Technical Note

Document Number CTN0007

Title Active/Magneto-Resistive Sensors
Approved By PM
Revision Date Prepared By Change History
1.0 23/3/2010 DPM

Active/Magneto-Resistive Sensors
Magneto-Resistive speed sensors (also called Active
Wheel Speed Sensors) offer the switching benefits of Hall
sensors with 2-wire connection. These rely on the
presence of an external magnetic field for sensing, and
are typically used for wheel speed, water speed, and
crankshaft position sensors (for example, Evo X wheel
speed, Seadoo RXP water speed).

These sensors are unusual in that they do not switch to

ground as a Hall sensor might be expected to do.
Rather, they operate as a variable current switch with
two levels, both of which require some current to pass
through the device. For this method to work in a 2-wire
sensor a pull-up is required on the signal line to ensure that sufficient current is available to power
the device.

Typically 3 to 6 mA might be passed, with corresponding output voltage levels dependent upon
the size of the supply pull-up resistor. In all cases the low level voltage is still considerably higher than
0 V.

Scope
This document describes the use of Magneto-Resistive Speed sensors with MoTeC hundred series
ECUs and Dash Loggers.

Using Magneto-Resistive Sensors with MoTeC Devices

Powering the Sensor

Magneto-resistive sensors must have current flowing even in the ‘low’ state. The standard pull-up
resistors which are used on MoTeC digital inputs (4k7 Ω to 5 V) do not normally provide enough
current for the sensors to operate correctly. Thus an additional pull-up resistor may be required to 5
V, 8 V, or 12 V. As the sensors are current-based, any of these supply voltages may be used so long
as the pull-up resistor is chosen to limit the maximum current.

Output voltage levels are a result of the series current through the device, in each of its two
switching states. The output voltage is never switched fully to the supply or ground.

Hundred Series ECUs

For MoTeC Hundred Series ECUs, digital inputs are used for speed measurements. These digital
inputs require a low signal below 1.0 V and a high signal above 3.5 V. To operate magneto-resistive
sensors, a suitable supply resistor value must be found to reach these values.

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MoTeC Pty Ltd CTN0007 Active/Magneto-Resistive Sensors

It is not likely to find a supply resistor to directly interface with magneto-resistive sensors. In some
cases MoTeC's DMC-F, Dual Magnetic Converter may be a simple solution. The DMC-F with an
internal pull-up resistor of 440 Ω to 5 V, a voltage threshold of 3.0 V and output switch might provide
a suitable signal for the ECU's digital inputs. You should test the sensor in order to determine If your
sensor can operate at these levels. The output is equivalent to a Hall sensor output and normal ECU
settings are used.

Example of DMC-F usage

The oscilloscope capture shows (in

blue) a Seadoo speed sensor
powered from 5 V via a 440 Ω pull-up
resistor. The output voltage levels are
2.46 V and 3.6 V from the sensor.

The Red trace is the switched output

from a DMC-F device. In this case,
the low voltage level is below 1.0 V
and the high signal above 3.5 V.

Dash Logger

For MoTeC's Dash Loggers (e.g. ADL2, ADL3) speed inputs are used. They have user-adjustable
voltage levels and can be used with a suitable pull-up resistor and appropriate settings. Dash
Logger digital inputs are not suitable for use with magneto-resistive sensors.

Testing unknown sensors

Determining Sensor Polarity

The sensor will frequently be in an existing harness where positive and negative wires can be
determined. If not, use a digital voltmeter to make resistance and diode voltage drop tests to
ascertain the likely polarity. Create a table as shown and replace the example values with your
actual measurements.
Resistance Measurements
Example Values
(+) probe to wire 1 (-) probe to wire 2 22.1 kΩ
(+) probe to wire 2 (-) probe to wire 1 22.1 kΩ
Diode Volts Drop Measurement
Example Values
(+) probe to wire 1 (-) probe to wire 2 0.645 V
(+) probe to wire 2 (-) probe to wire 1 1.164 V

In this case, the resistance measurements do not help to identify polarity, but the diode volts drop
measurement indicates that wire 1 is the negative and wire 2 is the positive wire. This is indicated by
the 0.645 V drop when the sensor is wired in reverse, a typical protection diode reading when the
+probe is connected to the negative pin and the –probe is connected to the positive pin.

