So you've got a problem with your fuel pump...or your turn signal flasher...or you're air conditioning compressor. The first thing you should do is turn your ignition switch to "ON" and measure the power supply at the part's wire harness connector. If you find 12v at your accessory, then it's broken and needs to be replaced or repaired. But if you find no current, then you have to go back to the source. Your relay is likely not switching (opening or closing a circuit) properly, thus the part that is not working is not being powered when it should. Of course, this could be a wiring issue. If the relay is getting no power at all, then you'll need to find what wire harness connector has been broken and fix that. But it's safe to assume that is your relay is being powered but failing to switch the accessory it is supposed to, then it needs to be tested on a multimeter and determine if replacement is necessary. Depending on the function of the relay, your car may simply not provide certain functions like turn signaling or air conditioning. But more important relays such as your fuel pump or fuel injector relays will keep you from driving the car altogether.
When your vehicle needs to control one or more circuits with just one signal, or when switching a circuit requires a full 12v but the operating switch uses much lower voltage, a relay is used. It is a way to electronically open or close circuits and supply them with the load they need to power specific parts of your car or truck. A small wire coil wrapped around an iron core steps up the voltage inside the relay, and the resulting magnetic field actuates a small armature that either creates or severs a contact that will supply power to or remove power from a load. Some relays perform this function for several contacts at once, managing multiple circuits (http://electronics.howstuffworks.com/relay.htm).
Changing a relay is super simple. Often times, it may be harder to locate the relay that needs to be changed than it is to actually change it. If you suspect a relay to be at fault, you've probably already tested the load and circuit to make sure those aren't bad. To test a relay, you will need to use the circuit diagram printed on the case, and run a multimeter through the contacts to see if it switches as it is supposed to. If it fails, you know it needs to be replaced. Your relays are usually going to be located in a box under your dashboard or in the engine compartment. They will simply pull right out of ther socket and plug back in.
When it comes to fixing electrical problems in your car, it can be a frustrating experience. Finding the correct part can sometimes be an exercise in trial and error. But when it's a new replacement relay you need, Car Parts Discount has the part for you. We offer original and aftermarket brand relays from all the top manufacturers in automotive electronics. So you can be sure that when your car is down with an electrical problem, we've got the part for you.