Where is the crank sensor on a 1991 Buick Regal - 31L?

The crank sensor on a 1991 Buick Regal with the 3.1L V6 engine is located on the passenger side of the engine block, near the oil filter. It's typically mounted on the engine block itself, not the bellhousing. You'll likely find it near the front of the engine, fairly low down.

However, without visually inspecting your specific vehicle, this is just a general location. A repair manual specific to your year and engine will give the most precise location. A picture or diagram in a repair manual would be ideal. You can often find these online through sites like Haynes or Chilton.