by 
The Curious Bag
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Though some custom systems have been built without using them, almost all cars utilize some sort of fuel pump relay switch. These simple devices are responsible for turning on the fuel pump, so a bad one can quickly render your vehicle useless.
History
The first primordial fuel pump relay switches were invented at the dawn of electricity in the 1890s. They've been used on practically all combustion-engined vehicles since.
Function
A relay takes a small electrical signal like the one from your ignition switch and uses it to close a more powerful circuit.
Appearance
Relays are usually made of black plastic, are shaped like cubes and measure approximately 1 to 1.5 inches on a side.
Location
Most fuel pump relays can be found in the black underhood fuse panel. The diagram label stuck to the bottom of the fuse panel lid should give an exact location within it.
Failure
A bad fuel pump relay will cause either a "dry" no-start condition or will cause the engine to stall if it fails while in use.
Source:
Allexperts.com: 1982 Corvette Relay Repair
Car Craft Magazine: 1969 Mustang Fuel Pump Discussion
VW Vortex: Fuel Pump Relay Failure Discussion
Resource:
youtube.com: Testing a Pump Relay
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