The control unit is faulty, I had that problem twice.. I have now bypassed the control unit and use a normal pump pressure switch to control..works fine..The Grundfos company at first said I had to have air in the lines then admitted the control assy. was at fault. after two new units in three years I did away with the control unit.

Have you checked that theres no leaks in your system? If your lines are short and with tap or taps open this could be that system will not stop as direct flow going straight in then out. When the line is full and tap not going then the pump should stop and stay stopped as they have a non return valve on your inlet side.

I am assumeing that this pump is for your fresh water for your house??? If this is your situation you probally have a bad pressuer switch or a bad diarpham in your pressure tank. good luck