Road speed sensor - Hall effect

The purpose of this test is to evaluate the correct operation of a Hall effect speed sensor based on the output voltage and frequency in response to road speed.

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and an acupuncture or multimeter probe on the test lead with the coloured moulding (positive).

Method 1: jack up the driven wheels of the vehicle and place on axle stands on firm level ground, then probe each of the three connections as illustrated in Figure 1 (the three connections being: the sensor's voltage supply, an earth and the Hall effect / road speed output). If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout lead or box if you have one available.

Monitor the Hall effect output - the frequency will increase with roadspeed and the timebase may require adjusting accordingly.

Method 2: identify the Hall effect output and roadtest the vehicle with the PicoScope and laptop computer inside the vehicle, taking care to route the test leads away from hot or moving components.

Figure 1 shows the sensor being probed at the gearbox speedo drive.

Example waveform

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.

rss road speed sensor

Figure 2

Technical information

These road speed sensors are now common place on many of today's modern motor vehicles, their function is to provide information to the Engine Control Module (ECM) monitoring the vehicle's momentum.
The control unit now has the ability to determine the idle speed when the vehicle is slowing or stationary and not at any other time during the vehicle's journey. The sensor will be located on either the speedometer drive output from the gearbox or to the rear of the speedometer head.

A typical road speed sensor or Vehicle Speed Sensor (VSS) will produce either an analogue output from a magnetic inductive sensor, or a digital square wave from a voltage driven unit. These sensors can be either a Hall effect device with 3 electrical connections or a Reed switch with 2 connections.

Figure 2 shows a typical road speed sensor which is mounted between the gearbox speedo drive and the speedo drive cable.

The testing of the inductive pick up is identical to a crank angle sensor, resulting in a sine wave and the normal resistance testing. The Hall and Reed switches will provide a square wave and like the inductive sensor, the waveforms can be viewed on an oscilloscope.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Multimeter Probes


  • Large Dolphin/Gator Clips


  • PicoScope Battery Clip


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Hall effect