If the led is hooked to the data line (tach line?) then it may be showing continuous glow due to hysteresis of human eye while it actually may be turning off and on with rpm. To illustrate what I'm saying, hook an led up to an AC transformer with the appropriate voltage drop resistor to properly wattage the led of course. Visually, the led will show continuous glow but one knows that it can only be rectifying half the AC cycle and therefore must be turning off and on. This can very easily be observed visually by moving the led across ones visual periphery where the led isn't positioned at the same spot to the eye where visual hysteresis comes into play. Now it will be clear it is blinking as is likely for the led on the data line.