Mazda Training manual — part 89

Dynamic Driving Safety Systems

ABS

Wheel Speed Sensor

•

Depending on the vehicle, the ABS uses passive or active wheel speed sensors. Each
wheel sensor sends a pulsed speed signal to the ABS control module. Based on those
signals, the control module calculates the speed of each wheel and the vehicle speed.
When a wheel speed difference occurs, while comparing the vehicle speed with each
individual wheel speed, this is used as an indication of changing grip on an individual
wheel.

•

Based on this information, the ABS control module calculates the slip of each wheel
during braking. With the aid of the ABS operation the ABS control module keeps the slip
of the wheels in the range of 15…30 % to ensure good steerability and high brake
forces.

L2003_02009

Mazda3

1

Wheel speed sensor

3

Wheel bearing

2 Steering

knuckle

4 Pulse

rotor

Curriculum Training

02-15

ABS

Dynamic Driving Safety Systems

Passive Wheel Speed Sensor

•

The passive wheel speed sensors are an inductive-type, i.e. the sensors produce an
analog voltage signal via induction due to a changing magnetic field, caused by the
turning wheel.

•

Passive wheel speed sensors are installed on B-Series (UN), Premacy (CP), MX-5
(NB), Mazda6 (GG/GY), MPV (LW), and RX-8 (SE).

•

The voltage output increases with increased wheel speed. With very low wheel speed
(below approx. 6 km/h), the voltage output is too low to deliver a proper wheel speed
signal to the CM.

•

The CM uses the number of pulses per second to calculate the wheel speed. The
vehicle speed is calculated by the CM, based on the wheel speed of all wheels.

NOTE: Strong vibrations can demagnetise permanent magnets. Hammer blows in the

vicinity of the wheel sensor (e.g. when repalcing a wheel bearing) could permanently
damage the sensor, if it is not previously removed.

L2003_02010

1

Wheel speed sensor

5

Pulse rotor

2

Permanent magnet

6

Air gap

3

Coil

7

Connection to ABS CM

4 Iron

core

02-16 Curriculum

Training

Dynamic Driving Safety Systems

ABS

L2003_02053

1

Wheel speed sensor

2

ABS HU/CM

Diagnostics

•

The inductive wheel speed sensor can be checked by:

–

Reading out DTCs

–

Checking the voltage signal

–

Measuring its resistance

–

Checking the air gap

–

Monitoring the corresponding PIDs (see below)

PID

Definition Unit/Condition

LF_WSPD

LF ABS wheel-speed sensor input

km/h, mph

LR_WSPD

LR ABS wheel-speed sensor input

km/h, mph

RF_WSPD

RF ABS wheel-speed sensor input

km/h, mph

RR_WSPD

RR ABS wheel-speed sensor input

km/h, mph

L2003_T02005

Curriculum Training

02-17

ABS

Dynamic Driving Safety Systems

Checking the Air Gap

•

If the wheel speed signal is faulty, or when replacing a wheel speed sensor (active or
passive), make sure to adjust the gap between the sensor and the rotor according to the
W/M. If the gap is too large, this can cause inaccurate wheel speed signals. If the gap is
too small, the rotor and the sensor could be damaged.

NOTE: An air gap exceeding the specifications could be caused by excessive wheel-

bearing clearance.

•

Keep the rotor and the sensor clean from metal debris and other dirt to ensure a proper
signal is generated.

L2003_02012

1

Wheel speed sensor

3

Pulse rotor

2 Gap

02-18 Curriculum

Training