Kmod-ledtrig-heartbeat problems

Hi, i want to use the kmod-ledtrig-heartbeat trigger to signal the CPU load of my archer C7 V2. My router blinks static, no matter what cpu load.
In the description i read:
The frequency is in direct proportion to 1-minute average CPU load.
What do i wrong ?

In other words, LED blinks always in the same frequenz, no matter what cpu load.
I forgot, i am using 19.07.1 with AR71xx architecture.