At its most basic, a short circuit is a fault in the wiring harness, which shunts electricity between circuits before getting to its destination. A short-circuit should not be confused with an open circuit, in which current does not flow at all. Though the symptoms of a short circuit can be similar to an open circuit, diagnosis is a bit different. There are several ways a short circuit can occur, and it isn’t usually easy to find and repair. To understand how to find a short circuit, though, we need to understand how a properly-functioning circuit works.

How Car Electrical Circuits Usually Work

There are many ways that electricity is carried around the car electrical system, and a short circuit could easily interrupt the proper flow of electricity in any of them. We can roughly divide the car electrical system into sensor and actuator circuits. Types of sensors include oxygen sensors, light sensors, switches, speed sensors, and the like. Actuators can be motors or lights, or similar.

• A typical sensor circuit, say engine coolant temperature, might be the wiring between the engine control module (ECM) and the engine coolant temperature sensor (ECT). The ECM might be located behind the glove box, while ECT is located on the engine. The ECM sends a 5 V reference voltage to the ECT, which changes resistance depending on temperature. When the ECT sensor is cold, it has higher resistance, so less voltage gets back to the ECM. As the engine heats up, the ECT sensor resistance proportionately drops, sending a higher voltage back to the ECM.
• A typical actuator circuit, say a headlight, includes the wiring from the battery, through fuses and relays, the headlight switch, to the headlight bulb, and then back to the battery. The headlight switch always has power going to it but doesn’t route power to the headlight until the driver turns the switch.

In either of these circuits, proper function is assured as long as the wiring is intact, but there are many ways that any circuit could be interrupted. Rodent damage, chafing wires, shoddy installation practices, water intrusion, and impact damage are just a few of the things that can interrupt your car's electrical circuits. Inadvertently driving a screw through a wiring harness is a great way to cause a short to ground or short to power or both.

Types of Short Circuits

There are two types of short circuits, short-to-power and short-to-ground, in which electricity takes an unintended shortcut without going through the intended sensor or actuator.

• Short to ground – A short to ground refers to a current flowing from the circuit to the car body. Wires may chafe and shed their insulation, contacting the body or engine. A short to ground can result in blown fuses, inoperative lights or motors, or “missing” sensors. For example, a chafed wire may short to ground, which could cause the headlight fuse to blow, protecting the circuit from overheating, but knocking out the headlights.
• Short to power – In the wire harness, with many circuits in close proximity, there is the possibility of a short to power fault. In this case, chafed or cut wires may connect to each other, enabling current to flow where is not intended. For example, someone installing an aftermarket device might drive a screw through a wire harness, inadvertently piercing and “connecting” multiple wires. Turning on the headlights might send current to the horn, or stepping on the brake might illuminate the reverse lights.

With all the technology in the modern automobile, from powertrain management to entertainment systems and everything in between, it should come as no surprise the amount of electrical wiring needed to connect it all. Metal recyclers estimate some 1,500 wires, about a mile connected end to end, keeps the average modern luxury car connected, for example. Short circuits can damage electronic components, set the check engine light, blow fuses, drain the battery, or leave you stranded.

It might seem complicated, but the best thing you can do is divide and conquer. Modern electrical wiring diagrams (EWD) are color-coded, which can ease diagnosis, though diagnosing a short circuit is still no walk in the park. 

How to Find a Short Circuit

Tracing a short circuit takes time and patience. To get started, you’ll need an EWD for your vehicle, test light or ​multimeter, and tools to access the wire harness. First, identify the circuit you’re looking at. You’ll need to see where it goes, what connectors it goes through, and what color the wires are.

When testing 12 V circuits, you can usually start with the fuse in the affected circuit. Remove the fuse and connect the test light across the terminals of the fuse socket. The multimeter, set to measure continuity, can be used in a similar way. Disconnect battery positive, set the positive probe on the load side of the fuse, clamp the negative probe to battery negative. If there is a short circuit, the test light will illuminate or the multimeter will beep. Now, divide and conquer.

• Disconnect the connector at the load or sensor. If the test light goes out (or the meter stops beeping), this might indicate an internal fault in the load (a burnt-out bulb or motor can do this).
• Reconnect the load connector and disconnect something halfway through the circuit, such as at the switch. If the test light goes out (or the meter, well, you get the idea), you know the short circuit is somewhere between the switch and the load. Focus your attention on that section of the wire harness.
• Gripping the wire harness and flexing it may break the short circuit, so you can identify at least its location. If the light goes out, you know you broke the short circuit.
• If the test light didn’t go out (or the meter) with the switch disconnected, that means the short circuit is still somewhere between the fuse and the switch. Look for another place to disconnect the wires and see if the test light goes out. Keep dividing the circuit by disconnecting connectors and watching for the test light to go out.

On 5 V circuits, such as those used by the ECM to sense and control the engine and transmission, disconnect the ECM and the battery, set the multimeter to measure continuity, and probe between the circuit and body ground or engine ground. Follow the same divide and conquer method to determine the approximate location of the short circuit.

Once you find the short circuit, then you can go about repairing it. Before reconnecting the battery or putting in a new fuse, recheck for short circuits with the test light or multimeter.