If your sump pump does not turn on and off as it should, you may need to replace the float switch. A float switch can become broken because of debris or lack of proper maintenance. Before you replace the float switch, you should let your sump pump run until all the water has been removed from the sump basin. After that has been completed, disconnect the power to the sump pump.
Steps to Follow
Consult your owner’s manual to find the location of the float switch on your sump pump.
Remove the float switch from the pump unit and then open the float switch housing.
Unscrew the connection on the float switch so you can see the electric switch inside.
Remove the old switch and then glue in a new one with waterproof glue.
Let the glue dry completely before you reassemble the switch and put it back in the sump pump.
After you put the float switch in the pump, seal any areas that were sealed previously.
If You Don’t Want to Replace The Switch
If you would rather not replace the float switch, another option is to add a new, external switch with a piggyback plug so you can bypass the existing switch. This is easier than replacing the float switch.
Run the sump pump until all the water is removed from the sump basin, then disconnect the 
power.
Bypass the circuit on the original float switch and use silicone sealant to thoroughly seal the housing.
Loosely attach the replacement switch to the pump housing with a zip tie. Plug the switch into the outlet and plug the bypass plug into the switch.
Fill the basin with water. Adjust the replacement switch attachment point so the pump will turn on at the right basin water level.
Then tighten the zip tie to secure the new float switch.
If you need to replace your sump pump’s float switch, SMD Fluid Controls has some of the best new switches on the market, like the FN20 Mechanical Sump Pump Switch.