3 Rookie Compressor Diagnosis Mistakes

A common rookie mistake is failing to do a proper visual inspection. In some cases, technicians diagnose a shorted compressor because it’s tripping the breaker, but they don’t actually take measurements at the terminals. To check for a short, you need to inspect the terminals, not just the contactor. If the terminals have been damaged, you can typically repair them.

Another rookie mistake is not allowing the compressor to cool off. Compressors can go into thermal overload because of a running condition, and they can stay in thermal overload for a long time. In many cases, the shell will also stay hot for a while. A locked compressor (such as due to a bad capacitor) will go out on thermal overload very quickly (and come out relatively quickly).

However, it may take a long time for the compressor to come out of thermal overload in cases that cause low suction pressure, low mass flow rate, and high compression ratio (examples include cases of a low refrigerant charge or a restriction). To speed up the cooling process, you can often use a hose and run cool water over a hot compressor.

Technicians also make rookie mistakes when they check the resistance from terminal to terminal instead of from a terminal to ground. You may find an open winding or thermal overload, but it’s very difficult to find a short due to the low resistance. Compressors have inductive reactance as a major part of their total resistance, which makes it very difficult to use Ohm’s law in practice. Instead, ohming a compressor to ground is more effective.

Then, you can use isolation diagnosis to isolate the compressor from the rest of the system. Once you’ve isolated the compressor from the system, reset the breaker and run the system. If the system runs without the compressor in the circuit, then you can be sure that the issue lies with the compressor. Using a megohmmeter is also often impractical, especially on scroll compressors.