There are two types of short circuits, which are short-to-power and short-to-ground.
- 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.
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.