How Turn Signals Work

By: Karim Nice

The Thermal Flasher

In this vehicle, the thermal flasher is located in the fuse panel.

This small, cylindrical device is sometimes located in the fuse panel under the dashboard of the car. It costs about $3 in the auto parts store and works reliably for years.

Inside the thermal flasher there are a few simple components:


  • An electrical contact that conducts electricity into the wire
  • A piece of gently curved spring steel to which the electrical contact attaches
  • A resistive wire wrapped around a smaller piece of spring steel
The thermal flasher

When you push the turn-signal stalk down, the thermal flasher connects to the turn-signal bulbs by way of the turn-signal switch. This completes the circuit, allowing current to flow. Initially, the spring steel does not touch the contact, so the only thing that draws power is the resistor. Current flows through the resistive wire, heating up the smaller piece of spring steel and then continuing on to the turn-signal lights. At this point, the current is so small that the lights won't even glow dimly.

After less than a second, the small piece of spring steel heats up enough that it expands and straightens out the larger, curved piece of spring steel. This forces the curved spring steel into the contact so that current flows to the signal lights unimpeded by the resistor. With almost no current passing through the resistor, the spring steel quickly cools, bending back away from the contact and breaking the circuit. The cycle then starts over. This happens at a rate of one to two times per second.

Let's look at the mechanism that cancels the turn signal when you finish turning.