I just installed some LED taillight and marker light bulbs on my fullsize Jeep since I had a couple of bulbs burn out and figured I'd upgrade while I was at it. Now there are times when the turn signals don't work at all, or they flash really fast. I've been told a different flasher module will fix it, but what I don't understand is why a normal flasher won't work with an LED bulb. What is different about a flasher that's specifically for use with LEDs?
The flashers used in older vehicles are thermal flashers that rely on a certain amount of electrical current to operate. Inside the flasher is a resistor and a small curved bi-metallic strip. When the turn signal switch is activated, current travels to the resistor and starts heating it up, which also starts heating up the curved little metallic strip. That metallic strip straightens out when it gets to a certain temperature, causing it to complete the circuit to the taillights and marker lights, lighting them up. When this happens, the current bypasses the resistor and allows it and the little metal strip to cool off. When the strip gets cool enough it snaps back into its curved shape, breaking the current flow to the taillights but starting to heat up the resistor again. And so the process repeats. The bending and straightening of that metal strip is the click you hear.
A key part of making a thermal flasher work is having the proper amount of current flowing through it. This is why a flasher will often flash rapidly if there's a bulb out in the circuit; there's less current flowing and so the heating and cooling cycle inside the flasher speeds up. LED bulbs require much less current to operate than normal incandescent bulbs, so there usually isn't enough current flowing to even trip the flasher, or if there is, it will act like there's a bulb out and flash rapidly. An LED flasher is all electronic and doesn't rely on a thermal element or current flow. Instead, an LED flasher is simply a timed circuit that connects and breaks the circuit at fixed intervals when activated, causing the bulb to flash.