Icemakers do not have a sensor to detect when the ice mold is properly filled. Instead, they use a theoretical time/pressure method. Basically the water fill valve has an orifice that is sized to allow 120 to 140cc of water in 10 to 15 seconds with an average water pressure of 40 to 45psi line pressure.
Somewhere on the icemaker control head there should be either a lever or a setscrew that is indexed +/- or increase/decrease. That lever or screw is used to make field adjustment to compensate for differences in actual water pressure at the field location. Normally one full turn of the screw will increase or decrease the water volume by 20cc.
If the unit has been working ok then suddenly has begun overfilling, there is one of three possible problems.
1. Some external condition has dramatically altered the line pressure to the icemaker fill valve.
2. The Icemaker water solenoid fill valve is sticking in the open position and is sluggish to reseat. (Solution is to replace the fill valve.)
3. The whirlpool modular style uses a rotating wiper switch on a printed circuit board to time the cycle. When the units get old some of the insulation on the circuit board wears away or chips off the board which results in an extended fill cycle time. (Whirlpool recommends replacing the powerhead or icemaker unit.)