Relays act as remote‑controlled switches that let a low‑voltage circuit control a higher‑voltage load. Because they can fail at the coil or the contacts, a systematic diagnostic test can quickly isolate the problem and keep the system running safely. Once a faulty relay is identified, replacement is straightforward—usually just a few screws.
Close the S1 switch and check that the relay applies 110 V AC to the test light bulb. With the meter set to AC voltage, confirm 110 V across the bulb terminals. If the bulb is not illuminated despite the voltage, the relay is likely defective.
Measure the supply voltages at the relay terminals: 110 V AC for the load side and 12 V DC for the coil side. If either input is missing, the relay will not operate correctly. Presence of both voltages indicates that the relay itself may be at fault.
With power isolated, measure the voltage drop across the relay’s contacts where the 110 V AC enters and exits. A zero‑volt reading shows proper contact closure; a high voltage indicates burnt or damaged contacts that must be replaced.
Remove the relay from its bracket and, with power off, use the multimeter to test the ground continuity on the pulldown coil circuit. A resistance of zero ohms confirms a good ground; any deviation suggests a faulty coil or wiring.
Measure the resistance across the coil itself. A reading of zero ohms confirms a shorted coil, while an open circuit (no continuity) indicates a broken coil that requires replacement of the relay.
Always exercise caution when working with electrical circuits. Ensure all power is disconnected before inspecting or handling components.
