Abstract: A warning device is disclosed for an evaporative cooler or swamp cooler. The alarm comprises a pair of closely spaced electrodes mounted at a desired position above the floor of a water reservoir in the evaporative cooler. During normal operation of the evaporative cooler, the electrodes are not submerged in the pater. Upon malfunction of the float valve in the water supply to the reservoir, uncontrolled amounts of water are added to the reservoir, and the water level rises. When the water level attains a specified depth, the electrodes become submerged in the water. An electrical circuit is associated with the electrodes, and when the electrodes become submerged in the water, the electrical circuit is closed. The closing of the circuit activates an audible alarm which can be heard by inhabitants in the building being served by the evaporative cooler. Preventive maintenance can then be undertaken before extensive damage is done by water overflowing from the water reservoir of the evaporative cooler.