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 200°F. 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 200°F. 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 250-290°F 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:
||When the system is next to the exhaust manifold
||250°F when the system is in perfect charge. Up to
300°F when the system is undercharged. Some specifications run up to
250°F when the system off
||Have measured 290°F at the compressor high pressure
||The system is designed to operate under 248°F during
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:
LACK OF USE:
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.
FAILURE TO FLUSH SYSTEM:
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
EXCESSIVE HIGH OR LOW PRESSURE:
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.
IMPROPER FREON OR OIL CHARGE :
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 220°F-240°F. 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 275°F, 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.