MK126 Not Turning Off Correctly

We have a customer with an MK126 reporting that it is not functioning correctly with his 1964 vehicle that has a dynamo/battery electrical system rather than an alternator. He says that the best he can achieve is to set the adjustment so the LED turns on correctly but it will only turn off when the engine is above 1,200 RPM. That makes driving at night difficult as the bright LED is constantly turning on and off.

Can you offer any solution to the problem.