The flash rate of incandescent indicator bulbs is determined by the resistance of the bulbs..... so the higher the resistance the slower the rate and the opposite lower the resistance the faster they flash, when LED bulbs are fitted to a system designed for incandescent bulbs they flash too fast as LED bulbs have a much lower resistance which is inherent in the nature of LEDs, to use LEDs in these older designs a resistor of the correct value must be used to add resistance to the circuit.

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