Irregularly emptied tank filling system is operated with two MIN and MAX sensors. The pump motor is switched-ON when the fluid level is below the minimum level (MIN = 0, MAX = 0) and is switched-OFF when the fluid level has reached its maximum (MIN = 1, MAX = 1). If the level is between the two sensors (MIN = 1, MAX = 0),the engine is running and the pump is switched-ON briefly. In case of (MIN = 0, MAX = 1) a tank or sensor failure has occurred.