An Open Thermal Overload on a Compressor

Often compressors are diagnosed as defective due to an open thermal overload. The purpose of the overload is to shut off the compressor when it gets extremely hot.

Imagine this scenario: A technician is called due to no cooling in the home. The tech finds the compressor is not running. He checks the continuity between wires using a multi-meter and discovers an open between the R&C and S&C. It’s a rare occurrence that both R (run) and S (start) windings are bad. The tech does a couple of other checks, one of which is the continuity between R&S. The reading is 2 ohms. That means the R and S windings aren’t open as first suspected. Why might this be the case?

Looking at the wiring schematic of a compressor, you will see there is a thermal overload in the common circuit. When the overload opens, you will not get continuity between the common terminal and the R or S terminals.

During the process of checking continuity, the tech did notice the compressor to be hot. You will need to let the compressor cool off, so disconnect power and wait until the next day (or if you’re in a hurry, spray it off with a hose) to recheck the windings of the compressor. This time the compressor ohms out correctly. Now the tech needs to determine why the compressor overheated.

An overload might open due to:

  • A lack of maintenance. If the outdoor coil gets dirty it can raise the head pressure and make the unit run hot.
  • A defective run or start capacitor. These two items need to be checked.
  • Inadequate voltage, especially on 3 phase.
  • A lack of refrigerant. Refrigerant helps cool the compressor windings. 

Do your due diligence to figure out why the compressor is overheating and you won’t misdiagnose the problem and replace a perfectly fine unit.