In this month’s troubleshooting situation, our problem involves a 4-pin compressor that uses both an internal and external overload protector. Two other factors in regard to this particular piece of equipment are: A single-pole contactor is used in the compressor circuit, and it is equipped with high and low pressure switches that act as protective devices.

Upon your arrival and conducting an initial inspection of the equipment, you find the following conditions:

1. The compressor is cool to the touch; and
2. The condenser fan motor is not operating.

As you begin your troubleshooting process, you make two quick voltage checks (see the partial diagram in Figure One) and you find the following:

1. 230-volts at L1 and L2 of the contactor; and
2. 0-volts at the contactor coil.

With these voltage tests accomplished, the next step you decide to take is to disconnect the power supply, remove the appropriate wiring, and use an ohmmeter to make resistance and continuity checks, which reveal the following:

1. Infinity from C to T;
2. 2.5 Ohms from C to R;
3. 3.5 Ohms from C to S; and
4. 6 Ohms from S to R.

Your troubleshooting question: What is the specific failure that is preventing this compressor from operating?