Why do my blinkers flash on an off so fast. but do not work .

When one turn signal bulb blows, it has the following effect.

Dashboard signal indicator will blink at approximately double speed. This is an easy way for the car/truck to inform the driver that one of the bulbs has failed. this is actually a Feature. Anytime blinker indicator speed increases it means that the electrical system isn't drawing enough current.

just check the side of the auto that has the fast blinking. i'm pretty sure you''ll find a blown bulb on that side of the truck. Remember that there are often more than just the 2 bulbs (front and back). many autos have a side marker light on the fender that also blinks.

now if you experience Fast blinking on both sides and don't find any blown bulbs, then the problem your issue could be with the actual blink controller. i know my car has the blinking controller embedded in the CAUTION BLINKER SWITCH, and in my case i'd have to source an Emergency Blinker Switch to rectify the problem.