Turn signals don't work w/ engine running

[Bill Goddard]

When the engine is off, the flashers work great - they are everything you would want. But with the engine running the turn signals stay on with a slight flicker.

I am using a 12v neg grnd alternator putting out 14.57 v,
LED tailights and LED front fender lights.
The flasher is an electronic heavy duty no. 90651 (Taiwan) marked for use with LED lights

Why does all this stuff work so well with the engine off and so badly when it is running?
Bill G