Greenheck MPX (472695) - Manual (Page 32)

Troubleshooting – Refrigeration Circuit
Symptom Possible Cause Corrective Action
Compressor
starts but
cuts out
on high
pressure
switch
High pressure
activates at
600 PSIG
Refrigerant overcharge. Check pressures, charge by subcooling.
Condenser fan motor defective. Check fan motor.
Condenser coil inlet obstructed or
dirty.
Check coil and inlet clearances.
Air or non-condensables in system.
Check high side equalized pressures, check thermal
expansion valves.
Defective high pressure switch. Replace.
Restriction in discharge or liquid line.
Check refrigerant line pressures, check thermal
expansion valves, replace any defective component.
Condensing fan relay not pulling in. Replace.
Reheat valve and bypass valve not
opening.
Check valves or valve circuit board.
Compressor
cuts out
on thermal
overload
Low voltage. Check voltage.
Sustained high discharge pressure.
Check running amperage and conditions described
under “low suction pressure” symptoms.
High suction and discharge pressures.
Check thermal expansion valve setting, check for air in
system. Check air conditions and cfm.
Defective compressor overload.
If compressor is hot, allow compressor to cool for two
hours. Recheck for open circuit.
Improper refrigerant charge. Check subcooling.
Improperly wired. Review wiring schematics.
Loose wiring. Check all connections.
Defective start relay. Replace relay.
Motor windings damaged. Verify amp draw.
Compressor
hums, but
will not start
Improperly wired. Review wiring schematics.
Low line voltage. Check voltage.
Loose wiring. Check all connections.
Defective start relay. Replace relay.
Motor winding damaged. Verify amp draws. Replace compressor if necessary.
Internal compressor mechanical
damage.
Replace.
Compressor
noisy or
vibrating
Refrigerant overcharge. Check pressures and subcooling.
Liquid floodback.
Check thermal expansion valve setting. Check for
refrigerant overcharge.
Tubing rattle.
Dampen tubing vibration by taping or clamping.
Carefully bend tubing away from contact where
possible.
Scroll compressor rotating in reverse.
(3-phase)
Rewire for opposite rotation.
Worn or damaged compressor. Replace the compressor.
Improper mounting on unit base. Check that compressor is properly isolated.
Always have a completed Pre Start-Up Checklist, Unit Start-Up Checklist and Optional Accessories Checklist prior to
requesting parts or service information.
32 Model MPX Make-Up Air Unit
®