OK I bet I know what it is. Behind that main drive wheel is a small copper plate. Every time the ice maker does a harvest right near the end that plate rubs against a probe coming from the printed circuit board. The probe is connected to a small adjustment screw which allows you to tweak the amount of time the water valve is energized. Over time the arcing associated with this builds up a nasty black mess that can cause the water to half fill or overfill. Cleaning it up is tricky. Nothing is mentioned in the manual about this but I have had limited success with cleaning this copper plate. Other than that you is gonna probably need a new head.