A bad speed sensor most often affects the shifting of the automatic transmission. However, in cars with an onboard computer, the sensor also affects the ABS system, speedometer, cruise control and power steering. Proper diagnosis and repair is essential for safe operation of the car.
The speed this sensor monitors is that of the engine. The sensor transmits the information to the computer, which automatically makes the necessary adjustments to keep the engine and other components operating at top efficiency. If the sensor sends faulty information to the computer, the computer returns faulty information to these other systems.
The speed sensor attaches to the engine's crankshaft. It is a metal disk with teeth around its circumference. The sensor also includes a stationary magnetic coil. The spinning of the crankshaft creates an induction current around the coil. The teeth on the edge of the disk cause obstructions in the magnetic field, allowing the computer to determine the speed at which the crankshaft and engine are turning.
For diagnosis of this type of problem, take the car to a general auto repair shop. Even though these symptoms mimic transmission problems, the general shop is equipped to make the diagnosis using a scan tool and then direct the customer to a specialty shop if necessary.