Turn Signal Lights
FIGURE 39-16
Typical turn signals illuminated.

Turn signal indicators are located on the extreme corners of the vehicle. They are usually amber in the front and can be either red or amber in the rear Figure 39-16. A column-mounted switch, operated by the driver, commands a pulsing current to the indicator lights on one side of the vehicle or the other. These pulsing lights warn other road users of the driver’s intended change of direction.

Once activated, they continue until the switch is cancelled either by the operator or by a cancelling mechanism in the switch. The cancelling mechanism operates to return the switch to its central or “off” position after a turn has been completed and the steering wheel is returned to the straight-ahead position. The circuit consists of the battery, fusible links and fuses, the ignition switch, the flasher unit, a three-position switch used as the direction indicator switch, the lights at the front and rear of the vehicle, indicator lights mounted in the instrument cluster to indicate to the driver which way the switch has been operated, wiring to connect all of the components, and the ground circuit to return the electrical current to the battery.

If the indicator switch is turned to indicate a right-hand turn, current from the battery typically flows through the fusible link to the ignition switch, where it is directed through a fuse to the flasher unit. The flasher unit uses a timing circuit to pulse the current flowing out of the flasher unit 60 to 120 times per minute. This pulsing current is directed through the indicator switch to the right-hand indicator lights at the front and rear of the vehicle, causing the lamps to flash on and off. An indicator light on the instrument cluster also blinks in sync with the turn signals. The operation of the flasher unit also produces a clicking sound to audibly inform the driver that the indicators are in operation.

When the turn signal switch is returned to the off position, no current flows through the flasher unit, so the timer circuit is switched off. When the turn signal switch is turned in the opposite direction, it directs the pulsing current to the left-hand lights at the front and rear of the vehicle as well as the left-hand indicator light on the instrument cluster. Older vehicles used a thermo-mechanical flasher unit that relied on heat from the current flow to cause the flasher unit to work. It is very important to use bulbs of the proper wattage on all types of flasher units, as the speed of the flash may be incorrect if incorrect bulbs are used.

These indicators can also be computer controlled. The BCM commands the appropriate turn indicators to come on as it sees an input from the turn signal switch, and flashes it at the proper rate. The computer turns off the turn signals when a steering angle sensor signals the steering wheel is being centered. It can also cancel the turn indicators if the vehicle is driven for a programmed amount of time or distance without the steering wheel being turned. Often a chime will alert the driver if the turn indicator has been left on for too long. On many computer-controlled turn signals, the turn signals will not work at all when the computer senses the wrong amperage flow.

Turn Signal Lights—Domestic and Import Systems

All turn signal lamps flash, but there are variations in layout and design. This is most noticeable when comparing some domestic and imported vehicles. Imported vehicles tend to have separate amber-colored turn signal lamps on both the front and the rear of the vehicle. Some domestic vehicles use the rear brake lamps as turn signals by flashing the brake lamp on one side to indicate the turn. Wiring schematics should always be checked because the dual function of the brake lamps means the wiring for brake lamps on vehicles with this feature is different than those where the brake lamps perform only during braking Figure 39-17. In such cases, current for the brake lights must flow first from the brake switch and then through the turn signal switch, and on to the individual brake lights. This design tends to make the study of the current path more complicated, since there are two inputs—one for the turn signals and one for the brake lights. So the turn signal switch is much more complicated on a domestic-style turn signal switch than an import-style switch.

FIGURE 39-17
Brake warning light circuit.