The wheel speed sensor on a 2001 Oldsmobile Alero is located
within the wheel hub assembly, at the back of the brake rotor. To access it, you'll need to remove the wheel, the brake caliper (and possibly support it with a bungee cord or wire), and possibly the brake rotor itself depending on the design. There's one sensor per wheel.
It's a crucial component and is usually a magnetic type sensor. Be careful when working near it to avoid damaging it or getting debris in it.
