The Electrical Schematic Is Your Friend

By Dan Sullivan, Mechanic, Teacher/Author and Electrical Troubleshooter | September 28, 2010

Rule 1: Read first to learn how the circuit works, then redraw to troubleshoot.

Rule 2: When tracing a circuit, always work from the load to the battery, or from negative to positive.

Rule 3: There is usually only one load component per circuit, because 99.9 percent of all circuits are parallel.

Rule 4: Use every clue the manufacturer gives you.








If there's one thing mechanics hate more than math, it's reading electrical schematics. Here's why. First you have to find it, which is not always easy. Once found, you have to figure it out — also not easy if you go at it the wrong way. And, most disconcerting, you have to put up with feelings of insecurity and self-consciousness as you study the schematic, worrying that someone (including your boss) might think you're weak, or incompetent, or stupid or just wasting time by doing something other than real work.

So, okay, if you're capable of memorizing hundreds of pages of data about thousands of circuits on scores of machines, then go for it! But if not, get the book, get the schematic, and get to work. And if you do, a year or so from now your friends in the shop will be calling you the "electrical wizard."

How to Troubleshoot On-Board Electronics

The primary disadvantage of electronic control modules (computer ECMs) is that they add wiring to the machine. Luckily, the black box does an exceptional job tracing circuit faults. Heres how to find a problem and get a balking machine back up and running.

Rule 1: Read to learn, then redraw to troubleshoot.

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Some schematics are drawn as a straight line, called "ladder diagrams." If not, you should take the time to read the schematic and learn how the circuit works. Then, redraw the whole circuit as a straight line. Reading the schematic and then redrawing is a very critical rule.


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Here are some added points:


  • If you try to draw at the same time you're reading (because you want to save time), all of the errors you might make — will make — while reading will be in your sketch.
  • Even if you don't draw a perfect picture, the effort will familiarize you with the circuit.
  • You can make notes on the drawing as you work.
  • The more you do this, the less you'll need to do it.
  • Your first effort to draw the circuit should be a simple version — positive, ground, switches and load.
  • I've learned that many people struggle with the detailed drawing, so if you make a simple sketch first, you'll at least know what the circuit does, then you can go back and add the details.

Rule 2: When tracing a circuit, always work from the load to the battery.

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Find the load component first (for instance, light, horn, coil, solenoid or backup alarm), then find its ground, and then find the battery. Trying to read a schematic from positive to negative doesn't work. Always work back toward the battery from the load. This rule is important because it gives you a correct direction. All grounds must eventually end at the battery (not inputs).


Rule 3: There is usually only one load component per circuit, because 99.9 percent of all circuits are parallel.

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Most circuits have only one load. Just because something is connected to a wire, it doesn't mean it's actually in the circuit. Just because you go to a load, it doesn't mean you must go through it. This rule is critical because it helps you know where to go — and where not to go. Remember, you have to think about how a system works. Current doesn't flow around in circles through each load, it flows from ground to battery. As you trace a circuit, if you hit another load, you're wrong. Turn around! Remember Rule 2: Always start at the ground and work to the battery. Note that in the accompanying diagram, there are four circuits in the system.


Rule 4: Use every clue.

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Always take advantage of every clue the manufacturer gives you — keys, legends, indexes, charts, notes, patterns, serial numbers and so on.


Construction Equipment thanks Dan Sullivan for sharing excerpts from his book, Fundamental Electrical Troubleshooting. Sullivan is a full-time trainer, helping technicians understand and apply electrical theory and problem diagnosis. He also is the inventor of the TESlite, a diagnostic instrument for troubleshooting electrical problems. Text and drawings are used with permission. You can reach Sullivan at Sullivan Training Systems, 877-WRENCH2, or at