How to Find a Short in a Turn Signal

Electrical circuits are one of those odd things that are usually a lot less complicated than they appear, especially if you look at it from the point of view of an electron. You -- the electron -- start at the battery, then travel through a wire to the junction box. From there, you might go to an accessory or switch, or you might stop off at a second fuse box on the way. If the switch lets you through, you go to the accessory, then back to the battery through the ground. All in less time than it would take to blink, if electrons had eyelids.

Things You'll Need

• Test light
• Digital multimeter

Instructions

1. Check the turn signal fuse; nine times out of ten, your short is a blown fuse. After checking the fuse, pull the fuse out, probe the fuse terminals with your test light or DMM, and turn the ignition key to the "on" position. If the light illuminates then you have power to the system. If not, then you're not getting power to the turn signal system. Next, unplug the wiring harness from the appropriate turn signal bulb, and connect your test light to the chassis-side wiring harness terminals. Activate the turn signal to power that bulb, and check the test light. If it illuminates, then you have a bad bulb.

2. Activate the turn signal, and listen for the flasher mechanism. The flasher mechanism defaults to closed; when a wire filament inside of it gets hot because of the current flowing through, the circuit opens with an audible click. More resistance in the turn signal circuit will cause the flasher to slow down; less resistance will cause it to cycle faster. If the blinker cycles on and off very quickly, you have a grounded wire in the wiring from the blinker to the turn signals. If it cycles very slowly, you have a damaged wire. If it doesn't cycle at all, you could have a severed wire or no power to the turn signal system. If you have power to the system, but the blinker doesn't flash, proceed to the next step.

3. Test the light wires directly. Connect your test light to the battery's negative terminal, and activate your turn signal. Probe the positive wire (usually the lighter of the two) on the turn signal bulb harness, and repeat on the other side. If one side illuminates but the other doesn't, then there's a short in the positive wire for that bulb. If neither side illuminates, connect your test light to the battery's positive terminal. Repeat this test, probing the wiring harness negative wire. If one side illuminates and the other doesn't, you have a problem with that positive wire. If neither the positive nor the negative wires on either side illuminate the test light, then proceed to the next step.

4. Test the turn signal switch. The difficulty of this will range from vehicle to vehicle, but you'll be league ahead if you can procure a wiring diagram. Most often, the turn signal switch wiring will run through the steering column and under the dashboard, and will connect to the chassis-side wiring harness using the same harness connector as the ignition switch and horn. But, this can be complicated, so you might try testing the blinker socket first. Unplug your blinker, and put your test light on the ground and power-input terminals in the fuse panel. Turn your signal switch on; if the light illuminates, the switch is working and you have either a bad blinker mechanism, or a short in the wiring from the blinker to the lights. If it doesn't, you have a bad switch or bad switch wiring.

5. Test the switch output directly. Sad to say, but the best way to do this is to disassemble your steering column to access the switch wiring harness directly. Test the power-in and ground wires (the latter of which you may not have) for power signal to the switch. If there's no power to the switch, but there's power to the signal fuse and a good fuse, then your problem is between the switch and fuse panel. If there's power to the switch, then test the switch's power-output wires. Probe the output wires, and turn the switch to the on position. If you get no output signal, but get power in, then the switch is bad. If you have power in and power out, but no power at the blinker mechanism input, then your problem is in the wiring between the switch and blinker.