I see a lot of "my ______ isn't working, what's wrong?" posts here. So instead of asking whats wrong, I will teach you how to diagnose the problem yourself. Electrics problems are usually not very difficult to figure out with the proper knowledge and tools.

You will need a test light, so go buy one. They are cheap, like $5 at any auto parts store. Just ground the lead and start probing. If the light lights up, there is power there, if not, then there is no power there. Always probe a known good power source first to make sure that the test light is working and your ground connection is good.

Now, on with the lesson....

P.S. This is a fictional circut with made up wire colors. Don't post "I can't find the blue wire in my car" or I will laugh.

 

Here is a basic headlight wiring diagram. You have a battery, fuse, switch, relay, and two headlights. Lets's assume that the headlights are not working in this diagram. Lets break out our test light and start testing circuts.

Hold on, the pics arent loading right....

 

Ok, here's that headlight circut. Once again, you have a battery, fuse, switch, relay, and two headlights. Remember, the headlights aren't working, both of them. Lets get started.

 

First, lets test the fuse. You can test it without removing by using the test light. Just probe the two metal inserts on the top of the fuse.


Lets look at the diagram. If there is power at the blue wire and not the other end of the red fuse, then the fuse is no good. Replace it.

 

Ok, lets assume that the fuse was good. Lets continue on and probe the back of the switch. If there's no power here at the green wire, you have a break in the green wire itself.

If there is power there, move on the the purple wire at the back of the switch. With the switch on, if there's no power then the switch is bad. If there is power there, then the switch is good and the problem lies somewhere else.

 

Ok, the switch was good. So lets test the relay. Leave the headlight switch on and test for power at the orange wire. Once again, if there's no power here then there's a break in the orange wire since there was power above at the purple connection at the back of the switch.

If there is power at the orange wire, test the other side of the control circut at the red connection. No power here means the relay is bad.

 

By now, I'm sure you are seeing a pattern. We are testing things in order- from positve to negative. So if there is no power at any point, then the problem is just before where we last checked. Here are the rest of the tests that need to be made. As soon as you stop seeing power at a certain point, then the problem lies in that colored component.




Here's a trick to speed up the process. Always check the fuse first, but once you do that, just go right to the relay. Unplug it, and with a paper clip, jumper the hi current side of the relay. This is a quick relay check, since the relay is often the source of electrical problems. CAUTION: The paper clip will get hot fast, so be careful

 

This is just an example of a circut. You can substitute the headlights for a cooling fan motor, or a power window motor, etc. Every circut will have a power supply, a switch, and an electrical divice. Some circuts will have relays and some won't. Some switches are controled automatically and some are controled by you. A good example of this is a cooling fan. You don't turn it on, the ECM does, so the ECM would be the switch. Just remember that every circut is basically the same.

Here are some helpful tips. First, broken wires are rare. It is far more likley that an actual component like a swtich, relay, or electricl motor has failed since it has moving parts. Moving parts have a lifespan, they rarely last forever.

Wires have no moving parts so they tend to last longer. If there is aftermarket wiring installed in your car and it is not properly secured to something, then it will vibrate and move, becoming a moving part, and can fail.

When probing with a test light, never pierce a wire with the probe. This will allow moisture to enter the wire, and eventually it will corrode, break, and give you electrical gremlins down the road. Always unplug the connectors to check them or backprobe the connector if you can.

NEVER BACKPROBE ECM CONNECTORS!

And above all, use common sense. If you are unsure of what you are doing, then stop and let someone else fix it. If you start smoking components and burning up wires, you are creating more problems and increasing the cost of the repair. Hope this helped!