System Temperatures


It is important to understand that operating temperatures in the actual underhood environment vary from vehicle to vehicle and are not the same as temperatures on the test stand. An air conditioning system designed for a mobile application would normally only see temperatures below 200F. Factors effecting this are the heat generated by the compressor and the normal pressure/temperature relationship of the refrigerant used in the system. When a system is operating on a test stand or in an application with few outside heat influences, then the system temperatures will be below 200F. Truck and agricultural applications would be good examples with expectations of the lower temperatures.

Unfortunately, automobile applications do not generally fall in the lower temperature range. Higher temperatures in the 250-290F are generated by external heat sources. Small engine compartments, exhaust manifold, engine block, and lack of ventilation are the general external heat factors. The major factor in external heat is the mounting of the compressor, hoses, and other system components and their proximity to the exhaust manifold. 

Several of the original equipment manufacturers presented some high temperature figures that will give indications of what certain vehicles and systems might see. I have listed them below: 

Manufacturer Higher Ranges Notes
Chrysler 250-270F When the system is next to the exhaust manifold
Ford 250-300F 250F when the system is in perfect charge. Up to 300F when the system is undercharged. Some specifications run up to 250F when the system off
General Motors 290F Have measured 290F at the compressor high pressure switch port
Volvo 248F + The system is designed to operate under 248F during normal driving.

The higher temperature ranges are on the high side of the system and are vehicle dependent. However, a rebuilt compressor or the replacement seals furnished to the repair shop could be put into one of these vehicle-specific applications. Normal engineering discipline would encourage design of the seal material for the highest temperature application.

Some common causes for compressor failures are:

Compressors not run regularly will be starved for lubrication when they are turned on. This causes excessive wear until the oil again reaches the compressor. It's best to use the A/C once a week for a few minuets.

Compressors and hoses wear causing bits and pieces to mix with the oil. This sludge gets into the compressor and can cause it to seize. Failure to properly flush or using flush that leaves a residue is a primary cause of failure.

Condenser fans can quit due to bad connections or defective motor windings that get too hot. This create excessive head pressure and heat that can lock up or damage a compressor. Loose connections at the coil can cause high resistance and low voltage causing clutch slippage. Low refrigerant charge will cause lack of lubrication getting to he compressor. Painting condensers will insulate them from efficiency removing heat and increase head pressure in the compressor.

Compressors manufactured after 1990 have smaller capacity, therefore use less refrigerant and oil and are extremely sensitive to inadequate amounts of oil. The wrong type of oil may break down in high heat conditions. R134a systems are more sensitive to lubrication than R-12 systems. An over charge of oil can clog the condenser, the orifice tube or expansion valve and starve the compressor of oil. R12 systems use mineral oil and R134a systems use PAG or Double End Capped PAG Oil. (always check the OEM specs.)

Blends can affect seals and O-rings, causing them to leak, swell or otherwise deteriorate and it can happen relatively quick.

What causes excessive clutch cycling?

It's common to assume low refrigerant is the cause for excessive clutch cycling. This often leads to adding refrigerant and thereby overcharging the system. Most vehicles have a compressor clutch cutout strategy based on an engine coolant temperature of 220F-240F. The vehicle computer relies on a coolant temperature sensor to provide the signal for the compressor clutch decision and if the sensor reads too high, the clutch will cycle off. Cooling systems with a 50-50 anti-freeze mix and the proper radiator cap should boil at around 275F, so the system may not be boiling when the compressor cuts out. Check the actual coolant mix and temperature by using an infrared thermometer to check out the coolant temperature and a Scan Tester to tell you what temperature the sensor is actually signaling to the computer.


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