How To Find A Short In A Car - J.D. Power

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Cars are more than just collections of integrated mechanical parts. They also use highly complex systems of electrical wiring and circuits. When working correctly, our vehicles’ electrical systems allow us to enjoy modern conveniences like our stereos, interior lights, and even holographic speedometer displays.

However, our cars’ circuits can also occasionally short. A short in your vehicle can cause long-term issues that can affect the overall control of the vehicle. They can even be dangerous sometimes, depending on the location of the electrical fault. Therefore, knowing how to find a short in a car is essential if you plan to repair the damage yourself, or if you want to determine whether it’s safe to drive your vehicle until you can get it fixed.

This guide will break down everything you need to know to locate an electrical short, even if you don’t have a lot of experience with car maintenance. So let’s get started!

What Is A Short?

In a nutshell, a “short”, or short circuit, is a fault in the wiring harness of your car—the wiring harness shunts or shifts electricity between different circuits before it reaches its final destination.

Short circuits are best understood when contrasted with open circuits, which don’t allow current to flow whatsoever. Both are electrical issues, but short circuits have distinct signs and cause different problems.

How Do Circuits Work In A Car?

To fully understand short circuits, you’ll need to know how electricity flows through your car in the first place.

Your car's electrical system can be broadly divided into sensor and actuator circuits. Sensor circuits are those that handle oxygen sensors, light sensors, speed sensors, and so on. Actuator circuits are those for motors or lights, for example.

One primary sensor circuit is the wire that runs between your vehicle's engine coolant temperature sensor (ECT) and the engine control module (ECM). These two components are located behind the glove box and engine in most vehicles, respectively.

So long as the wiring is fully intact, electricity can flow freely between both components. In the above example, the ECM can send a 5V reference voltage to the ECT, which causes the ECT to adjust its resistance based on the temperature.

While this all sounds very technical, don’t worry. The bottom line is this: your car's electrical system only works properly so long as the wiring is intact and uninterrupted. When a short circuit crops up, electricity can’t flow properly, meaning that the electricity goes somewhere else (potentially causing damage) and/or that certain components won’t work correctly.

What Does A Short Circuit Look Like?

Short circuits are categorized into two types:

• Short-to-ground circuits occur when a current flows from the circuit to your car’s body. This can happen if a wire sheds its insulation or chafes, allowing electricity to transfer from the wire to your vehicle. When a short-to-ground circuit occurs, you might see blown fuses, inoperative components or lights, and so on.
• Short-to-power circuits occur primarily in the wire harness, where there are numerous circuits grouped close together. When a cut or chafed wire contacts another, current can flow where it is not intended to. This can cause the headlight switch to send power to the car horn unintentionally, for instance, or even cause your headlights to illuminate when you step on the brakes.

It goes without saying that both types of short circuits are undesirable. But understanding the difference between them will help you identify what type of short circuit you’re facing and assist you in locating the problem more quickly. 

How To Locate An Electrical Short In Your Car

Finding an electrical short in your car is a time-intensive process, no matter your experience level or the number of tools you have at your disposal. 

You’ll need a few major pieces of equipment to start, including an EWD (electrical wiring diagram). An EWD is a color-coded chart that helps you navigate your car’s electrical systems. We’d also recommend getting a multimeter or test light, as well as any other tools necessary for opening your car’s wire harness and interior panels (i.e., screwdrivers and wrenches, etc.).

Map Out Your Car’s Circuits

To begin troubleshooting the circuit, open your car’s wire harness and consult your EWD. Identify the different wires and circuits you’re looking at so you know where to go next. Wire colors should correspond to colors noted in the EWD, although they may vary depending on where you sourced your EWD. 

Check Fuses One-By-One

Your wire harness should have a collection of fuses for your car’s different wiring systems.

To check fuses and see if there are any easy-to-reach short circuits:

• Remove a fuse at random and connect your test light to the terminals for the fuse socket (the place where the fuse connects to the electrical system). The test light will light up if an electrical current is detected. If you don’t have a test light, you can use a multimeter, which measures electrical continuity in the exact same fashion.
• With this example, if the test light fails to illuminate, it may indicate that current is not flowing to that fuse, so the faulty wire is likely somewhere along that particular path. Locate the wire using your EWD and carefully examine it.
• Repeat the process for each fuse to help narrow down your problem area

Checking For Faults Along A Wire

If you suspect the issue is tied to a particular wire, you can disconnect the wire’s connector at either the sensor or load endpoints. Use your test light and see when it goes out, or look for when your multimeter stops beeping. Either way, you can use this process to narrow down where the fault has occurred – does electricity stop flowing at the sensor or closer to the load?

For example, say that you reconnect a wire and fuse for the headlights. If you find that the test light goes out halfway through the circuit, such as closer to the headlight switch, you’ll know that the short-circuit problem is between the switch and the load instead of the headlight and the load. You can then open up your car’s paneling and take a closer look at the wiring to see if you can locate the issue.

Inspect Any Visible Wiring

You can also save yourself a little effort and inspect any easily visible wiring first. You might get lucky and find a frayed or chafed wire that you can start repairing right away. However, most cars’ wiring is carefully hidden, so odds are you’ll need to open up at least a few panels to find where the problem lies.

5V Circuit Testing

If you suspect the short-circuit to be a 5-volt circuit - which are circuits used by the ECM to control the transmission and engine - you can disconnect your battery and the ECM one after the other. To measure the continuity, use the multimeter and probe between the circuit and the car body - or the circuit and the engine. This will allow you to detect electrical fluctuations and roughly figure out where the short-circuit is located.

After Finding A Short

Remember, locating a short in your car is just the beginning of the process. Once found, you’ll still need to repair the electrical wiring that is causing the issue. If you’re lucky, this will be a relatively quick fix and just require you to repair the wire’s casing. Other problems may require you to replace the wire entirely.

Summary

Knowing how to quickly find a short circuit can save you a lot of headaches and ensure that you maintain a safe, fully-functional vehicle. While it can be tedious, using the guide referenced above should equip you with the information you need to identify and repair any short that occurs in your car.

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How To Find A Short In A Car - J.D. Power
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