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Road Speed Sensor Waveform Notes
The Electronic Control Module (ECM) has the ability to adjust the engines idle speed when the vehicle is slowing or stationary by using information from the Road Speed Sensor (RSS). The sensor is a 3 wire device and will have a supply at battery voltage, an earth and a digital square wave output also switching at 12 volts.
With the appropriate electrical connection made to the RSS output, raise one wheel with a trolley and place an axle stand under the suspension unit. Start the engine and select a gear, a waveform switching from 12 volts to zero should be seen. As the road speed is increased the frequency of the switching should be seen to increase. This change can also be measured on a multimeter with frequency capabilities. The sensor will be located on either the speedometer drive output from the gearbox or to the rear of the speedometer head.
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