How to Find an Electrical Short in Your Car
A car electrical short creates an unintended path for current and can manifest in different ways:
- Blown fuse
- Burned fusible link
- Open circuit breaker
- Burned wires
- Burned components
- Wires or harnesses with melted insulation
- Inoperative component(s) (such as a light, sensor, or motor)
- Components operating in unexpected ways
Any of these conditions may be caused by a hot (current-carrying) wire that:
- makes contact with the car's chassis (ground) due to damaged insulation.
- contacts another wire due to damaged insulation.
- is pinched between two parts.
- becomes frayed.
- has been incorrectly connected when installing an accessory.
- has become loose and found its way to ground.
- has the wrong gauge for the intended application.
For most cases, you'll be dealing with simple electrical shorts that you may diagnose using a test light or a digital multimeter and the vehicle repair manual for your particular vehicle make and model.
The electrical wiring diagrams in most aftermarket manuals usually show the color of the wires (sometimes gauge size) and connectors and components in the circuit. However, they don't have the exact location of wires and components. But for most cases, this will be enough.
If you don't have this manual yet, you can buy an inexpensive aftermarket copy through Amazon. come with step-by-step procedures for many repairs, maintenance, and troubleshooting procedures. So most likely, you'll keep using the manual and recover your small investment soon. Haynes manuals
How Do You Locate an Electrical Short in Your Car?
Here are the troubleshooting methods I'll be covering in this article:
- Preparing to Troubleshoot the Circuit
- Finding a Short Circuit
- Using a Short Finder
- Short Circuit Diagnostic Made Easy
I. Preparing to Troubleshoot the Circuit
Before you start troubleshooting the circuit, identify the circuit you need to troubleshoot. For example:
- If a particular fuse blows whenever you replace it, look up the fuse number in your car owner's manual or vehicle repair manual.
- Then look up the electrical diagram for that circuit in the manual.
- Identify the wires, connectors, switches, or components in the circuit feeds.
- Locate the power source and ground.
Keep in mind that not all short circuits will blow a fuse, but they can discharge the battery overtime or make one or more circuits behave in unexpected ways.
A circuit can be affected intermittently, if a loose or bare hot wire touches ground (chassis) only when the car is moving.
A quick analysis of the diagram can tell you how the circuit is supposed to operate; what points in the circuit you can actually access for testing; components you may disconnect; connectors you can unplug to isolate portions of a circuit; and switches and splices you may access for testing.
Look for the most likely problem sources. This may include connectors (usually power and ground wires in the same connector that may have become loose), splices, and components.
This will save you time and help you develop your troubleshooting strategy.
For example, if one or more lights don't work and you can't trace the wiring, maybe you could check the wiring and connections at the switch, power feed, and load.
If the diagram says ground is shared by more than one component, check the operation of those components. If they operate correctly, you know the problem lies in the part of the circuit that works directly with the affected component; otherwise, see where those components share a connector.
Sections of the circuit can be isolated this way for easier troubleshooting.
II. Finding a Short Circuit
When possible, before you start, inspect the part of the wiring in those sections of the circuit that you have access to, and check for burned wires, wires with melted insulation, or loose wires. Wiggle wires to make sure they are connected.
Remove the fuse or protective device from the circuit with the electrical short.
Check your repair manual, and see if you need to turn the ignition key to the On position to activate the circuit. Also, turn on any switches in the circuit, if flipping the switch On blows the fuse. This pretty much tells you the short is in the leg of the circuit the switch powers On.
Set your voltmeter to the DC voltage scale, at around 20 volts, and connect your meter across the fuse holder. If there's a short in the circuit, you meter should read some voltage.
If you are using a test light, you can also connect the light across the fuse holder. When there is a short, the light will illuminate.
Now remove the load from the circuit and check your digital multimeter (DMM) again. For example, in the example diagram above, we could remove the light bulb first and then the solenoid.
- If your meter still reads voltage, the short circuit is located between the load and the fuse holder or protective device.
- If our meter doesn't read voltage, the short is located either in the load itself (perhaps a bad motor) or in the section of the circuit after the load.
Turn off the switch and check your meter.
- If the meter reads O volts, the short is located between the switch and one of the loads, or the switch itself. If you are dealing with a relay, you can check this other post on testing relays.
- If the meter still reads voltage, the short is located between the switch and the fuse holder or protective device.
Removing loads, disconnecting switches, or unplugging devices is a practical way to isolate the section of the circuit where a short circuit is located.
However, there will be times when you won't have access to a load or a connector in a circuit for this purpose.
In a situation like this, you could use a jumper wire to connect a meter lead to the input and output voltage side of a component or device, or probe a section of a wire close to the load, switch or connector.
Use any combination of strategies to help you isolate sections of a circuit as you gradually move towards the power source from the suspected trouble points.
III. Using a Short Finder
A is another practical method to locate shorts in a circuit. This device includes a circuit breaker and a compass or Gauss gauge. However, some car manufacturers recommend not using a circuit breaker in place of a fuse when testing for short circuits. While both fuses and breakers are used to protect an electrical circuit from overload, fuses are quicker to react (melt) to the presence of unwanted high electrical current. Still, a short finder is a popular and practical tool in use today. short finder
To operate the short finder:
Plug in the circuit breaker into the fuse holder that protects the circuit with the electrical short.
When you activate the circuit, the breaker will cause the circuit to cycle on and off.
As the circuit is activated, follow the hot wire's path in the test circuit with the compass. The high-current in the hot wire will create a strong magnetic field, causing the needle in the compass to fluctuate.
When the needle stops fluctuating, you have passed the short in the circuit.
The advantage with this method is that you don't have to dig through a harness or remove panels to follow a hot wire in a troubled circuit. As long as you can move the compass close to the wire with the short, the needle will keep deflecting, even if the wire is located behind a firewall or panel.
The only problem with this method is that, depending on the circuit and the location of the short, it may be difficult to trace all sections of a circuit using the compass. A wire may suddenly go into a path buried under engine components where you can't reach.
Still, you may find a circuit finder useful in combination with the methods described here in the previous section. It certainly can be of help.
IV. Short Circuit Diagnostic Made Easy
Using the electrical wiring diagram for the circuit you intend to test and understanding how the circuit is supposed to operate are key to finding a short circuit.
The strategies described in this guide can help locate shorts faster. Even if you don't have access to every point in the circuit, you still may learn what section of a circuit is affected.
Remember to isolate a circuit in sections: disconnect components, unplug connectors, and use jumper wires as necessary.
And don't forget to participate in our poll below. Let us know about your favorite tool to diagnose car electrical shorts.
Do you know that some car manufacturers recommend using a sealed beam headlight as a diagnostic tool (as a test light) when all you have is a circuit breaker? The sealed beam light serves as a load and as a limiting current device in the circuit.
We've included the sealed beam headlight in our poll to see how many of you use this type of light as a tool. And thanks for participating.
What is your favorite tool for diagnosing car electrical short circuits?
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
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© 2018 Dan Ferrell