Refrigerator Compressor Testing

With the compressor being the most expensive component in the system, it is wise to be sure it is bad before you condemn it. For this reason, you should learn a systematic method of diagnosing the compressor. You will need a good ohmmeter that can measure from one ohm through 20 ohms. A meter such as this can be purchased in the price range from $20 to $125 depending upon the quality. The homeowner can get by with the less expensive one due to the fact his will not be subjected to the amount of usage the ser­vice technician will give his.

This procedure is followed if the compressor doesn’t operate when called to do so. Always remember safety comes first. Before opening the condensing unit, turn off the electrical power supply to the unit. With the service panel removed, look with your eyes before you touch anything. There should be some type of flash cover enclosing the terminals of the compressor. This is a basic safety de­vice to protect the service technician from electrical shock when the unit is operating, and it protects the technician from pressure-driven oil if one of the terminals should fail and blow out of its mount. Don’t assume that oil in a unit is clean. In most units the refrigeration oil is clean; however, in some instances, the oil has become contaminated. The formation of sulfurous acid might have occurred inside the inoperable unit. This acid can be dangerous to your skin and eyes. For this reason, don’t assume anything; be sure and careful. With the cover removed, you will notice that the terminals are arranged in the approximate order as shown in Fig. 3-11. The following is a list of electrical failures that you will test for.

a—Grounded compressor (short circuit). This condition takes place when the insulation of the drive motor windings leak the electricity to the steel compressor body. Blown fuses result.

b—Open-winding. A condition that occurs when the conductor of one of the motor windings parts.

c—Locked rotor. This condition happens when either the crankshaft bearings seize due to lack of lubrication, or a compression component breaks within the compressor shell jamming the crankshaft. In the case of a single-phase unit, the same locked-rotor condition will be witnessed if the system has a defective starting component.


The multimeter enables us to diagnose internal electrical problems of the compressor. “Ringing out a compressor” means taking continuity tests. Make sure the power is off. If necessary, turn your meter to ac volts and check it out. Sometimes a disconnect leaves a blade engaged that has broken loose from the main control bar. After you check for voltage, mark the wires that are connected to the terminals so they may be returned to the same position when re-assembled. There are many ways to mark them: different color tape, black bands of electrician’s tape, notch with knife blade. What­ever works for you is suitable. The wires must be removed from the compressor to prevent voltage from other circuits. For instance, a 240-volt compressor might have a 120-volt condensing fan motor. It is possible that you might read the neutral leg of the 120-volt circuit as a grounded compressor.

Zero your meter. Make sure that it rests at infinity. This is done with the little screw at the base of the indicator needle. Then touch the two probes together and turn selector switch to X1000 scale. The needle should deflect to zero ohms. If not, make adjustments with the little knob to set your meter to zero.

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