage being supplied at the field coil: Take an amp draw of the electrical system to make sure you have the proper voltage that is needed for the field coil. The correct coil resistance is 3–5 ohms.
- The air gap between the compressor face plate and pulley is wrong: Check with OEM manufacture specs. Use a feller gauge and add or remove a face plate shim.
- Bearing failure: For worn-out pulley bearing, check for pulley play or listen for noises around the compressor clutch area.
- The compressor drive belt is the wrong type or width or is misaligned.
- Lock compressor: Due to excessively high head pressure. Make sure the system is not overcharged with Freon or oil.
Once upon a time, only size mattered when choosing the correct O-ring. Now, it’s a matter of material. R-12 used BUNA-N. They were mostly black in color. Today’s refrigerants use an HNBR material that is green in color because of the higher head pressures and type of oil being used. Use HNBR O-rings for both R-12 and 134a.
WHY MUST I USE A VACUUM PUMP?
Moister and air are two of the most common problems in an A/C system. Moister will break the lubricant down and cause contamination and icing inside the system and form a blockage, restricting refrigerant flow. By using a good vacuum pump, you will help eliminate this problem. Twin Eagle Distributing recommends using a 2-stage 6 cfm vacuum pump. Always replace the receiver-drier when an A/C system is open or a new compressor is being installed.
The whole purpose of a filter drier is to keep the moisture from mixing with the lubricant and causing contamination and debris to enter the system. Most OEM manufacturers recommend removing and replacing the drier or accumulator in addition to evacuation. This will ensure better performance. Make sure your new drier accumulator has the XH7 or XH9 desiccant that is recommended for all refrigerants.
Black Death is a term used by A/C service techs. It is a result of the breakdown of the refrigerant oil in the system. This causes excessive wear in the compressor and, as a result, causes a black residue that is made up of solder flux and aluminum shavings from the old compressor. When this mixture cools, it hardens in the condenser. Flushing the A/C system several times will not remove this black death. You must replace the condenser, or you will cause this mixture to liquefy and flow out to the liquid line, restricting the orifice tube or expansion valve and damaging the new compressor.
WHY FLUSH THE SYSTEM?
- Burnt-Out Compressor
- Plugged Expansion Valve or Orifice Tube
- Drier Desiccant Let Loose
Flushing removes any debris oil that may be in the system, getting it to ground zero. So when you install that new compressor, expansion valve, or orifice tube, it will not fail on you. Make sure you have the correct amount of oil in the system.
SAFETY SWITCHES IN AN A/C SYSTEM
High and low-pressure switches are used to protect the compressor from becoming damaged due to excessive head pressure or from too little refrigerant in the system, causing very high superheat and not lubricating or sufficiently cooling the compressor. (Never bypass these switches, or severe damage to the A/C system can and will accrue.)
THE THERMOSTAT SWITCH
The thermostat switch is used to help regulate the outlet air temperature by cycling the compressor clutch on and off. It is also a safety device to ensure that the evaporator coil does not freeze up on cooler heat load days. If this switch should fail, you may find that you have no voltage at the field coil, or the compressor may never cycle out and cause the evaporator to freeze up.
There are typically two types of thermostat switches used in off-road A/C: preset thermostat, in which you do not adjust the temperature setting, and manual, where you will have a knob so that you can adjust when to cycle the compressor on and off.
The thermostat probe should be placed 3 to 6 inches from the expansion valve. Make sure you have enough probes in the coil. If you place the thermostat probe at the top of the evaporator coil, it should be at least in the middle of the coil. If you place it in the front, also make sure it’s in the middle of the coil. Make sure the probe does not go through the evaporator coil and into the heat coil.
Also called TXV, they may be one of two types: externally or internally equalized, and they can be either right angle or block style. The expansion valve’s job is to properly meter the right amount of refrigerant entering the evaporator coil. You will find a drier/receiver when these types are used. To troubleshoot problems with the expansion valve, you must have the proper set of refrigerant gauges attached to an operating A/C system.
An orifice tube is a fixed metering device located inside the liquid line between the condenser and evaporator. The orifice tube is enclosed with plastic housing and protected by a fine mesh filter. The filter helps prevent debris from entering the system. Orifice tubes are usually color coded. An accumulator-only or accumulator and drier receiver will be used with an orifice tube.
Typical Orifice Tubes
Also called a heat exchanger, the evaporator coil removes heat from the air that the fan motors pull in, then discharges the cold air out of the louvers. The evaporator usually operates between 30 degrees and 40 degrees Fahrenheit, and the louver temperature is about 10 degrees warmer than the coil. Long duct work may increase the temperature a few degrees more.
