Always Braking Suddenly

December 16, 2008 | By | Reply More


One of the first major advancements that used a computer to assist with vehicle safety is the anti-lock braking system. The premise is simple:  To slow down the vehicle as fast as possible by maintaining traction between the tires and the road.  If the braking force overcomes traction and the wheels lock up, it reduces hydraulic pressure to the brakes. Once the wheels regain grip, it quickly reapplies the pressure. Obviously, it’s important to keep this system functioning properly for the safety of your customer. Depending on what is wrong, a repair may be expensive, but the cost of an accident will far outweigh the money saved in not repairing the system. Let’s see how we can keep the diagnosis and repair profitable, and at a cost your customer sees as a wise investment.

Who Is Involved

Knowing how the components function will help you diagnose any problem. As with any computer-controlled system, by definition a computer processor must be present. This is the main brain of the ABS. It receives inputs from sensors, processes the information, and, depending on programming, manipulates outputs. These output controls actively change the way the vehicle behaves. In the case of the ABS, it controls the hydraulic pressure applied to the calipers. It applies fluid pressure when the wheels have traction with the road. When lockup occurs, output controls will reduce brake pressure and allow the wheel to regain grip.

For decades, BMW has incorporated self-diagnostic features into the control unit for the ABS. When the ignition key is cycled on, the ABS control unit goes through a “dynamic circuit check.” That is, it monitors input circuits for incorrect voltage readings, such as electrical shorts to power or ground. It does the same for output controls. It monitors the voltage on the wires going into and out of the control unit. If there is a problem, the computer activates a warning light indicating to the driver that the ABS will not function as designed. The ABS light will come on for a bulb check with the ignition key turned on, and it will remain on until all circuit tests are complete. At this point, if all the tests are passed the light will go out. If a test fails the light will remain on.


front speed sensor

Pictured here is a new left front wheel speed sensor. Notice how the wiring is positioned around the strut assembly. This way, as the wheel is steered the wiring is not stressed. Make sure the wiring is not damaged when replacing the sensor. Damaged wiring may have been the real cause of the problem.

Input sensors are critical to ABS operation. The main sensors that allow the ABS to function are the wheel speed sensors. These are generally mounted on the spindle of each wheel, and they read the movement of a toothed ring gear mounted on the rotating hub assembly or axle. One type is the AC pulse generator, or inductive speed sensor. These have a winding or


Login to continue reading this article.

Not a member? Register for FREE access.

Lost your password?

Category: BMW TechDrive, Brakes

Leave a Reply

You must be logged in to post a comment.