This is a quick, real-life diagnosis/find by Kalos Services senior commercial HVAC/R tech Eric Mele
Improper Defrost Timer installation
While servicing this walk-in cooler for an unrelated problem, I noticed the defrost timer installed in a way that will almost certainly cause early compressor failure.
Because this is a cooler (and not a freezer) it is set to defrost by simply shutting off the compressor four times per day while still running the evaporator fans. If you look at the timer you will see that the little pins are flipped four times per 24 hr period.
If you are not familiar with walk-in coolers, and freezers, they are designed to pump down into the condenser/receiver at the end of each cooling cycle by closing a liquid line solenoid and shutting off via low-pressure control. The reason for this is because the areas they control are kept so cold a good amount of liquid refrigerant would migrate to the evaporator in the off cycle.
The timer was installed at the condensing unit and would stop the condensing unit/compressor from running during the defrost event, but would not cause the unit to pump down during defrost. This will likely cause a flooded start at the end of each defrost when the liquid refrigerant rushes into the compressor on startup.
There are no interconnecting wires between the evaporator and condenser, which is common on remote walk-in systems. The easiest way to correct this problem is to relocate the defrost timer into the evaporator, and break power to the liquid line solenoid valve during defrost.
The diagram below shows first how it is currently connected and then how we rectified the situation.
— Eric M.