A relay is both a control switch and a control object (actuator). Taking the fuel pump relay as an example, it is the control switch of the fuel pump, but the coil of the fuel pump relay can only form a circuit through the grounding point of the electronic control unit when the driving transistor is conductive in the electronic control unit.

The working principle of automotive relays mainly utilizes the principle of electromagnetic induction, which generates a magnetic field through the electrification of the coil, thereby controlling the closure and disconnection of the contacts. Specifically, when the coil is energized, it generates a magnetic field, and the moving iron is attracted to drive the contact spring, causing the static contact point to separate and the moving contact point to close. When the current of the electromagnetic coil is cut off, the iron core loses magnetism, the moving iron core resets under the action of spring force, the moving contact opens, and the stationary contact closes.