If the coil becomes plugged with dirt, mud, or ice, the air being discharged from the louvers will get warmer. Blow out the coil with compressed air (about 60–90 pounds), or wash the coil with water or coil cleaner to ensure that there is good airflow throughout the coil. Use a fin comb to straighten evaporator coil fins, not a screwdriver.
Also called a heat exchanger, it removes heat from refrigerant that is in a very high-pressure, high-temperature state by passing cooler ambient air over the condenser fins. This changes the state of the refrigerant from a gas to a liquid before going to the drier/receiver or accumulator.
If the condenser fins get plugged with mud debris or even a fine coating of dirt, you will have poor or no cooling out of the louvers. Be sure to clean the condenser coil with compressed air or wash it with water or coil cleaner. Keep coil fins straight with a fin comb, not a screwdriver.
FAN SWITCH & RESISTORS
You will find the fan switch located in the evaporator blower system, as in a lot of your aftermarket A/C units, or in the cab console for drop-in coils or OEM systems. The fan switch controls the blower motor speeds.
On the back of most fan switches, you will find five posts: “B”= power, “C”= clutch, “L”= low speed, “M”= medium speed, and “H”= high speed.
The resistor is used to cut the amount of current to the blower motor, so you will have high, medium, and low. It will be found in the blower unit that houses the blower motor. (See the section on blower motors and fan wheels for more information on resistors.)
Typical Fan Switch
BLOWER MOTORS & FAN WHEELS
Blower motors control the amount of air across the evaporator coil. If the motor rotation is not correct, or the motor, fan wheel, or resistor (if used) malfunctions, not enough air will be drawn across the evaporator coil, and it may freeze up. Broken fan wheels or housing will cause the motor assembly to sound very loud and will need to be replaced.
There are usually two types of motors used in a blower system: Field Wound, which has four wires and is operated by a fan switch only, or a Permanent magnet motor, which has only two wires and requires a fan switch and a resistor. Always make sure you have the right motor and resistor for the voltage.
RECIRCULATION AIR FILTER
The recirculation air filter will be found inside the operator’s cab. It will be located in the evaporator box. It is very important to keep this filter very clean. Most of these filters are made of open-cell foam, though some of the new filters are made of paper. Most foam and paper filters can be blown out with 20 to 30 pounds of air pressure. Most foam filters can be washed out with water. Failure to service will cause the evaporator coil to plug up with dirt, mud, and debris, causing poor or no cooling.
FRESH AIR FILTER
The fresh air filter will be found outside the operator’s cab. It allows clean, fresh air to enter the cab area. Most of these filters are made of paper and filter particles, down to about 19 microns. They must be removed and cleaned daily if the machine is operated more than 10 hours a day. Remember that if your equipment has the capability of bringing fresh air into the cab, it also brings in the ambient temperature as well.
DRAIN PAN AND DRAIN LINES
The drain pan collects water from the evaporator coil and channels it out of the cab. If the pan leaks or drain lines become plugged with mud, the water that has condensed off the evaporator coil will enter the cab area. They will need to be flushed with water or compressed air. Check the drain lines for kinks, holes, sharp bends, flat runs, or uphill runs.
THE REFRIGERANT OIL
The internal components of an air conditioning system must be properly lubricated with the correct refrigerant oil. There are several types on the market today: PAG, Ester, and mineral. Make sure you choose the right one for the type of refrigerant that is in the system.
The oil is absorbed into the refrigerant that carries it through the compressor as a mist. An oil charge that is over or under can damage the compressor. The compatibility of the refrigerant oil is determined by its ability to remain oil when mixed with the refrigerant and not become separated by a chemical reaction. R-12 refrigerant uses a mineral oil 5GS made for that refrigerant and cannot be used in R134a systems. An R134a system uses either PAG oil or Ester oil. Note the following: PAG oils absorb moisture 100 times greater than mineral oil (in effect, 1% water within 15 hours). Ester oil will mix to 50% with all PAG oil types.
Refrigerant leaks can cost lots of downtime and money if not found and repaired. A good halogen leak detector will pick up leaks under static conditions; however, some will leak only when the system is in operation.
There are other ways of finding a refrigerant leak. Use visual inspection; look around the refrigerant fittings and hoses for spots of refrigerant oil and dirt. Check the front clutch plate of the compressor for oil and dirt. If the air conditioning system is completely empty, you may want to use dry nitrogen to pump up the system to hear a leak. Also, they have a fluorescent dye that can be used to detect a leak. Note, to see the dye stain, you need a UV lamp, and the leak has to be where the light can reach it.
