Start by inspecting the system. Look for any obvious problems or clues that will determine the cause of the failure. This includes looking for what is running and what is not running. The evaporator fan motors should be running, because they are designed to be operational all the time. The compressor and condenser fan motor should be running if the case temperature is above the cut-in value of its temperature control. On these cases, the compressor and condenser fan generally operate together and are powered by the same voltage source. This may not always be true, though; occasionally, the condenser fan motor will be wired separately and have voltage applied all the time. Sometimes this is done by the manufacturer, and sometimes it is done by a previous service technician.

If the compressor is running and the condenser fan is not running, this is likely the cause of the system problem. The compressor will not be running for long in this state, because without the condenser fan motor running, the compressor will overheat and shut down. If the compressor is not running and the condenser fan motor is running, see if the compressor’s body is extremely hot or at ambient temperature. If its body is extremely hot, the compressor is likely cycled off on its overload, and the reason for this will need to be determined. For now, continue with the visual inspection to obtain more information.

Next, look at the condition of both the evaporator and condenser coils. It may not always be easy to examine the evaporator, but it is an important part of the troubleshooting process. An iced evaporator is a common problem and is most often caused by a system that is low on refrigerant or has a temperature control issue.

The icing pattern of a coil can sometimes determine the cause of the problem. Generally, a low refrigerant charge will cause the inlet of the coil to be heavy with ice and, depending on the level of the lost refrigerant and how long it has been left in this condition, the ice will start to thin along the length of the coil. In many cases, there will just be a ball of ice on the inlet of the evaporator only. A completely and uniformly iced evaporator is normally the result of a temperature control issue.

Next, check the condition of the condenser coil. If it is extremely dirty, the compressor will be running hotter than normal. Don’t forget to inspect the case hardware for issues such as damaged hinges, worn door gaskets, etc. Problems with the door and its hardware can lead to excessive air infiltration into the case, leading to high case temperatures and excessive frosting of the evaporator.

If all the motors are operational and the case temperature is high, there is likely an issue with the refrigerant flow throughout the system. Either there is not enough refrigerant in circulation (low refrigerant charge), or the compressor is not providing the required pressure differential to circulate the refrigerant throughout the system. At this point, attach the pressure gauges and measure the operating pressures to verify the exact issue. Since these systems use a relatively small amount of refrigerant, it is best to start off using a single-port pressure gauge. This helps prevent an excessive amount of refrigerant from being released. A saddle valve may also need to be added to the system’s low-side and high-side piping. Generally, these systems come from the factory without any access valve installed.

When possible, repairing these coolers without attaching a pressure gauge to the sealed system can avoid creating unnecessary service issues with the cooler in the future.

Publication date: 7/1/2019

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