A relay coil has voltage but will not energize. What might be the issue?

When a relay coil has voltage but does not energize, one of the primary concerns is the integrity of the coil itself. If the coil is open, it means there is a break in the wire winding, preventing electrical current from flowing through the coil. The coil is designed to create a magnetic field when energized, and without a complete circuit, this field cannot be established, which is why the relay will not activate.

Having voltage across the coil indicates that power is being supplied; however, if the coil is open, no current can flow. This is a common failure mode in relay coils due to wear, damage, or manufacturing defects.

The other options suggest different issues but do not directly correlate to the symptom of having voltage without activation. A defective switch would prevent voltage from being applied in the first place. A short circuit would result in too much current flowing, potentially damaging the relay or blowing a fuse. High resistance would limit the current as well, but typically not to the point of having no energization unless the resistance is extremely high or the coil is open. Thus, the most straightforward explanation for the relay coil not energizing, despite the presence of voltage, is indeed an open coil.