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The ABC's

Once you understand how to read and follow a wiring diagram, you are able to make the basic electrical tests for power and continuity. This enables you to tackle complaints, using a wiring diagram, and the following steps.
1. Figure out the test sequence.
2. Find the specific test points in the car.
To make fast and accurate corrections of faults follow this plan.
1. Verify the complaint
2. Locate the inoperative component on the schematic.
3. Identify other components on the same power source.
4. Isolate the problem.
5. Repair the problem.
6 Test the operation of the component.
When it comes to the electrical system on a car, a lot of people shake their heads and walk away. Even experienced mechanics can be fearful of electrical work for fear of the unknown. While some components do use computers and advanced circuitry, solving most electrical problems just isn’t rocket science.
Using simple tools and some common sense, it’s possible to diagnose and solve nearly all-electrical problems quickly and easily. Among these tools are a test light, a voltmeter and a good understanding of electrical basics and common problems. When you go to tackle an electrical problem, remember that you’ll be wise to consult a service manual for its specific electrical information.
Use Logic
1. If a single component fails in a multi-component system, start testing the failed component.
2. When several components fail in the circuit, start testing at the common power source or ground.
If you have a choice of testing at accessible or non-accessible points, chose the accessible points first.
3. Make visual inspections look for lose or corroded connections.
Intermittent problems
1. Intermittent problems are generally caused by an open in the circuit, this can be true on either the ground path or the power supply path.
2. To find an intermittent problem sometimes all it takes is moving the wires around, which is called a wiggle test.
Substitution Testing
1. When the trouble symptoms indicate a failed component substitute it for a known good part. Where repeated failures occur to fuses or bulbs, look for overloads or short circuits.
Open Circuit Testing with a Jumper
Jumper wires can be used to temporarily substitute for components such as switches, fuses, breakers, relays, wires, and other low resistance components that are suspected of being defective.
1. Connect the jumper wire to by-pass the suspected defective or open component.
2. If the circuit is operative the removed component is defective or open.
Testing for Power
The easiest instrument to use is a 12-volt test light.
1. If you are looking for power in a circuit hook the test light to ground and probe the component or wire.
2. If the light glows you have power, get to know your test light it should glow brightly, just like hooking it straight across the battery terminals. If it is dim break out your voltmeter. A voltmeter will show you how much power is actually at the test point.
3. Like a jumper test, the test for power can be performed at different points in the circuit to find the fault.

Testing low voltage electrical systems is a step-by-step process. The most common problems we'll run into are wiring faults: either grounds or opens. Grounds are usually caused by chafed wires in contact with metal or moisture at a connection point causing contact with metal. Opens are simply lack of continuity. Opens can be a break in a power carrying wire, an unplugged connection, a corroded or poor connection ... this can apply to the return part of the circuit as well. An open (circuit designed) ground wire, corroded or poor ground connection. Faulted components and their individual test procedures are usually covered in the Bentley manual.

The single most valuable tool for shooting wiring faults is a volt-ohm meter. I prefer the old analog (with the wildly swinging needle) meter. Whether you use an analog or digital meter, make sure it is capable of measuring high resistance trouble. The easiest way to verify this is to set the meter (yeah ... right there in the store before you buy it) to ohms on the highest resistance setting (Rx100, Rx1000, etc.) and hold the tips of the probes pinched in your fingers of both hands. Watch the display and make sure it registers differing readings as you roll your fingers on the probe ends. The moisture in your fingers and thumbs should provide a good enough connection to give you a good meter reading. Also check to see if the meter has the capability to be "zeroed". This will allow accurate readings as the batteries inside start to tire. You zero it before each use.

Finding wiring faults is easy. Remove the power, remove the load, test the wire. Using your ohms scale, you want to look at just the wire. Hook up one lead to your suspect wire, and hook up the other lead to bare metal of the body. Remember ... you've removed the voltage on the circuit you're testing (either removed the fuse or removed the wire from the fuse block) , and you've removed the load from the far end (removed the bulb/radio/switch/whatever that wire provides power to). With everything removed and your meter hooked up, you should see no reading. If you see a reading ... you've got a ground. If there's no reading, that's good. You now need to check for an open. With the meter hooked up, just as above, touch the far end of the wire to bare metal. You should see a low reading (lower the ohms, better the connection). If you see no reading ... you have an open.

Remember ... with an ohm meter you're looking at just the wire. If you don't de-power and de-load the wire you can damage your meter or get bogus ground readings. More later.

I commend both you gentlemen for covering this subject , and i understand that to some people this electrical thing will always remain black magic .

Those of us that do this every day to earn a living, sometimes can't understand why other people can't understand these principals

It comes down to one simple thing .....experience and a the ability to feel comfortable with what you see happening in front of you

So if i may, i would like to add one small piece of advice, to people starting out in the weird and wonderfull world of electrics ,piece of advice given to me when i was still a very young lad .

It deals with your meter .... Go grab your meter and play with it. learn what your meter does in different situations , read its manual get to know it like the back of your hand

that way when you finally need to use it to trouble shoot you are not asking yourself ....is it supposed do that? this unfamiliarity with the meter only puts additional strain on the trouble shooting process . it is the same principal that applies to any tool, and learning how to use it well and use it correctly . after all if you don't know how to use the tool you can't expect to do a good job with it

Excellent point