McQuay MWM Service Manual Book page 59

By means of diagnostic flow chart:
Generally, there are two kinds of problems, i.e. starting failure and insufficient
cooling/heating. "Starting failure" is caused by electrical defect while improper application
or defects in refrigerant circuit causes "Insufficient cooling / heating".
i ) Diagnosis of Electric circuit
Faulty
:
No cooling/heating ( Unit fails to start )
Evaporator
fan motor
Unit power
supply
Fuse for
operation
Running
Over current
relay for
evaporator
Evaporator
fan contactor
Condenser
fan motor
Over current
relay for
condenser
Running
1
:
Stop
Faulty
Blown
circuit
Tripped
fan
Faulty
Stop
Tripped
fan
2
Check Cause
:
No voltage or low voltage
Wrong phase/missing
phase
Components shorted
Connections loose
High or low voltage
Wrong phase/missing
phase
Coil burned
Contact faulty
Faulty fan motor
Defective operation
No voltage or low voltage
Wrong phase/missing
phase
Remedy
:
Get the right voltage
Repair the power line
Repair or replace
the components
Tighten the connections
Get the right voltage
Check the power
supply to the motor:
repair when necessary
Change the contactor
Repair the contacts
Repair or change
the motor
Repair or change
the switch
Get the right voltage
Check the power
supply to the motor:
repair when necessary
58