The answer to 'my sump pump smells bad' is that a smelly sump pump is usually caused by bacteria buildup or rotting organic matter in the pump or pit. Common symptoms include a foul odor, slime, or discoloration in the sump pit. To fix this, you'll need to clean and disinfect the sump pump and pit.
Start by unplugging the sump pump and removing it from the pit. Use a wet/dry shop vacuum to suck out any standing water and debris. Then scrub the pit and pump with a stiff brush and a diluted bleach solution (1 part bleach to 10 parts water). Let it air dry completely before reassembling. You may also want to treat the pit with a sump pump cleaner or enzyme product to help break down organic matter and prevent future odors.