Not surprising that the LED's are being turned on and off by the modulator rather than actually modulating. LED's have a cutoff voltage so if the voltage is reduced too much, they simply turn off rather than dim like a halogen would. It may be a matter of not reducing the voltage as much when the modulators are used with LED's. Not sure how well that would work either but someone will need to experiment to know for sure. I'm in the camp of not being a fan of modulators but rather adding additional really bright lights to increase conspicuity.