Copyright Goodheart-Willcox Co., Inc. 392 Motors and Electric Control Systems 17.4.2 Electrical Troubleshooting Example Troubleshooting procedures evolve as a technician gains experience in the field. Methods such as IS-IS NOT diagrams are helpful while learning the HVACR trade, or when stuck on a very difficult problem. As the technician gets better and better at electrical troubleshooting, it is no longer necessary to write down the IS-IS NOT diagram. The possible failure modes become routine and can be done in your head. To demonstrate this process of elimination, we will troubleshoot a packaged air conditioning unit with a “no cooling” problem. As in the IS-IS NOT example, the ther- mostat display is blank. The technician begins at the power panel, measuring the circuit voltage and checking for tripped circuit breakers. See Figure 17-25. The technician begins the troubleshooting process by first checking for input voltage to the transformer. Volt- meter probes are placed at the primary side of the trans- former terminals, Figure 17-26. The voltage measurement confirms that there is cor- rect voltage at the primary side of the transformer. The technician then measures the voltage on the secondary side of the transformer, Figure 17-27. This measurement confirms the expected 24 V measurement. The technician has confirmed that there is power to both the high-voltage and low-voltage circuits. But there is no power to the low-voltage side to the thermostat. The thermostat is powered from the circuit board in the unit. The technician measures 24 V being supplied to the circuit board, but measures 0 V coming out of the circuit board. The problem has been narrowed to the circuit board. The technician removes the fuse from the circuit board (Figure 17-28) and measures the fuse’s resistance. The resistance of the fuse measures OL (open loop), mean- ing the fuse is open and no current can flow through it. The technician replaces the fuse and confirms that there is now power out of the circuit board. The technician will finish the service call by checking for potential causes of the blown fuse and then starting the system and taking measurements to ensure proper operation. Goodheart-Willcox Publisher Figure 17-26. Measuring voltage on the primary side of a transformer. Note that the measurement is being uploaded to a smartphone using a Bluetooth transmitter. Goodheart-Willcox Publisher Figure 17-27. Measuring voltage on the secondary side of a transformer. This voltage supplies power to the control circuit. Goodheart-Willcox Publisher Figure 17-28. Circuit board fuse being pulled for a continuity check. Goodheart-Willcox Publisher Figure 17-25. Checking power to the distribution center and circuit breakers.