Troubleshooting the Open Faults in Series Circuit

One of the most commonmodes of failure is the “open” circuit. A component, such as a resistor, can overheat due to the power rating being exceeded. Other more frustrating problems can happen when a “cold” solder joint cracks leaving a wire disconnected from a relay or connector. This type of damage can occur during routine maintenance after a technician has accessed an area for inspections. In many cases, there is no visual indication that a failure has occurred, and the soon-to-be-frustrated technician is unaware that there is a problem until power is reapplied to the electronics for scheduled operations.

The first example is a simplified diagram shown in Figures 165 through 167. The circuit depicted in Figure 165 is designed to cause current to flow through a lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter should be used.