Copyright Goodheart-Willcox Co., Inc. 390 Motors and Electric Control Systems • From control inward. This method begins with testing the control devices and then progressing to the load device. • Divide and conquer. Divide and conquer can be helpful on complex systems. This method is similar to the hopscotching method, but instead of checking each switching device from left to right, the divide and conquer method begins with a voltage measurement at a switching device near the middle of the ladder diagram rung. If the correct voltage is measured, the technician knows that the switching devices between the load voltage (left rail on ladder diagram) and the measurement location are closed. Thus, the technician can focus on testing the switching devices on the right half of the ladder diagram rung. The following sections describe another troubleshooting technique and provide a basic example of electrical trouble- shooting. However, technicians use many other troubleshoot- ing methods. Good troubleshooting methods use a systematic approach to identify the root cause of the problem. Bad trou- bleshooting methods often involve making decisions based on incomplete information, such as “swapping out” parts without addressing the root cause of problem. 17.4.1 Convergence Method One method of troubleshooting developed during the early NASA space missions is known as the convergence method. In this method, the operation of a system is broken down by each component and its relationship to the component before it. For example, in a simple circuit with a switch and a light, the source voltage must be present, then the switch must be closed, and finally the light bulb must be operating correctly. An IS-IS NOT chart is a tool used in the convergence trou- bleshooting method. An IS-IS NOT chart includes the problem Lifelong Learning HVACR systems and equipment are always evolving due to the integration of technology and the continuous effort to improve system efficiency. Systems developed in future years will have different service needs from the needs of today’s systems. Throughout your HVACR career, continue improving your troubleshooting skills by remaining informed about the latest systems through company training, manufacturer training, and online sources of HVACR industry information. Pro Tip OFF A COM V Ω A V Ω Fault (open) is in this part of the circuit Measurement #1 240 V Measurement #3 240 V Hold probe in place while moving other probe from terminal to terminal Measurement #4 0 V Measurement #2 240 V Goodheart-Willcox Publisher Figure 17-21. To use the hopscotching method, keep one probe connected to the load side and then use the other probe to “hopscotch” over each switching device moving from left to right on the ladder diagram. A 0 V measurement indicates that the switching device that was just “hopscotched” (or the wiring or connections between the checked point and the previous checked point) is open.