To determine if there is a discharge valve leak,
connect the gauge manifold and start the unit. Run it
until the low-side (LO) pressure gauge indicates
normal pressure for the unit. Stop the unit. With an ear
near the compressor housing, listen for a hissing
sound. Also, watch the gauges. When leaking caused
by an obstruction is present, the low-side pressure
rises, and the high side decreases until the pressures are
equalized. A quick equalization of pressures indicates
a bad leak that should be repaired immediately or the
The oil level in the compressor crankcase should
be checked by the procedure in the following
manufacturer's manual. This procedure normally
includes the following steps:
Attach the gauge manifold to the suction and
discharge service valves.
Pump the system down.
Close the suction and discharge valves, isolating
Remove the oil filter plug and measure the oil
level as per the manufacturers manual.
When the compressor knocks, you may have to
disassemble the compressor to determine whether the
cause is a loose connecting rod, piston pin, or
crankshaft. Sometimes a loose piston can be detected
without the complete disassembly. In cases requiring
disassembly, you should take the following steps:
First, remove the cylinder head and valve plate to
expose the top of the piston. Start the motor and press
down with your finger on top of the piston. Any
looseness can be felt at each stroke. The loose part
should be replaced.
Check the oil level because oil levels that are too
high often cause knocks. Always make sure that a low
oil level is actually the result of a lack of oil, rather than
a low charge.
Stuck or Tight Compressor
A stuck or tight compressor often occurs as a result
of poor reassembly after a breakdown repair. In such
cases, determine where the binding occurs and
reassemble the unit with correct tolerances; avoid
uneven tightening of screws or seal covers.
INSPECTION OF COMPRESSORS
An inspection should be performed on a
refrigeration unit from time to time for knocks,
thumps, rattles, and so on, while the unit is in
operation. When any of the external parts have
excessive grease, dirt, or lint, they should be cleaned.
Before cleaning, you should always ensure the power
A careful check of the entire system with
instruments or tools is essential to determine if there
has been any loss of refrigerant. NO LEAK IS TOO
SMALL TO BE FIXED. Each leak must be stopped
Some specific conditions to look for during the
inspection of a refrigeration system are as follows:
Inadequate lubrication of bearings and other
Rusty or corroded parts discovered during the
inspection should be cleaned and painted.
Hissing sounds at the expansion valve, low
readings on the discharge pressure gauge, and bubbles
in the receiver sight glass, all indicate a weak
Loose connections and worn or pitted switch
contacts result in inoperative equipment or reduced
reliability. Thermostats with burned contacts may
produce abnormal temperatures in the cooled
Fans difficult to rotate by hand, with bent blades,
or loose or worn belts are a source of trouble easy to
locate and correct during inspection.
Air filters clogged with dirt should be cleaned or
replaced during the inspection.
Hermetically sealed units should be inspected for
signs of leaks and high temperatures and for too much
noise or vibration.
Q43. On compressors, refrigerant leaks most often
occur at what location?
Q44. Hissing sounds at the expansion valves, low
discharge pressure, and bubbles in the receiver
sight glass during inspections indicate what