I don't really have any knowledge of the MC5100 series, but I would surmise that LG1 is a GND connection, try metering it to LV common or the chassis.
As to the 12pin HD2 the PWM normally goes to a small value resistor then to a 4 pin opto, on the MC2100 it is pin 1 (Common) and pin 4, PWM in...