Tuesday, March 5, 2013

ABS Brake Sensors Analysis

I saved 3 used ABS brake sensor assemblies cables. One from a BMW which had indicated it was bad. One from Andre’s vehicle as a new ABS sensor was provided when he replaced the left front wheel bearing assembly. The third when Paul replaced his front wheel bearing assembly.

To test these brake sensor assemblies I used a 9 volt power supply and connected the minus lead to a 1,000 ohm resistor then to one of the two leads to the sensor.
The other lead of the sensor I connected to the plus side of the 9 volt supply.

I then placed an oscilloscope across the resistor. By running a screwdriver past the sensor I would get a nice negative pulse on the oscilloscope when connected to the sensor one way and no pulse when connected in the other polarity.

The pulses I observed are shown below.

This all makes good sense as the bad sensor was known by the BMW’s computer fault code to be bad. The two sensors that I could get a pulse from were replaced only because a wheel bearing went bad. (The sensors I suspect are just hall effect sensors.)

The Antilock Brake System (ABS) works something like this, I suppose. For some interesting physics phenomenon, a tire that is sliding has less traction than a tire that is not quite sliding. So if we keep a wheel turning slightly in an emergency brake we can stop sooner than if the wheel is completely stopped. Also one can continue to steer the car. If you are in a complete slide the car will not turn. So enter ABS. Using a thin light cogged disk which is attached to the wheel they place a sensor near the cogs at the outside edge of the disk which senses when each cog rotates past the sensor. At each sense a little pulse is created by the sensor. Now if the wheel is turning the sensor will send out a series of pulses which relates to how fast the wheel is turning.

This string of pulses goes to the computer which does nothing if there are pulses. However if the vehicle is moving and the one wheel’s pulses stop the computer (ABS) interrupts the hydraulic pressure going to the brake caliper at that wheel in a pulsing motion which ensures that wheel continues some spinning which increases traction for faster stopping and also providing some steering control.

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