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All Info About Auto Repair: Troubleshooting the Electrical System: Part 2
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Troubleshooting the Electrical System: Part 2

What tools you'll need to have and how to do it successfully.

Okay, we have our tools and our wiring diagrams, now what? The most important thing is to fully understand what, exactly, is the problem. Once you fully understand what's happening, take a few minutes to study the block diagram and the schematic. Get a clear understanding of how the system is supposed to work and how the parts interrelate with each other. Now you need to form a plan of attack. Take note of where the connectors are and break the system into blocks. Then test each block and as they check out, go to the next one. Open circuits are fairly easy. An open circuit is when there is a break in the wiring between point A and point B.

Let's go through an open circuit in the brake lights. The brake lights are always hot, that is to say they are powered all the time, key on, off or in your pocket. The best way to troubleshoot an open is from the battery back. In the typical brake light circuit power goes from the battery to the fuse box to the brake light switch to the bulb and then to ground. Take the test light and put the clip on the negative battery terminal and touch the point to the positive battery terminal. This does two things, it checks the battery for voltage and it insures your test light is working properly. There's nothing worse then the feeling you get after spending an hour looking for a problem and finding out the bulb in your test light is burnt out.

Next you check the fuse. The fuse box is usually marked with the fuse amperage and the circuit. For example, Brake lights 10 amps. If the fuse box is not marked, it will be in your owners' manual. Clip the test light to a good ground and touch the point to each side of the fuse. If the light lights on both sides, then the fuse is good. If it doesn't, then the fuse is bad. Replace it before you continue. If neither side lights up, you have an open between the battery and the fuse box. Repair the wire before you continue. Now by looking at the wiring diagram we see from the fuse box it goes to the brake light switch. The brake light switch is a simple on-off switch. When the brake pedal is up, the switch is open. When the brake pedal is down, it closes, completing the circuit. Touch the test light to both terminals of the brake light switch, one side will be hot and the other side will not. If both sides are hot then the switch is not opening and the brake lights are on all the time. As with the fuse, if neither side lights, you have a broken wire between the switch and the fuse. Repair the wire before you continue.

Now push the brake pedal down and touch both sides. Both sides should be hot. If they aren't, the brake light switch is not closing and is no good. You'll need to replace the switch before you go on testing. Looking at the wiring diagram we see the wire goes from the brake light switch to a connector in the back of the car. Go back there and locate the connector and unplug it. Now you see there are 5 wires in that connector. Which one is the brake light wire? Looking at the wiring diagram tells us that the blue wire is the brake light wire. So touch the test light to the blue wire and if it lights, you have power to that point. If it doesn't, then you have an open between that connector and the brake light switch. Repair the break before continuing.

Now we see it goes from that connector to the brake light bulb socket. Take the bulb out and touch the contact inside the socket with the test light. If it lights, then you have power up to that point. Put the bulb back into the socket and from the back, pierce the black (Most automakers use black for the ground wire. German automakers use brown for ground.) If the wire is hot, then the bulb is good. If not then replace the bulb before you continue.

Now by looking at the wiring diagram we see that this black wire goes to ground. Skin a little insulation off and connect a jumper wire from there to a good ground. If the brake light lights, then you have a bad ground. Repair the ground wire. Now you have tested the whole brake light system.

This is a simple circuit to troubleshoot, but the troubleshooting technique is applicable to any circuit. A circuit may branch off here and there, but if you look at each section and test it as you go along, you will find the problem fairly quickly and easily.

Now the other type of electrical problem is the short circuit. It's called a short circuit because instead of the voltage taking the "long" way through the circuit to ground, it takes the "short" way to ground. These can be a little tougher to troubleshoot. Since all the battery amperage wants to go through the wire, it will blow the fuse. This is kind of a Catch-22, you can't find the short unless there's power, and there's no power until you find the short. There's two ways to remedy this situation, the first is to buy a self-resetting circuit breaker and the other is to put a load in the circuit so the fuse will not blow. I have a self-resetting circuit breaker, and it works quite well.

I prefer the other option, putting a load into the circuit. Now I'd like to say this was my idea, but I got the idea from my Uncle who was a mechanic for over 40 years. It was so simple and elegant that I adapted it for myself and used it with great success for many years. What you need is a old fuse, a bulb and socket and two lengths of wire. They can be as long or short as you like. I took off the plastic from around the end of the fuse and soldered a piece of wire to each end. Then I soldered the socket to the other ends of the wire. I taped it up and I got a little fancy and made up a small cage to go around the bulb. Then I simply plug my modified fuse into the fuse box and there's my circuit load. Now I can troubleshoot the circuit and find the short.

In looking for a short, the first thing you need to do is look at your wiring diagram and see what is on that circuit. Then, one by one, unplug every component and look at the light as you do so. When the light goes out, you found the part that's shorting out. If, after all the components are unplugged, the light is still on then the short is in the wiring.

Now what you need to do is cut up the circuit into blocks as you did in looking for a open circuit. Look at your diagram and locate the connectors, and working from back to front, unplug them one at a time, checking the light. When it goes out, you have isolated the area that is shorted.

These two techniques will work for the home mechanic to find most common electrical problems. More advanced problems require the use of a voltmeter and ohmmeter to locate. Most service manuals you buy will have a troubleshooting tree to guide you through of electrical troubleshooting. The important thing is not to shortcut the procedure. The result of one test usually relies on the validity of the previous test. By shortcutting you will invalidate the testing and will have to start all over again or give you erroneous results.

To recap, when you troubleshoot a circuit, understand it first. Use your schematics and diagrams to see how it works and is laid out. Then work in sections until the problem is found. These points are the key to successful electrical troubleshooting.

Additional Information provided courtesy of
ALLDATAdiy.com and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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