If a relay coil is good, what should the voltage reading be when it is energized?

When a relay coil is energized and functioning correctly, you should observe a zero voltage reading across the coil terminals. This occurs because, in an energized state, the relay coil acts as a near short circuit, thereby allowing full current flow through it. The voltage across the coil drops to nearly zero due to the low resistance of the coil itself, which is characteristic of a good relay.

In typical scenarios, if you are measuring the voltage across the relay coil when it is activated, the potential difference should be very low, close to zero, indicating that the coil is successfully drawing current.

The other readings suggest different states of malfunction or concern. A high voltage reading would indicate that the relay is not properly energizing, which is not ideal. A low voltage reading may suggest that the coil is partially functioning but not to specification, while no reading at all would imply either an open circuit or that no power is being supplied to the coil. Thus, observing a zero voltage across a properly functioning energized relay coil is the expected accurate outcome.