The Auto compressor is the heart of the refrigeration circuit. Its duty is to pump and pressurize the refrigerant to move it via the A/C system. Compressors work hard and get hot, up to several hundred degrees and several hundred pounds per square inch of internal pressure. They depend on only a few ounces of lubricant to keep their components moving. If the lubricant is lost because of a leak, or the lubricant breaks down because of contamination, the compressor will not last. Sooner or later, the compressor will sure call it quits.
The most common sign of a compressor failure (besides no cooling) is a seized compressor. It will not rotate when the magnetic clutch engages, and you may hear squeals of protest from the drive belt. Or, the belt may either have already broken or been thrown off its pulleys.
Loss of lubrication is undoubtably the most common cause of compressor failure. This can occur when there is a refrigerant leak somewhere in the system that lets refrigerant and oil to escape. Typical leak points are hoses, hose and pipe connections (O-rings and flange gaskets), the evaporator, condenser or the compressor shaft seal. An electronic leak detector or dye should be used to find the leak so that it can be repaired.
A restriction inside the A/C system can also starve the compressor for oil. Oil circulates with the refrigerant, so if the orifice tube or expansion valve is blocked it may make the compressor to run dry and seize.
Even if a compressor is still turning, it may have to be replaced if it is leaking, making excessive noise or not working correctly. Some compressors are naturally noisier than others, but loud knocking noises can sometimes be produced by air in the system (the cure here is to vacuum purge the system to remove the unwanted air, then to recharge the system with refrigerant). Metallic noises and bearing noise are often signals that the compressor is about to fail.
A new compressor may be required if the unit is leaking internally or not producing enough pressure due to bad reed valves, worn piston rings, or worn or scored cylinders, etc.). A worn compressor or one with internal issues will not be able to develop normal operating pressures with a full charge of refrigerant. This kind of issue can be diagnosed with an A/C gauge set.
Poor cooling can equally be caused by a lot of things other than a bad compressor, so do not change the compressor until you have ruled out other possibilities such as a low refrigerant charge, too much oil in the system, air contamination, a clogged condenser, plugged orifice tube, inoperative electric cooling fan, etc.
Compressor operation can be touched by sensors in vehicles with automatic temperature control systems. Some have an A/C pressure transducer (often mounted in the high side line) to monitor refrigerant pressure and shut off the compressor if pressure gets too high; a compressor temperature sensor to turn off the compressor if it gets too hot; and/or a compressor rpm sensor to monitor belt slippage. Mitsubishi, for example, uses a “belt lock controller” to disengage the compressor if the drive belt slips or the compressor seizes.
On 1996 and newer Mercedes-Benz E-Class vehicles, the A/C control module will disengage the compressor if the refrigerant temperature and pressure sensors do not indicate a rise when the compressor is being driven.
COMPRESSOR CLUTCH PROBLEMS
If the compressor is not rotating, make sure the magnetic clutch engages when energized. Underlying issues here may include a bad relay, fuse, wiring problem or a defective clutch. If the clutch fails to cycle on and off when the A/C is turned on, jumping the clutch lead with a jumper wire from the battery will indicate if the problem is in the clutch or elsewhere. If the clutch engages, the issue is the clutch power supply (relay, fuse, wiring, switch or control module). Refer to a wiring diagram and work backward toward the battery to discover why the voltage is not getting through.
Many A/C systems have a low-pressure cutout switch that keeps the compressor clutch from engaging if system pressure (the refrigerant charge) is too low. This is designed to protect the compressor from damage in the event of a leak. So if the clutch is not engaging, check the refrigerant charge and the cutout switch. The clutch air gap is also vital for proper clutch operation. If the clearance is not correct, the clutch may slip and burn or not engage at all. The specs can be seen in a service manual along with adjustment procedures. Generally speaking, most clutches call for a 0.015 to 0.040 inch press fit clearance.
How often will a compressors fail as a result of “manufacturing defects?” Not quite often. According to one compressor manufacturer who examined 75 compressors that had failed and were returned under warranty, only two were discovered to have manufacturing defects. The rest failed due to problems such as too little oil in the system, air in the system, contaminants in the system, or “installer error.” The latter category also included using the wrong type of compressor lubricant, not using enough lubricant, using non-approved flushes to “clean” system parts, and using cross-contaminated refrigerants. Debris left over from a previous compressor failure was the most common cause of repeat compressor failures.
Always use the type of lubricant made for specific compressors. This is especially vital with rotary vane and scroll-type compressors. A replacement compressor may or may not have lubricant from the factory. In some cases, the shipping oil must be drained before the compressor is mounted. In other cases, the compressor may contain a POE or lubricant that may or may not be compatible with the car requirements. Follow the compressor suppliers installation instructions to the letter to prevent warranty problems later on.
Before including fresh oil to a system, all the old oil should first be removed. This will stop cross-contamination of lubricants and reduce the risk of overcharging the system with too much oil (which can cause cooling problems). Always refer to the OEM oil capacity chart for the vehicle application. The following is a list of recommended lubricants for R-134a import compressors:
- Behr/Bosch rotary compressors – Ester 100;
- Behr/Bosch piston compressors – PAG 46;
- Calsonic V5 – PAG 150;
- Calsonic V6 – PAG 46;
- Diesel/Kiki (Zexel) DKS, DKV & DCW – PAG 46;
- Hitachi (all) – PAG 46;
- Keihin (all) – PAG 46;
- Matsushita (all) – Ester 100;
- Mitsubishi FX80 – PAG 100;
- Mitsubishi FX105 – PAG 46;
- Nihon (all) – Ester 100;
- Nippondenso 6P, 10P, 10PA, 10P08E – PAG 46;
- Nippondenso SP127, SP134 & 6E171 – PAG 46;
- Nippondenso TV series – PAG 125;
- Panasonic (all) – PAG 46;
- Sanden SD500 & SD700 – PAG 100;
- Sanden SD710, SDB, TV & TRS – PAG 46; and
- Seik-Seiki (all) – Ester 100.
