Opel "Astra"
Brake Master Cylinder
Hydraulic module, Electronic Control Unit, Hydraulic pump
Hydraulic Control Unit
Two lines from master cylinder (in), Four lines to wheel cylinders or calipers (out)
In order to test a speed sensor, we need to locate the connection plug by following
the wire from the speed sensor it self.( In different cars-different places)
changing magnetic poles.
We can see how the wheel speed sensor tone ring is built into the
wheel bearing assembly, unlike other wheel speed sensor setups
where the tone ring is on the outside of the outer CV joint housing,
axle or hub.
wheel bearing assembly, unlike other wheel speed sensor setups
where the tone ring is on the outside of the outer CV joint housing,
axle or hub.
Wheel speed sensors consist of a toothed rotor and a pickup.Wheel rotation sends input signals to the ECU, which processes them and controls the hydraulic control unit.
A wheel sensor consists of a toothed rotor that rotates with the wheels, and a pickup. As each tooth of the rotor passes the pickup, a small voltage is induced in the pickup. These pulses are sent as input signals to the electronic control unit which processes them, to operate the hydraulic control unit.
A magnetic pick-up has a magnet which changes reluctance as the
magnet is near or far from the iron on the teeth of the ring.
Slow rotation of wheel
Faster rotation of wheel
More faster rotation of wheel.
The faster the wheel turns, the more voltage it produces.
The signal pulse is generated when a tooth passes under
the magnetic tip of the sensor. The tooth passing through the
sensor’s magnetic field causes the sensor to act like a little generator
and produce an alternating current signal that increases in frequency
and amplitude in direct proportion to wheel speed. If we look at the
signal on an oscilloscope, it should look like a nice even sine wave
with all the up and down humps in the waveform evenly spaced
and the same height.
A good wheel speed sensor will typically produce an alternating current (AC) voltage reading of 50 to 700 MV when the wheel is turning one revolution per second.
If the scope pattern produced by the sensor is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal
can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.
can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.
Salt at Fault
In the case of the Chevy/GMC recalls, salt water was apparently working its way into the hub assembly at the sensor hole and attacking the tone ring. In many instances, the problem was not bad enough to set a fault code and turn on the ABS warning light. But it was bad enough to generate misleading wheel speed readings and trick the ABS system into kicking in and pulsing the brakes when ABS was not needed.
In the case of the Chevy/GMC recalls, salt water was apparently working its way into the hub assembly at the sensor hole and attacking the tone ring. In many instances, the problem was not bad enough to set a fault code and turn on the ABS warning light. But it was bad enough to generate misleading wheel speed readings and trick the ABS system into kicking in and pulsing the brakes when ABS was not needed.
Magnetic sensors can also pickup metallic wear debris from the rotors and pads, causing the sensors to read erratically and confuse the anti-lock brake system. Cleaning the sensors and resetting the air gap can often restore normal operation. But if the tone ring is corroded or damaged, it must be replaced.
Reading ABS codes requires an ABS code reader, scan tool or scanner software that can access the ABS system. An inexpensive OBD II code reader or an entry-level scan tool designed for a do-it-yourselfer won’t work here. You need a professional tool designed for ABS diagnostics, or a digital storage oscilloscope to look at the wheel speed sensor waveforms.
If there are no codes, but the vehicle owner complains about the ABS system engaging when braking (noise, vibrations and pedal pulsations), the problem is likely a bad wheel speed sensor. But which one? That’s where a scope can really help you identify which sensor is acting up. Connect the scope to the sensor leads and spin the tire by hand. If you get a good clean signal, move on to the next WSS sensor and so on until you find the one that is generating a bad signal.
You need to make sure you have diagnosed the fault correctly on vehicles with ABS wheel speed sensors because the replacement parts are not cheap.
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