What Is a Fuel Pump Relay?by Contributor
The fuel pump relay is a small unit covered in a plastic case, which has several sprockets on one side that are clipped onto the ignition system. The fuel pump relay is what turns on the fuel pump when you start your car and powers it down when you shut off the ignition.
The engine control module (ECM) uses the fuel pump relay to control when the fuel pump is on. The fuel pump relay supplies electricity to power up and power down the fuel pump.
Check your owner's manual for the location of your fuel pump relay. It's located in different places on different cars. It could be under the hood, on the firewall, on one of the fender covers or near the steering column, to name a few locations. If you don't have an owner's manual, you may be able to find the information you need online by checking automotive and car part forums.
When you turn the key in the ignition forward to crank the car, the "start cycle" begins and fuel pump relay is engaged long enough to turn on the fuel pump. It shuts off again when the engine is running. Once the engine is running, the fuel pump relay shuts off, and the current for the fuel pump is supplied by the oil pressure sending unit. The other time the fuel pump relay is engaged is when you shut off the ignition. It engages long enough to shut off your fuel pump.
If engine takes a long time to crank up, it probably is due to a bad fuel pump relay. This is because the fuel pump relay isn't turning on the fuel pump during the start cycle. The engine eventually will start, but not until the oil pressure builds up and the oil pressure switch kicks in to power up the fuel pump. The oil pressure switch acts as a backup to the fuel pump relay. Another way you can tell the fuel pump relay is bad is when you don't hear the fuel pump run for a couple of second when you turn the key in the ignition forward to start the car.
Replacing the fuel pump relay may not fix the problem because the source of the trouble may be the fuel pump. Here are some common signs of a faulty fuel pump: the fuel pump turns on, but fuel pressure is low; a high-pitched sound occurs when the engine starts; acceleration is uneven (this is also a sign of a bad fuel filter, so have the fuel filter checked first, in this case); the car acts like it's out of gas even though the tank is full; the vehicle sputters, dies and won't restart.