Technical FAQs

Ask a Question

INTERNAL:How to troubleshoot compressor issues on all APC Cooling units



How to troubleshoot compressor issues on all APC Cooling units.

Product line: 
All APC Compressor Equipped Cooling Units. 

All Serial Numbers.     




Possible Causes

Possible Corrective Steps

Compressor will not run

1) Main Switch Open

1) Close switch


2) Fuse Blown

2) Check electrical circuits and motor winding for shorts to ground.  Investigate for possible overloading.  Replace fuse after fault is corrected.


3) Thermal Overloads tripped

3) Overloads are automatically reset.  Check unit closely when unit come back on line


4) Defective contactor/coil

4) Repair/replace


5) System shut down by safety devices

5) Determine type and ause of shutdown and correct it before resetting safety switch


6) No Cooling required

6) None.  Wait until calls for cooling.


7) LL solenoid will not open

7) Repair/replace LL coil


8) Motor electrical trouble

8) Check motor for open winding, short circuits, or burnout


9) Loose wiring

9) Check all wire junctions.  Tighten all terminal screws.


10) Trouble in starting switch or control circuit

10)  Close switch manually to test
power supply. If OK check control
circuit including temperature and
pressure controls


11)  Seized compressor

11) Repair/replace


Compressor noisy/vibrating

1) Check for liquid or slugging

1) Check expansion valve(s); check oil levels


2) Improper piping supports

2) Relocated, add/remove hangers


3) Worn compressor

3) Repair/replace


High Discharge Pressure

1)  Air or non-condensable gas in

1) Remove non-condensables


2) Insufficient cooling of air-cooled condenser

2)  Check fan operation, cleanliness of
condenser, and for adequate source of air flow


3) System overcharge / Too much liquid in receiver

3) remove excess


4) Discharge service valve partially closed

4) Open valve


5) Fan not running

5) Check electrical circuit


6) Head pressure control setting

6) Adjust


7) Dirty Condenser coil

7) clean

water/glycol only

8) Insuffiecient water flow/failed pump

8) Increase gpm/repair/replace

water/glycol only

9) Inlet water warm

9) Adjust/fix/repair

water/glycol only

10) Fouled HX

10) Clean/flush, replace

water/glycol only

11) Actuator/3-way valve failed

11) Repair/replace


Low Discharge pressure

1) Faulty condenser temperature regulation.

1) check condenser control operation


2) Insufficient refrigerant in the system

2) check for leaks, repair, add charge


3)Low suction pressure

3) see corrective steps for low suction pressure.


4) Variable head pressure valve

4) check valve setting


5) Hot gas by pass operation

5) check operation


High Suction pressure

1) excessive load

1) Reduce load or add additional equipment


2) Expanve valve overfeeding

2) Check TEV bulb, regulate SH.


Low Suction Pressure

1) Insufficient refrigerant in system

1) Check for refrigerant storage


2) Expansion valve power element dead, malfunctioning, or weak.

2) Replace expansion valve power element


3) Low refrigerant charge

3) Locate and repair leaks. Charge refrigerant


4) Improper adjustment of expansion valve

4) Adjust valve to give more flo


5) Condensing temperature too low

5) Check meas for regulating condensing temperature


6) Clogged suction line

6) Clean


7) Clogged LL filter drier

7) Remove/ replace


Compressor Short Cycles

1)  Low refrigerant charge

1) Leak check and add if necessary


2) No load

2) To prevent short cycling, if objectionable, need to add false load


3) Unit is too large for load

3) Reduce compressor speed; Install false load system


Compressor runs continuously

1) Shortage of refrigerant.

1) Repair leak and recharge system.


2) Leaking discharge valves.

2) Replace discharge valves


Additional comments to review in the event of a high head pressure event:

No control voltage feeding the swtich  /  meter and review the control transformer check for loose wires and that grounds are secure
Obstruction in the discharge piping  / Severe overcharge of refrigerant and or excessive subcooling
Condenser fan stops  / (motor overheats, bearing failure, fan blade damaged, blown fuse, lighting strike, loss of control signal, pressure transducer failure)
There are several things that can cause a HHP event to occur such as loss/lack of water flow, air in the system, clogged strainer, fowled HX, loss of voltage to the HHP switch and/or input, Loss of drycooler fan, debris/dirty drycooler coil, power loss at the drycooler, LAC valve backing up refrigerant, excessive subcooling to name a few.  Before pulling the charge I would first evaluate the system with all compressors running for SH and SC readings as the switch was tripped on "pressure" for a reason. 





Was this helpful?
What can we do to improve the information ?