If you need assistance, complete this table and send it to MoTeC for evaluation.

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MoTeC Pty Ltd CTN0007 Active/Magneto-Resistive Sensors

Determine Pull-up Resistor

Connect the sensor with a pull-up resistor to a sensor supply voltage pin:

M800 supply pins

Supply voltage pin no
8V A12,
5V A21, A9
Ground B14, B15, B16

ADL2 supply pins

Supply voltage pin no
8V 62
5V 18,28,44
Ground 51, 56, 61

Power up the sensor and monitor the signal voltage levels on an

oscilloscope or voltmeter. As the target is moved, the output voltage
should vary from high to low voltage levels.

Start with a 5 V supply and 4k7 (4700 Ω) pull-up resistor. If the pull-up
resistance is too high, (insufficient current to operate the sensor) the
output voltage will not change, and may be around ½ the supply
voltage.

Reduce the resistance until there is enough difference in low and high
output voltage to enable a correct configuration on the ECU or Dash
Logger.

Note – if the pull-up resistance is too low, excess current may flow and damage the sensor. These
sensors should never be connected directly to a supply voltage without a pull-up resistor.

A table of results should show supply voltage, pull-up resistance, and high and low output levels.

Supply Pull-up Example Values*

Voltage Resistance High output V Low output V Comment
5V 4.7 kΩ 2.7 V 2.7 V Unsuitable – not enough current
5V 2.0 kΩ 2.7 V 2.7 V Unsuitable – not enough current
5V 1.0 kΩ 2.92 V 2.74 V Just usable with Dash Logger.
5V 560 Ω 3.22 V 2.88 V OK for Dash Logger, not for M800
8V 2.0 kΩ 3.0 V 2.76 V OK for Dash Logger, not for M800
8V 1.0 kΩ 3.8 V 3.0 V Good for Dash Logger, not for M800
8V 560 Ω 5.0 V 3.64 V Good for Dash Logger, not for M800
* These figures are example results from an EVO X wheel speed sensor with integral hub magnet
ring. Replace these values with your actual measurements

If you need assistance, complete this table and send it to MoTeC for evaluation.

None of the pull-up tests shown above resulted in a voltage change which would trigger an
hundred series ECU DIG channel.

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MoTeC Pty Ltd CTN0007 Active/Magneto-Resistive Sensors

In this case, the sensor was used with a Dash logger and a pull-up of 1 kΩ to 8 V was chosen, with a
midpoint of 3.4 V.

Example of Sensor Setup

Wheel speed sensors into Speed inputs - pins 63-66 for Speed channels 1-4.
Input Spd1 pin no 63 Channel Wheel Speed FL - repeated for other 3 speed inputs

Calibration
Sensor Type Magnetic
Magnetic Levels calibration 3.4 V: 0 kmh
3.4 V: 300 kmh*
Pulses Per Revolution 48 *
Rolling Circumference 2000 mm*
* Replace the example values with your values

Note – some sensors (for example BMW wheel speed sensors) generate three output levels rather
than two (see scope capture).

In this example, the high level of 3.64 V represents a logic

high and the low level of 1.04 V a logic low for speed
measurement. The middle level of 1.92 V is used for
status/diagnostics. For correct triggering in a Dash Logger
the voltage threshold would be set to 2.4 V, so all of the
status data would be ignored.

Connection Diagrams

Some vehicles (for example Hyundai i30)

use an alternate connection method for
these sensors, where the series resistor is
used as a pull-down to ground, with the
positive side of the sensor wired direct to
12 V. This is functionally the same, but
would produce different sensor output
voltages to a pull-up configuration. In
both cases the polarity of the sensor is
the same, as is the direction of current
flow in the two switched states.

Pull-Up configuration
Pull-Down configuration

Summary
• Hundred series ECUs are not likely to directly interface with Magneto-Resistive Sensors. A
DMC-F may be suitable once confirmed by testing.
• Polarity may be determined using a diode test on a voltmeter, but OEM schematics are
preferable.
• Pull-up resistor values should be determined by starting with higher values (example 4k7 Ω)
and reducing the resistance until a suitable voltage change shows for the two states.
• Sensors with multi-level outputs and triggering threshold voltages must be set to ‘filter’
unwanted data.

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