I bought white 15-LED refractor turn signal bulbs (7440) and replace the rear turn signal stock bulbs of my accord 2004. The blinking becomes faster than the stock bulbs, even the front turn signals (stock bulbs). however, these blinks normally when the Emergency button is turned i.e. both left and right turn signals blink at the same time.
Does anyone of you know why it happens?

 

There is a resistance differential going on there with your new part, guarantee it.

 

agreed. does the vendor mention it in the documentation?

 

this wont be 100% accurate but it will improve the situation. Take a voltimeter and measure the Ohms across the stock bulbs and the new bulbs. find the difference, then go buy a resistor from radioshack with that difference in ohms, and crimp/solder it in series with the new bulbs.

The reason its inaccurate is because the current draw from a light bulb doesnt have a linear relationship with voltage running through the bulb. In other words, as voltage goes up, so do the ohms, whereas a resistor DOES have a linear relationship, and as the voltage goes up, the current draw goes up at the same rate, resulting in the same number of ohms.