This article presents an example of a systematic troubleshooting method incorporating the symptom-cause method.
Figure 1 illustrates an electrical schematic diagram showing a time clock controlling a defrost circuit and a refrigeration circuit. Notice in the refrigeration circuit that the compressor’s run winding has been opened from a motor overheating problem.
The service call is a “no-cooling” call for a low-temperature walk-in cooler. Once the technician quickly looks the system over and listens for any clues of what the problem may be, he should study the electrical schematic, if available. Understanding the logic or sequencing of the circuits before diving head over heels into the problem is of the utmost importance.
In this scenario, an open run winding gives certain symptoms that will not exist for other possible system problems. For example, the technician listened to and examined the refrigeration system, then studied the electrical schematic drawing. He then listed the symptoms.
The technician then turns power off to the refrigeration system to let the motor cool down. After studying the electrical diagram again, the technician lists some possible causes that will correlate to every symptom listed. If a possible cause does not correlate to every symptom listed, it cannot be a possible cause.
Notice that every possible cause listed correlated to all of the symptoms. All the technician has to do is check only these six possible causes to find out which one is causing the symptoms, instead of blindly checking out the entire system.
With the power off, the technician takes a wire off of the start winding and ohms the winding. The technician finds that it has 4 ohms, meaning that it is not open. The technician now takes a wire off of the run winding and ohms that winding. The technician finds that it is an open winding because of the infinity reading on the ohmmeter. The compressor has to be replaced. With either winding open, the compressor has no phase shift for starting and will lock its rotor, drawing LRA until the overload trips.
If either capacitor was bad, the motor may not have had enough phase shift to start. This is especially true if the motor was under a high load. In certain cases, the motor may slowly turn. If the compressor was mechanically stuck (for example, if something was wedged between the piston and cylinder), the motor would lock its rotor and draw LRA.
If the contacts between terminals 1 and 2 of the potential relay were stuck open for some reason, the start capacitor would be out of the circuit. This again probably will not cause enough phase shift to start the motor turning. The motor would again draw LRA.
Notice that in every case, all symptoms were met.
IMPROBABLE CAUSESWhat about an open overload, or an open potential relay coil between terminals 2 and 5 of the potential relay? If the overload were opened at the beginning, maybe from too high of a compressor amp draw, the compressor would not hum or draw LRA. This would not correlate with all of the symptoms listed and could not be a possible cause.
If the coil of the potential relay was open, the contacts between 1 and 2 of the potential relay would stay in their normally closed position and would not open. This would cause the start capacitor to be in the circuit all the time. The motor would turn, drawing higher-than-normal amperage, and eventually open the overload. These symptoms do not correlate with the original symptoms listed, thus they cannot be a possible cause.
Once the technician has replaced the compressor and the system is up and running, it is important to run a system check to see what caused the compressor overheating that opened the winding. Evaporator superheat, total superheat, condenser subcooling — along with suction pressure and head pressure — must be taken for the system check.
In this case, the technician took a system check and found the evaporator superheat to be very high at 40 degrees F and the total superheat to be a very high 90 degrees. Condenser subcooling was fine at 12 degrees. Both suction and head pressures were low (see Table 1).
THE SOLUTIONThe problem that caused the overheating was a faulty thermostatic expansion valve. The valve would not open enough and the entire low side of the system was being starved. The compressor was refrigerant-cooled. This caused the compressor to overheat and cycle on its overload until the run winding finally opened.
Without this final system check, the new compressor surely would have failed in a short time.
Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and the author of Troubleshooting and Servicing Modern Air Conditioning & Refrigeration Systems, published by ESCO Press. To order, call 800-726-9696. Tomczyk can be reached at firstname.lastname@example.org (e-mail).
Publication date: 06/03/2002