A fuel pump relay's job is to take some of the voltage load off of the wiring harness to allow a better current flow from the battery to the fuel pump. Determine whether a fuel pump relay is bad and needs to be replaced with help from an auto mechanic in this free video on testing car performance.
How to test a fuel pump relay. A fuel pump relay -- its job is to take some of the voltage load off of the wiring harness and allow a better current flow directly from the battery to the fuel pump. Fuel pumps draw a pretty significant amount of voltage and the relay simply allows that voltage to flow easier. There are many different styles of relay. This is a pretty common one. This is a Bosch 12 Volt cube-style relay. Some of the relays are going to be round, some of them are square. Again, they all function basically the same. What you're going to want to find on the relay -- there's four pins that you need to be concerned with. One pin is going to be the ground pin, one pin is going to be the input voltage pin, and then one pin is going to be your load pin -- that goes to the fuel pump -- and one pin is going to go to the battery. And what you want to do is using your multi-meter, you want to put it to resistance mode if you have some kind of audible alarm, that's nice to have. It gives you a...an audible indicator when you have continuity. And you want to put your probes on the connector that goes to the fuel pump and the connector that goes to the battery. And then you need to find a 12 Volt power supply. A battery works fine. Anything that's providing 12 Volts, just for test purposes, is all you need. I have a small voltage regulator that I use that I...that I will put on the relay. And so you connect your ground connection from your 12 Volt power supply to the ground on the relay, and when you touch your 12 Volt trigger to the 12 Volt input, the relay will close and you'll get continuity between the two posts that the meter are connected to. You will also hear a clicking sound as the solenoid inside the relay, which is just a magnetic winding, pulls the relay shut and allows it to connect those two posts together. So if you do not get continuity and you don't get that clicking sound, then the relay is most likely bad and needs to be replaced.