Abstract: A control for a motor driven rotating arm crop irrigation system of the type having a low pressure control for stopping the water supply pump includes a normally closed discharge valve adjacent the supply connection to the arm. A control responsive to malfunction of the arm drive opens the discharge valve to release water and thereby decreases the pressure in the supply line; this lowers the pressure at the low pressure control of the supply which then operates to shut off the water supply. The power for the malfunction control including the discharge valve operation is supplied by a battery located adjacent the arm pivot and no electric power supply or control wires are required between the pivot area and the water supply equipment.