Probable Cause:
Insufficient oil in system.
Suction riser too large.
Insufficient traps in suction risers.
Suction superheat too high.
Lack of oil separator on systems operating below – 30°F suction.
Low refrigerant charge.
Refrigerant flood-back.
Ruptured suction line to the liquid heat exchanger.
Crankcase heater burned out.
Iced-up evaporator coil.
Distributor and/or expansion valve too large (oil dilution creating crankcase “foaming” – loss of oil pressure).
Liquid refrigerant feeding thru oil separator.
Evaporator fan motor(s) not operating.
Defective oil pump.
Plugged oil pump inlet screen.
Worn bearings, pump, or compressor.
Oil pressure control trip out due to compressor short-cycling.
Control voltage too high, causing premature trip-out.
Oil pressure control defective.
Defective compressor overload.
Oil pressure control improperly wired.
Remedy:
Add oil, per the compressor manufacturer’s operation specifications.
Check line sizing at design conditions and change piping if incorrect.
Install a suction P trap in the vertical riser per manufacturer’s specifications.
Adjust superheat.
Add oil separator.
Add refrigerant.
Adjust superheat 200F minimum at the compressor. (Consult compressor manufacturer
for minimum superheat.)
Replace heat exchanger.
Replace crankcase heater.
Clean evaporator coil.
Check sizing at design conditions and change if incorrect.
Check oil separator heater.
Check fan motor(s) and replace if necessary.
Replace oil pump.
Clean oil pump screen.
Replace bearings, pump, or compressor.
Check high and low pressure settings.
Check refrigerant charge.
Check for noncondensables.
Check for dirty condenser.
Replace burned-out condenser motors.
Adjust control voltage to correct voltage.
Replace oil pressure control.
Replace compressor overload.
Check wiring and correct, if necessary.