How to Diagnose a Car Electrical Problem

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Electrical problems should be diagnosed by looking at an electrical load to determine if there is a source of power. Discover how to check a car's fuses if it's experiencing electrical problems with help from an ASE-certified master automobile technician in this free video on diagnosing car electrical problems.

Part of the Video Series: Auto Repair & Maintenance
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Video Transcript

Today we're going to talk about the basic procedure of diagnosing an electrical problem. Sometimes something in your car won't work. And you need to determine why. And in most cases it's either an electrical supply problem of some kind or the component itself doesn't work. So the component itself we refer to as an electrical load. So a light bulb, the part of a circuit that's supposed to do the work in making light is a load. A motor could be another load as well. So one of the first steps that you'll do in these circumstances is actually go to the load and find out whether you've got power there. In some cases you'll check the fuses first. But it'll depend on the accessibility of the load. In this case the load is pretty easy to get to that so we're going to check that first. So what we'll do is we'll take a typical automotive test light and we'll hook it in parallel with the load to determine if there is the presence of both battery power and ground. So in this case I can see where the wires come in. And a light circuit like this is pretty simple. So we'll first take and hook it up to the ground lead on one side and the test light on the other. And it wont make our test light light up so that's the indication that there's something missing. So then we need to determine what it is that's missing. And in this case I have a tool that is hooked up to the battery directly so we can determine if we have power or if we have ground. In this case I hook up to the ground connection and the green light and the tone indicates that we do have ground on the ground wire. Yet when we go to the positive side where it should make a different tone and light up red it still shows a ground. So that's an indication that we do not have battery positive making it to the loaded cell. So we need to check upstream. In this case we need to check the fuses. Our next step of the process is actually to check the fuses in the car. And so I'll remove the top of the fuse cover to gain access to the fuses inside. And I'll demonstrate in this case, this type of a fuse has two little test ports where we can check and see what we have for a connection. And on this one we're showing 12 volts on one side and 12 volts on the other. And that's an indication of a good fuse. When we check the fuse for the suspect circuit we find, in this case we check one side and we still show the battery positive. The other side of the same fuse is showing a ground. And this is an indication that what we have is a bad fuse. So if we remove that fuse and we take a look at it we can see that there's actually a missing gap of fuse. A fuse is designed to melt when too much current flows in the circuit and that appears to be what's happened here. So in this case we'll get another fuse and reinsert it and check. We now have battery positive on both sides of it and the trunk light is returned to working. But that's not actually the end of the story. Fuses don't blow for no reason. What we have here is an intermittent short. But that's for another video.

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