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by Bob O’Neill and Bob Lincoln
The speed/distance sensor sends a signal to the ECU to determine the speed of the vehicle. There are many functions that use this information, including the idle speed control, cruise control and radiator fan relay. This information is also used as diagnostic for other sensors and then generates fault codes for them.
Because so many systems use the speed, a faulty speed/distance sensor can generate codes other than code 15. It can cause the engine to lose control of RPM to race the engine or cause it to stall at idle, and can cause an automatic transmission to go into limp mode, and not shift above second gear. If the speed sensor fails, the speedometer and odometer will stop working in vehicles which use electrical signals to drive the speedometer (phased in during the 1990s), while the tachometer will still work. (Many cars had both a speed sensor and a physical speedometer cable. In the Shadow this was changed in 1992 or 1993.)
The construction of the sensor was changed after 1992. Through 1992, the sensor has a glass enclosed reed switch: an eight pole magnet rotates which causes the reed switch ‘pulse’ once for each pass of a pole of the magnet. The speed/distance sensor is mounted on the transmission and the gear pinion gear rotates the magnet. One end of the reed contact is grounded and the other is connected to a 5v supplied by the ECU. The magnet closes the contact 8 times per turn of the revolution of the center shaft of the sensor. This means that it rotates 1000 times per mile or 8000 pulses per mile. Because the earlier style used a glass enclosed reed switch they are prone to failure.
In 1993 and later cars, the sensor uses solid state Hall Effect components to replace the contacts of the earlier style.
A code 15 is set when there is no signal from the speed/distance sensor. The power limited light will not light and limp mode does not apply. The code is set if the speed sensor is not producing a signal, if the throttle has been closed for 7 seconds and the engine speed is 768RPM higher than the target idle speed.
Poor connection or bad wiring – Read ‘How to troubleshoot drivability issues.’ Like other sensors which communicate with the ECU a good ground connection and good power feed is important. Before diagnosing any ‘bad’ sensor first be sure that their electrical connectors and wiring is good.
Defective speed/distance sensor - If you have earlier style sensor, 1992 and earlier, they are prone to failure. The sensor is located on the transmission on top of the long casting where the right axle connects.
Sensors, Switches, and Other Systems | Main Repairs Page | EEKs
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