The process of testing a speed sensor involves removing the sensor, connecting the sensor's electrical leads to a multimeter and then powering the sensor using a handheld drill. Viable sensors should show a voltage increase proportionate to the speed of the drill. Replace speed sensors that fail to show a difference in voltage when tested at various speeds.
Speed sensors are attached to the crankshaft of an engine and assess the speeds at which it spins. As the crankshaft rotates, induction current travels up around the magnetic coil, and the serrated edge of the shaft obstructs the production of a magnetic field. The sensor measures the amount of current produced as the crankshaft spins, which is used to determine the speed of the vehicle's engine. Problems associated with a bad sensor may include problems with the engine's fuel-to-air ratio or issues with the cruise control.
Some sensors are designed to be self-powered and have a third electrical wire. Test sensors that have three electrical leads by connecting the sensor to the multimeter and using a power source in place of a drill. Ensure the sensor is properly connected to both the power source and that the multimeter is attached to the red electrical lead and piggybacked to the black ground lead.