Relays are electromechanical devices, meaning they have moving parts and are susceptible to failure. Eventually either the spring or the contacts wear out. When this happens, although the relay may still make its classic "clicking" noise when the coil is energised, it no longer completes the electrical connection between its terminals. Often a relay is at fault when a technician is troubleshooting some larger electrical circuit. Knowing for certain if the relay is the culprit will save a lot of time in the repair process.
Identify the terminals on the relay module. There will usually be five, but many types exist and different styles may be more common for certain applications. Symbols for the terminal name may be present on the plastic enclosure of the relay. NC, NO, and COM, stand for normally closed, normally opened, and common, respectively. Additionally, the coil or electromagnet terminals (usually two) may be marked graphically as a symbol appearing as a loopy coil.
Apply voltage from the 24 volt battery to the coil terminals on the relay using alligator clip jumpers. There is no polarity sensitivity for the relay coils, simply connect the positive side of the battery to one side of the coil and the negative side of the battery to the other. An audible click sound should immediately be heard, as the coil will be energised and attract the relay's internal spring-loaded "common" terminal. This clicking sound is a good sign, but does not necessarily signify a functional relay.
Set the ohmmeter to continuity test mode. Use the meter's probes to measure for continuity through the relay's contact under different circumstances. When the relay's coil is energised, continuity should be available through the common and normally open terminals. When the coil is inactive, continuity should be present through the common and normally closed terminals. If different readings occur or if no readings change between active and inactivate states of the coil, the relay is damaged and should be replaced.
When removing power from the relay's coil, a surge of energy will rush back as the magnetic field around the coil begins to collapse. This phenomenon can occasionally result in a painful, high-voltage electrical shock, even though only low voltages may have been applied to the coil. This is because coils can act as voltage multipliers, so be careful.