Bilge pumps, automatic or manual, are designed to remove liquids from the bilge. An automatic bilge pump should always be a simple system with a direct, continuous and reliable connection to power.
This is, usually, the case when the boat rolls off the production line. Unfortunately, different owners make modifications over time without considering the importance of this system.
It is important that you know when the bilge pump is working. There should always be an indicator of some kind that tells you if the pumps are running or if there is a high water warning. You must always know if there is water inside the boat and how often your bilge pump comes on. Depending on the amount of liquid in the bilge, you can allow the automatic bilge to kick in or you may choose to use a manual override. With the flip of a switch the automatic feature of the system is overridden and the pump is made to work regardless of the level.
Over the years of troubleshooting bilge pumps, we have noticed that quite often it is the float switch that fails. Some of these switches are built with a small drop of mercury within an enclosure. When the lever that floats reaches a certain position the mercury makes contact with the two wires thus closing the circuit and starting the pump.
In larger systems, with bigger pumps, the float switch is activated by a spark because of the inrush current of the pump. Over time, this spark can wear out the conductors within the enclosure causing the switch to fail. When trouble shooting your system, don't forget to test the switches.
The connection from the battery to the bilge pump is one of the few connections that should bypass the house switch. Only under special circumstances should there be a break of power to the bilge pumps and even then this break should be temporary. i.e.: when you are conditioning your batteries, checking the electrical system and you need to isolate the batteries or when the boat is on dry dock.