A controller with an operational display but fails to energize one of it loads (e.g., compressor, fans, defrost heaters) is also generally easy to troubleshoot. Using the wiring diagram from the controller or the equipment wiring, you should be able to electrically isolate the failed section on the controller and confirm the failure by temporarily bypassing the failed relay or triac on the controller.

However, there are definitely times when working on these controllers can be frustrating. A controller that randomly turns a load on or off can be tricky to determine whether the controller needs to be replaced or if one of its programmed parameters is set incorrectly. For these problems, you will need to look at the programmed settings of the controller and see if a parameter is incorrectly set or if a sensor is causing the issue.

Find the Manual

To do that, you will need to get the manual for the system or the controller, which can be difficult — although hopefully the right manual will be online. Sometimes there are several variations of these controllers used by manufacturers, and finding the right manual for the controller you are working on can be a problem. This is especially true when you are working on older equipment, and the information is no longer available. In this case, you may need to call the manufacturer to try to get this information. This can also be very frustrating, as the information they send you is often generic and not helpful at all.

Recently, I was working on a reach-in freezer that had randomly been turning off the evaporator fans. Many of these controllers will turn on the evaporator fans at a specific evaporator temperature. I needed to get into the controller’s setting and see the actual temperature of the evaporator’s temperature sensor to determine whether the evaporator was not getting cold enough or if the sensor was not reading the correct temperature. It took a while to find the right manual. This particular controller manufacturer had a lot of information online, but they also make a lot of different variations of these controllers, so finding the right manual took some time. After finding the manual, I was able to determine that it was a faulty controller.

Another time, I was working on reach-in freezer that would not initiate a defrost cycle. At first, it seemed a simple diagnosis: a defective controller. However, after finding the right manual, I was able to determine the evaporator sensor was the issue. The controller was “seeing” an evaporator sensor value of 99°F and would not allow the initiation of a defrost cycle. After changing the sensor, the defrost cycle worked fine.

Obtaining the right manual for these controllers is very important. Using the wrong manual can lead to wasted time on the job and possibly misdiagnosing a problem. Once you find the right manual, save it. It could be very helpful on another job or if you need to work on the system again.

Making the Repair

Changing out these controllers can also be problematic. It is always best to use the OEM controller, as it will make the repair quicker and easier. However, the OEM part may not be readily available or may cost an excessive amount of money. As another option, you can use a universal replacement; however, you will need to very carefully assess if the controller you select will work.

Many of these reach-ins use a cube-style controller, and changing these out to a universal replacement is generally easy if you follow both the directions and the wiring diagram of the universal controller. However, some cabinets use very specific OEM parts with a separate PC board and display. Replacing these controllers with a universal replacement is doable but will require major rewiring of the system and mounting of the new controller, and it may not have all of the features of the original controller.

So be careful — what seemed to be a doable repair becomes a monster of a job to complete. Sometimes, it is best to either wait or pay for the OEM controller.