How Do Fuel Pump Relays Work?

Fuel pump relays control the electrical circuit that is connected to fuel pumps, regulating when a car's systems need refueling. The essential purpose of this relay is to act as a kind of automatic switch in the electrical system on your vehicle that can handle high current loads and help extend the life off your pump. They are generally good enough to handle the high load on power that is needed for a fuel pump carrying around 30–40 amps. On most factory setups, the relays switch on and power up the pump during cranking but then keep it powered while the engine is running.

Photos courtesy of PicoScope2) RE: Alternator & Charging, The relay oper-ates with an electromagnetic coil and switch mechan-ish. When cranking the engine over, power briefly goes to that coil and creates a magnetic field (in turn closing or opening the switch inside this relay) and allows current flow through that same line from ECU -briefly energizing the fuel pump. This configuration ensures that the ignition switch does not bear a heavy current load (ensures long life) and protects electrical components in the vehicle from overheating or failure. In performance automotive applications, may be necessary to have a reliable relay allowing the fuel pump to run at the 255 liters per hour needed in high flow turbocharged engines every time.

Relay failures can cause problems like stalling the engine or starting difficulties. If a relay is stuck open, the pump won't receive power; if it's closed, even with the engine off and keys out of the car (can drain your battery);. This is why regular relay inspections every 50,000 miles can help prevent such issues and in time replacements guarantee the fuel pump gets constant power.

The details outlined above for fuel pump relays show there are certain nuances in the design, such as making sure it doesn't melt or stop working when everything is hot under a hood. Fuel Delivery—An unreliable relay can result in fuel pump efficiency losing as much as 15% effectiveness, which affects the amount of power used to flow through and consequently results in engine performance. Automotive impresario Carroll Shelby is famously quoted by saying "Power is everything. A healthy relay and the power it supplies a Fuel Pump is vital for prolonged life accompanied by good operation, especially when forces taxing on your ride are at play.