A fuel pump relay is an electrical device much like an automotive fuse. It directs current to the electric fuel pump and ensures that the part receives enough power to operate properly, but not too much as to damage it. Fuel pumps require a lot of power to operate and this could cause overheating of the wiring system and the pump if were not for the relay.
In most automobiles, the fuel pump relay is underneath the vehicle's hood and attached to the firewall. It is a small black plastic box with a wiring harness plugged into the bottom of it. The relay operates as a monitor for the power going to the fuel pump, activates it when the ignition is turned on, and turns the pump off when the vehicle is turned off. The fuel pump can be heard humming when the ignition switch is first turned to the on position. This provides fuel for the vehicle to start up, and once the vehicle is running, the vehicle's computer sends a signal, and the fuel pump relay turns off. The vehicle's fuel pump will now be powered through another source, typically in the oil pressure sending unit.
To test the relay in the event of a non-starting vehicle, drivers can turn the key to the on position and listen for a buzzing or humming sound coming from the vehicle's fuel tank. If there is no noticeable sound, the problem could be the fuel pump or the relay. It is much easier to test the relay than to change the fuel pump.
In most vehicles, there will be two or more relays in the same location. Most are the same, and the vehicle's owner's manual will provide information about where they are. A driver can unplug the relay to test if it's causing a problem. If there is another relay next to it, he can unplug it and plug the fuel pump wiring harness into it. He should then turn the vehicle's key to the on position and again listen for the sound of a running pump.
If the pump runs, it is time to buy a new relay at the auto parts store. If the pump still isn't working, the driver needs to check the fuel pump. This will require removing the fuel tank. Individuals should use caution when working around fuel and be sure to keep the work area clean to prevent accidents.