HOSES & FITTINGS
Refrigerant hoses carry the refrigerant from one component to the next. The hose should be of the barrier type (Twin Eagle Distributing recommends the Goodyear Galaxy hose) for R134a refrigerants and the beadlock-style fittings. Most air conditioning systems use the #6 (5/16” ID), #8 (13/32” ID), and #10 (1/2” ID) refrigerant hose. Hoses need to be inspected occasionally for blistering, bulges, dry rot, chaffing, swelling, and hardening. Check around hose fittings for traces of oil and dirt that may indicate a refrigerant leak. Keep hoses away from hot spots, moving parts, sharp edges, and oil leaks, and watch hose routings on machines that articulate.
One Piece Beadlock Fitting
Bubble Circular Crimp
Typically Utilized with 1 piece Beadlock Type Fitting and Barrier Nylon/Rubber Hose
The complaint: “It used to blow harder.” Things to check are:
Obstruction at the air return, plug, or dirty filter
Debris in the evaporator coil that blocks air circulation
Icing of the evaporator coil due to bad thermostat switch
Air leak or obstruction in the air duct hose
Defective blower relay
Blower wheel loose on the motor shaft
Wrong blower wheel rotation
Wrong blower motor shaft rotation
Low blower motor voltage due to poor ground
Using a 24-volt motor on a 12-volt system
MUST I RECOVER THE HFC-134a REFRIGERANT?
YES! Most definitely. EPA has issued regulations under Section 609 of the Clean Air Act, establishing standards and requirements regarding the servicing of Motor Vehicle Air Conditioners (MVAC).
Technicians who repair or service Motor Vehicle Air Conditioners must recover the refrigerant and either recycle it on-site or send it off-site to a reclamation facility so that it may be purified according to ARI Standard 700. Technicians must use EPA-approved equipment to perform refrigerant recovery and recycling.
Technicians who service MVAC or MVAC-like appliances (“Farm and Heavy Equipment”) must be properly trained and certified by an EPA-approved technician certification program set forth in section 82.34, and the description of the specific requirements for approved training and testing set forth in section 82.40.
CHARGING A/C SYSTEM
After you have followed the rules set by EPA rule 609 and made your repair, you will need a refrigerant manifold set. Attach the low and high side manifold couplings to the service ports on the A/C system. Attach the middle hose to a good 2-stage vacuum pump. Turn it on, and open both the low and high side manifold wheels. Check the low side gauge to make sure you are going into a vacuum, which is achieved when the needle goes below 0. You want to be close to 29.9 inches of the vacuum. Let it run for at least 30 to 45 minutes. Note that at higher altitudes, you will not achieve -29.9 inches of vacuum. If you are above 3,000 feet, let the vacuum pump run at least 15 minutes longer.
After you have achieved a good vacuum, close both high and low side manifold wheels. Check the low side gauge and note where the needle is. After at least 5 minutes, recheck the needle. If you are losing the vacuum, that will indicate a leak in the A/C system. Repair and start evacuation again.
When ready to charge, remove the center hose from the vacuum pump and attach it to the refrigerant keg. Make sure you purge the center hose at the manifold gauge to let any trapped air out. Just crack open and close. Charge with vapor only, not liquid.
Next, open the low side manifold wheel. You will notice that the high side gauge needle will start to move up away from 0 after the low and high side needles are equalized or close to it. You are now ready to charge.
Start the engine and turn the fan speed on high and the A/C switch on. Check with the A/C manufacturers for how much refrigerant the system requires. If you can’t obtain that information, see the section on charging an A/C system using a pressure temperature chart.
CHARGING AN A/C SYSTEM USING A PRESSURE TEMPERATURE CHART
After following the steps in the section CHARGING AN A/C SYSTEM, you will need to take the fresh air intake at the condenser. Make sure you are reading the cool air coming in, not the warm air leaving the condenser.
Add 35–40°F to the temperature you took at the condenser. For example, let’s use R-134a, and the temperature reading was 90°F. You add 40°F. That gives you 130°F.
Look at the R-134a Temperature Pressure Chart. Find 130°F. You will find that equates to 200 PSIG. When your high side gauge reads 200 PSI, close the low side manifold wheel that will stop the refrigerant flow from entering the system. Take your louver temperature, then check the high side gauge. If it’s below 200 PSI, add a little more refrigerant to the system till it reaches 200 PSI. Be careful not to overcharge the system. Check your louver temperature. If it rises with the compressor on, then that could indicate overcharging.