FLUSHING AFTER A COMPRESSOR FAILURE
When a compressor fails, it may spit metallic debris into the A/C system. Most of this debris ends up in the condenser where it can clog tubes and interfere with efficient cooling. Some of the debris may be carried to the orifice tube or expansion valve and create a blockage. Debris can even be blown back into the suction tube. If not removed by flushing, it can be sucked back into a new compressor and make it to fail.
Flushing the hoses is always recommended following a compressor failure. Flushing the condenser is equally recommended. But with many condensers, change is the only sure-fire way to get rid of contaminants. Older serpentine-style tube-and-fin condensers can often be flushed successfully, but parallel flow condensers are very hard to clean. So too are newer style condensers with extremely small extruded tubes. For these kinds of applications, the condenser should be changed. It is expensive, but not as expensive as ruining a new compressor because of residual debris or sludge in the old condenser.
After flushing, mount an in-line filter after the condenser to trap any debris that might still be inside. The filter will keep anything that works loose from being carried to the orifice tube.
You should also mount a filter screen in the suction hose at the compressor inlet to protect the new compressor from any debris that might be upstream inside the suction hose or evaporator.
Another reason for flushing is to take out residual oil from the system. This is necessary when retrofitting an older R-12 system to the new ozone-safe R-134a refrigerant, but it is equally a good way to make sure the system contains the right amount of oil. Simply adding oil to the system to change that which has been lost is a guess at best, since there is no way to know how much has been lost due to leakage. Estimating a couple of ounces here and there for replacing an accumulator, receiver/drier, condenser, compressor or hoses is not a very accurate means of knowing how much oil needs to be added to the system when it is recharged with refrigerant. Flushing gets rid of all the oil so the exact amount specified by the car manufacturer can be added back to the system.
What happens if there is too little or too much compressor oil in the system? Not enough oil in the system will lower compressor lubrication and may lead to premature failure. Too much oil in the system can puddle in the condenser and stop the flow of refrigerant causing a drop in cooling performance.
Other parts that should also be changed following a compressor failure include the accumulator or receiver/dryer, and the orifice tube or expansion valve. The former contains a bag of desiccant that traps moisture and performs as a filter to protect the system. A new orifice tube or expansion valve is recommended because the small hole in this metering device can become easily clogged with debris. An aftermarket “variable orifice tube” can improve low-speed cooling.
EVACUATING & RECHARGING THE AIR CONDITIONING SYSTEM
After the compressor has been mounted and the hoses are reconnected, the A/C system must be thoroughly evacuated with a vacuum pump to pull out air and moisture. If not purged from the system, air will lower cooling efficiency. Moisture will react with refrigerant oil and produce acids and sludge. Moisture can also freeze and plug the expansion valve producing noise, restrictions or a complete blockage.
A pump capable of achieving high vacuum must be used to pull out all of the contaminants. When air is pulled out of the system, it produces a vacuum that causes residual moisture to boil and evaporate. For this to happen, the vacuum pump must be capable of pulling at least 29 in. Hg of vacuum throughout the evacuation process (which normally takes about 30 minutes).
One of the best ways to monitor the evacuation process is with a Thermistor Vacuum Gauge that reads in microns (one inch of Mercury equals 25,400 microns). It takes a highly accurate instrument to measure vacuum since even a little pressure left in the system can keep all the residual moisture from boiling out. Only a 1/2 inch of mercury of pressure (12,700 microns) can lower the boiling point of water by more than 20 degrees F. Pulling out the last fraction of an inch of pressure is the most critical step in the evacuation process to ensure complete removal of all air and moisture.
After pulling a deep vacuum on an A/C system, close all valves and shut off the vacuum pump. A slow rise in pressure (which you can see on the Thermistor Vacuum Gauge) will occur as the residual moisture continues to boil off inside the system. Pulling additional vacuum will get rid of this moisture. The evacuation will not be complete until the system can keep a stable vacuum reading below 700 microns for at least three minutes.
The time it takes to completely evacuate an A/C system can be lowered by preconditioning the evaporator prior to hooking up the vacuum pump. Preconditioning raises the temperature so the moisture will boil off faster. The easiest way to raise the temperature of the evaporator is to run the engine with the heater on HOT in the RECIRC mode. Turn the blower fan to HI and close all doors and windows. When the engine gets to its normal operating temperature, the evaporator will be thoroughly preheated and ready to evacuate.
If you have problems maintaining a stable deep vacuum, there may be a leak in the A/C system, the vacuum pump or the equipment connections. Leak testing should be performed prior to evacuating the system because evacuation is not always a reliable way to find or even identify a small leak in an A/C system. Seals and O-rings that leak under pressure may move under evacuation and not leak.
Lastly, recharge the system with the recommended amount of refrigerant and compressor oil. Do not overcharge and do not add too much oil. Inspect the cooling performance to confirm that everything is working properly and that the new compressor is performing its job.