It could be any of a number of things:
Low refrigerant. An A/C system requires a certain amount
of refrigerant to cool properly. If low, it works less efficiently
and does not cool well.
Dirty condenser -- The condenser is the heat exchanger mounted
in front of the radiator. It cools the high pressure refrigerant
vapor after it exits the compressor so it can condense into a
liquid. If the condenser is full of leaves, bugs and road debris,
air flow through the unit may be blocked to the point where little
cooling occurs. Cleaning the condenser should cure the problem.
Inoperative condenser cooling fan. The condenser often has
its own separate electric cooling fan. This fan should come on
and remain on when the A/C system is operating. If the fan motor,
motor relay or wiring is defective, the fan may not be working.
Air or moisture contamination -- For the refrigerant inside
the system to do its job properly, it must not be contaminated
with air or moisture. Air reduces the cooling efficiency of the
system while moisture can freeze and form ice that causes blockages
in orifice tubes and metering valves. Air and moisture contamination
may be the result of unrepaired leaks in the system, or failing
to vacuum purge the system prior to recharging it with refrigerant.
Blockages -- Debris, rust or debris in the system may plug
up the orifice tube or metering valve that admits refrigerant
into the evaporator. If this vital point becomes obstructed,
the flow of refrigerant may be restricted or blocked causing a
loss of cooling -- and possible compressor damage as well in systems
that rely on oil circulating with the refrigerant for lubrication.
Mechanical problem -- These include things like metering valve
failures, compressor wear, a compressor clutch that fails to engage,
bad pressure switches, etc. Pinpointing the problem will require
the skills of a competent A/C technician
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