Carl’s Corner – Auto Repair Help

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DIAGNOSE – THE TURN SIGNALS DON’T WORK OR DON’T WORK PROPERLYBy Carl O’Reilly

SYMPTOM SUMMARYThe turn signals do not flash or they may flash in only one position or one side may remain on constantly with the switch in the off position.

USUAL CAUSEThe turn signals use a flasher unit that acts as a repetitive circuit breaker. This unit makes then breaks the electrical circuit that turns on the turn signal bulbs. The cycle will continue as long as the directional switch is turned on. This unit relies on heat build up caused by electrical current flow to determine when to break the circuit. When the switch is initially turned on, electrical current flows through the flasher unit to the bulbs. When this occurs, heat builds up in the flasher unit. When the heat reaches a predetermined temperature, a metal strip within the unit stretches due to the heat and breaks the electrical circuit turning the bulbs off. With the bulbs disconnected from the circuit, the flasher unit cools off. Once it has cooled, the metal strip re-contracts and re-closes the electrical circuit turning the bulbs back on. The usual cause of inoperative turn signals is a defective bulb or flasher unit.

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CORRECTIVE ACTIONPrior to performing extensive diagnosis, check the vehicles fuses to ensure they are not blown. Some vehicles utilize two fuse boxes, one in the passenger compartment and another under the hood. Consult your owner’s manual for fuse box locations and fuse ratings. If the directionals only flash in one position and are off constantly in the other direction, the usual cause is a burnt-out bulb. When a bulb blows, the current flow in that side of the circuit will be reduced. The reduction in current flow prevents the flasher unit from reaching the needed temperature to switch the bulbs off. Turn the directionals in the position where they are inoperative and perform a visual inspection of the exterior bulbs. Replace any bulbs that are not lit. The flasher unit is used to switch the current on and off for both right and left directionals. If the turn signals are inoperative in both directions, a defective flasher unit or blown fuse is the usual cause. On most vehicles the flasher unit is located on the front or on the back side of the fuse box located in the passenger compartment. Some vehicles use a solid state flasher module that contains transistors and diodes. On these vehicles, you should consult a vehicle specific service manual for proper diagnosis.

PRECAUTIONS, TIPS and NOTES You should always use a replacement bulb of the same part number as the one being replaced. Your owner’s manual will indicate the correct bulbs for each location on the vehicle. Using the incorrect bulb can create too slow or rapid turn signal operation.

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Manufacturers often use grease to repel water from the bulb sockets. Do not remove this grease from the bulb socket when replacing defective bulbs.