Search Autoparts/Motorage/Drivability/

The ABCs of electrical diagnostics

Friday, February 1, 2019 - 08:00
Print Article

We’ve all learned the ABC song, and how to count to 10 as one of our first “organized” instructional classes, even if we didn’t know that’s what we were doing.  Now, as a grown up, we’re still learning the same basic golden rules. Be it just a bit differently than our ABCs. As an adult, we follow a flow chart — the basic fundamentals of an electrical circuit — which is like following along with the traditional kindergarten ABC sing-a-long song.

Motor Age Magazine Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.

SAVE 20%

Save 20% on DrivabilityTraining Videos, ASE study guides and more using ART20.


A Ford repair manual covering 1933 to 1947 for the generating and starting system consisted of only 41 pages to cover every detail of the system. 41 pages is just the introduction to today’s complex charging and starting systems.

As with the ABC song, nearly every type of repair scenario starts with the right approach, and the right starting point can make all the difference. If you start on the wrong end or somewhere in the middle, it’s like trying do the ABC sing-a-long song backwards. (OK, go ahead, try it.) It isn't so easy, huh? In this article we’re going to go thru the ABCs of basic electrical diagnostics from easy to a somewhat complex electrical circuit diagnosis. 

Early systems  

Throughout the history of the automobile, electricity has been a part of its makeup. For a time, 6-volt systems were the norm. Then in 1955, the 12-volt systems became the standard. Positive grounded vehicles were popular for a while due to the fact of the woven fabric covered wire, which had the tendency to absorb moisture. The positive ground reduced the galvanic effect and corrosion that was common on the negative grounded vehicles of that era. Then during WWII, a plastic-coated wire (PVC) was developed which greatly improved the wire quality and integrity tremendously, and the galvanic problems with the copper wires was nearly completely eliminated. This led to the standardization of the negative grounded vehicle.

Computer systems

Now with computer systems and high-tech components, the complexity of the electrical systems in today’s cars have certainly increased. But, the basic principles of electricity haven’t changed at all. Voltage, amperage and resistance are still the three main concerns. However, the critical nature of each have been greatly increased and are by far more susceptible to environmental issues and circuit condition than ever before.

Simple circuit diagnosis – beginners only

Let’s use a simple bulb, two wires and a voltage source as an example. Voltage runs from the battery through the bulb filament and back to the negative terminal lead, completing the electrical path. Thus, making an electrical circuit in its simplest form. Now let’s look at what would happen if we took the negative lead off of the battery terminal. Of course, as you would expect, the bulb goes out because current flow has ceased. But what’s happening to the positive voltage? Has it gone back to the battery and will decide at a later time to flow down the wire? No, not hardly. 

A voltage drop can be as little as a loose connection at the battery.  Examine the battery clamp connections carefully. Just because the post is secure doesn’t mean the post terminal to negative wire connection is good. Check both.

This is a unique characteristic of electricity. Each polarity will reach out as far as it possibly can to find its opposite polarity. (Talk about opposite attraction). The disconnected lead is nothing more than an extension of the battery positive terminal. (OK, technically there is a touch of resistance added by way of the bulb filament.) Keep in mind, the positive voltage is still at the end of that wire lead, and if that wire lead happens to find another pathway to ground it won’t hesitate to take it. For the novice technician, when a scattering of electrical spark should appear it is usually followed by the complementary convulsive reaction to the sparking wire.  

Open (incomplete) circuit issues      

One common occurrence is the open circuit problem. An open is exactly what was used in the previous example. In other words, an incomplete path of electrical flow. In most circuits, a loss on the positive side (such as a blown fuse) basically brings the entire circuit to complete halt. (We’ll cover a blown fuse a bit later.) But, on a few occasions, you’ll run across the dreaded “feedback” affect after the original positive signal has been compromised and another leg of the same circuit becomes the voltage source for the remainder of the circuit. 

Not in every case, and not that every manufacturer labels the fuse box the same way. But, it’s still a good idea to check the fuse box lid against the wiring diagram and make sure you’re on the right fuse.
Article Categorization
Article Details

< Previous
Next >
blog comments powered by Disqus