Diagnose Turn Signal Problems

by Richard RoweUpdated July 17, 2023

Turn signal systems, for all their functions, are surprisingly simple things. Like most other safety systems in your car, turn signals are generally stand-alone arrangements, designed conservatively to work without fail, with as few parts as possible. That way, there isn't much to go wrong. For this reason, turn signal failures are generally pretty easy to diagnose. An electrical tester can come in handy, but you probably won't need one.

System Basics

Power for the turn signal system comes from your junction box, and flows through the fuse and then the flasher unit plugged into it. The flasher contains what's known as a "bi-metallic spring," which is a flat strip with one kind of metal on one side, and another kind on the other. The flexible strip connects the electrical supply from the box to the wiring that goes to your turn signal switch. Electrical current flowing through the strip heats it up, causing the metal on the bottom layer to expand more than the metal on the top. This causes the strip to arc up and away from the electrical contact. When the strip cools, it drops back down again. This cycle is what causes your lights to flash.

All On or All Off

These kinds of failures are fairly rare, considering -- and they usually go back to one of two things. If all four of your turn signals come on and stay on when you trigger either side or the hazards, the bimetallic strip in the flasher isn't arcing up and away from the electrical contact. Most likely, it's because the flasher is bad, but it could also be because the entire electrical system is weak, and can't provide the current necessary to heat it. This might go back to a bad ground or bad connection somewhere in the system. If none of the lights come on, no matter what you do, it's a power supply problem. Bad grounds and connections are a possibility, but a bad fuse or flasher is more likely.

Bimetallic Definition

A bimetallic object is one that is made of two different metals. The two metals are usually different colors, and they are bonded together. Bimetallic objects are often used in automotive applications, such as in bimetallic strips that are used to control car doors.

Signalling Problems

Turn signals on either side of the vehicle can be wired in series, much like the bulbs on a strand of Christmas tree lights. If one goes bad, or it isn't properly grounded, it can affect the corresponding light on the same side. Check for damaged or burned-out bulbs in all turn signals on that side. Alternately, the ground on either light could have gotten damaged or come loose. Corrosion in one of the bulb sockets causing bad contacts is another possible cause. This doesn't always happen with a single bad bulb, though; many vehicles use turn signals wired in parallel, so that if one goes out, it doesn't take another with it. In this case, only one bulb will go out. But, if one goes out, it's smart to replace all of them; they all have similar lifespans.

Fast or Slow Flashing

If your turn signal flashes too fast or too slow, it's a problem with the heat rate transfer through the flasher strip. Most often, if all four turn signals are going too fast or too slow, it means the flasher is bad. If only the signals on one side do it, it may be one of the bulbs. The bulbs determine how much power goes through the flasher, and thus how quickly it heats up. Many well-intended persons have substituted a heavy-duty, high-wattage bulb or powerful aftermarket bulb, only to find the signals on that side flicker on and off rapidly. The more powerful bulbs are drawing more amperage, causing the flasher strip to heat up and cycle faster.

Switch and Dash Indicator Problems

Turn signals that don't work in one direction or the other may be due to a faulty switch, particularly if the non-working side starts functioning when you use engage the hazard lights. This is also a good point to check your parking lights, which usually utilize the turn signal bulbs. If the parking lights and hazards come on, but that side doesn't illuminate when you hit the switch, then it's a circuit problem as described in Section 3. Many vehicles now use turn signals integrated into the brake housings, using the brake lights themselves as turn signals. On some of these vehicles, it's possible to put the bulb in backward; this can cause a circuit failure in the lights and dashboard indicators.

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