This sensor is used by the logic module to determine the vehicle's speed. This information is used for several functions including idle speed control, radiator fan relay control, and speed control. It is also used to diagnose other sensors and generate fault codes for them. As a result, a bad speed/distance sensor can generate other codes as well or even cause the engine to race or stall at idle.
1992 and earlier speed/distance sensors use glass-enclosed, magnetic reed contacts next to an 8-pole, round magnet. The magnet is mounted to a shaft which is driven by the right output shaft on the transmission (some later models with mechanical speedometers have the sensor mounted behind the speedometer). One contact is grounded and the other is pulled up to 5V by the logic module. The contacts close 8 times per revolution and the magnet rotates 1000 times per mile. Therefore the sensor sends out 8000 pulses per mile. These contacts are prone to failure, and in 1993 a solid-state hall-effect sensor was put in place of the contacts. The sensor still behaved the same, but an additional pin was added to the connector to provide 8V to power the sensor. See my Speed/Distance Sensor Modification page for details on how to upgrade your mechanical sensor.
The sensor is diagnosed during vehicle deceleration. If the throttle has been closed for 7 seconds, the engine speed is is still at least 768 rpm above the target idle speed, and the speed sensor is not producing a signal, a fault code 15 is stored. There is no limp-in mode for this sensor and the logic module still treats any pulses from the sensor as valid.
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Updated 12/19/2003.
Copyright © 1996-2003 Russ W. Knize.