Everyone takes the turn signals for granted. They serve an important safety benefit, of course, and seldom fail to operate - save a burned bulb filament or two. Turn signal circuits are a marvel of simplified electrical engineering, although many are confused by the wiring. The entire circuit, aside from wires, consists of the following parts, so let's look at each one:
The basic turn signal circuit starts at the flasher, not at the switch. Inside the assembly is an electrical contact that brings current into the flasher from the fuse panel. A piece of curved spring steel is attached to the electrical contact. Next, a piece of resistance wire is wrapped around a smaller piece of spring steel, which is along side the larger one.
When you select either a left or right turn signal you electrically connect the flasher to the signal bulbs on the car. This completes the circuit, which allows current to flow through the resistance wire, causing it to heat the spring steel around which it is wrapped. In a very short period of time the spring steel expands, straightening out the larger piece of spring steel. This forces the now-straightened piece of steel into direct electrical contact with the signal bulbs, bypassing the resistance wire and sending full current to the bulbs.
The bulbs turn on, of course, followed quickly by the cooling of the spring steel. It curves away, breaking contact and turning off the bulb filaments. The current then passes back to the resistance wire, starting the whole process over again. The clicking noise you hear is the spring steel bouncing back and forth.
The stalk on your steering column is nothing more than the toggle that moves the switch contacts to the correct position for indicating a turn. The switch assembly is also mechanically designed to "self cancel," but there are so many different designs around that we won't go into that discussion here. We'll focus on the electrical function.
The switch brings in current from the flasher (on a wire, of course) to its center contact. Once "live," the center contact can distribute current to the right or left signals by the movements of the switch. When selecting a left turn, for instance, the stalk is pressed down and the switch's center contact rotates up to contact the wires leading to the left front, rear and dash indicator bulbs.
Since all the bulbs are in grounded fixtures the circuit is completed. When this happens the filaments glow until the flasher's internal contact breaks, at which time they turn off. As the flasher's contacts open and close the lights turn on and off.
The front turn signal lights also typically function as "parking" or "running" lights. Therefore, the fixtures are designed to operate on two different levels of brightness. A high brightness is needed for the turn signal and a lower for the running lamp. Rather than have two bulbs — or two fixtures — the manufacturers long ago came up with dual-filament bulbs.
These bulbs are simply made with two filaments, one brighter than the other. Separate wires lead to each filament, but both are grounded at the same point. The turn signal switch's current travels down its dedicated wire to the bright filament and the running light's current is supplied to a separate wire leading from the light switch.
At the rear are also dual filament bulbs. The bright filament is usually shared between the brake lights and the turn signals.
The emergency flasher (post-1965 cars) isn't really an additional flasher. It's nothing more than a switch that's wired in such a way as to override the turn signal switch and deliver current to both sides of the car's signal system, along with the indicator bulbs in the dash. It utilizes the existing flasher.