How Does a Turn Signal Flasher Work?


Hand signals are the norm for drivers using a vehicle that is not equipped with turn signals. Automatic turn signals have pretty much eliminated this practice, save for some motorcycles and antique autos. A simple electrical device flashes the turn signal lamps on and off, eliminating the need to stick your hands out the window to signal other drivers your intended direction. Activating the turn signal lever or the hazard light switch of your vehicle activates the flasher and causes the needed lamps to flash on and off, signaling other drivers where you intend to go.

Flasher Operation

  • A turn signal or hazard light flasher relies on current from the vehicle battery. The flasher gets the needed current when the turn signal or hazard light switch is activated. As is, this current is insufficient to light the signal lamps because of a resistor within the flasher. Once a bi-metal wire within the flasher unit heats up from the battery current, it expands and touches a contact within the flasher, shorting out the resistor and flowing through the armature coil, which illuminates the signal lamps. A magnet forces the contact apart and the bi-metal wire cools off and contracts, turning the lights off. This cycle continues until the turn signal or hazard light switch is turned off.

Locating the Flasher

  • Both the turn signals and hazard lights use a flasher. The flasher may be shared between the two functions or there may be a separate flasher for each. The flasher is cylindrical in shape and has metal terminals that protrude from one end. The terminals fit into a wiring plug or the vehicle fuse box under the driver side of the dashboard. If the vehicle owner’s manual does not indicate the placement of the flasher, switch on the turn signals or hazard lights and locate the source of the clicking noise under the dashboard. This should lead you to the flasher.


  • All turn signal or hazard light problems are not caused by the flasher unit. If one signal lamp is malfunctioning, the problem is most likely the bulb, lamp assembly or electrical wiring to the particular unit. If the turn signals on both sides of the vehicle flash erratically or not at all, you may have a flasher problem.

Flasher Replacement

  • A flasher is an electrically controlled mechanical device that will eventually wear out. Replacing a flasher can be as easy as pulling the old one out and pushing in a new one. A perfectly good flasher may malfunction if it is overloaded. This may occur when the flasher is asked to flash additional lights, such as when towing a trailer. In this case, a heavy-duty flasher may be needed. Check with your dealer or auto parts store to see if this type of flasher is available for your vehicle.

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