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I wanted to add LED flashers to my motorcycle (I previously used 10W classical bulbs). After I installed both on the right side (front and rear) I noticed that they won't blink. They will turn on, dim for 1 second, and then stay on forever (without ever dimming again). I thought the problem was a relay, but then I hooked up one of the old flashers on the rear, and now the front led one worked fine too... Why does this happen? Would attaching an extra load solve the problem? (I was thinking of a resistor in parallel with the led flasher, so that it would mimic the classic bulb, though dimming the led brightness) Is this a good idea? Do I have other alternatives?

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They will turn on, dim for 1 second, and then stay on forever

This sounds very much like the new LEDs aren't drawing enough current thru the old mechanical flasher. These things work by having a heating element wrapped around a bi-metalic strip. When cold, the bi-metalic strips bends in a way to open the contact. The heating element across the switch. When the bulbs are off, they have low resistance and most of the applied voltage is across the heater. That heat up the bi-metallic strip, which causes it to bend to close. That turns on the lights fully, but kills the current thru the heater, allowing the strip to cool. The switch opens and the process repeats.

Your flasher is intended for use with incandescent bulbs. These have two properties the LEDs don't. First, they have much less than normal operating resistance when not fully on. Second, they draw a lot more current.

The solution is to replace the flasher with one designed for your relatively low current LED lights. I haven't looked, but with LEDs being common today, there are probably replacement flashers specifically intended for driving LEDs. If not, it wouldn't be hard to make a simple circuit that switches the voltage to the LEDs on/off over time.

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It is common in vehicle electronics for lamps to be problematic when they are replaced with LEDs due to the current being much less. Sometimes a resistor can be added in parallel to increase the current to a level that will work or to a level that will not indicate the lamp as blown.

Alternatively, in your case, use another flashing circuit.

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