What would cause the turn signals to not work in a 1999 Ford F-150?

Answer 1

Check the turn signal relay under the dash. As a check, turn on your hazard (emergency) blinkers. The turn signals lights should all start blinking. These use a different relay and says the lights are okay. If this works, follow the relay clicking noise. The two relays are next to each other.

I have a 97 f150 and the blinkers did not work. They would always work for the mechanic....go figure. We finally figured out that I put the tilt steering down which separated the switch in the column...when the steering wheel was up for mechanic they would always work.

You may want to check the fuses. If the truck is 4x4, there is a 4x4 differential fuse that also controls the turn signals. I had the same problem with a 98 F150, signal lights would not work, but the hazard lights would. The 4x4 light comes on but the front axles wouldn't engage.

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How Turn Signals Work

by Karim Nice The Thermal Flasher 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.

Copy & Paste Link below for more info, and pictures

auto.howstuffworks.com/turn-signal2.htm