Abstract: A modification of a fuel filter for use with an internal combustion engine is described. The modification provides a switch element that, in conjunction with the water level sensor conventionally provided in the filter, enables detection of the presence or absence of a filter insert or whether an improper insert has been installed in the filter housing. When the filter insert is inserted or removed, the switch element makes or breaks an electrical circuit that is part of the water level sensor. The control unit is thus able to detect switch states of three different variables in the filter: the water level is/is not too high; the filter insert is/is not installed; or the correct filter is/is not installed. The control unit accordingly allows or prevents operation of the motor, depending on the detected switch state.

Abstract: The invention relates to a fuel filter comprising a housing, an interchangeable filter cartridge that is located in the housing, a water collection chamber and a water level sensor. Said water level sensor has two electric conductors, which can be electrically interconnected by means of water if there is a correspondingly high water level. The filter is provided with a switching element that can move between a contact position and an interruption position, said switching element making contact with the two conductors in the contact position and the contact between the switching element and at least one of the two conductors is interrupted when the switching element is in the interruption position. The electric resistance that occurs between the two conductors has a first value, when the switching element is in the first contact position and a second value that differs from the first, if the switching element is in the interruption position.