SUBJECT VEHICLES: 2003 to 2006 Mercedes E, CLS, and SL Class vehicles.
COMPLAINT: Message indicating worn brake pads.
CAUSE/SOLUTION: Front/rear brake job needed on vehicle with Sensotronic Brake Control. See following directions for correct steps in servicing this system.
They’re just brakes, right? I still remember the day some years ago when a customer came into the shop where I was working. He wanted to know why a warning message was displayed on the instrument cluster of his 2003 Mercedes E-class. The message indicated worn-out brake pads. Further inspection determined that a front and rear brake job was needed. The customer declined the work and took the car home.
The next day went a little like this: A customer went to the parts department and purchased front and rear brake parts for a 2003 Mercedes E-class. Within a few hours, the parts department received a call asking how much new front and rear brake calipers cost for that same vehicle. This was immediately followed by a call to the shop asking how much labor would be involved to replace all the brake calipers.
Anyone who is not fully aware of the type of brake system that is installed on the vehicle can make this mistake. The customer failed to read the safety warnings that came with the brake pads, or he would have noticed that this is not your average do-it-yourself job.
The Sensotronic Brake Control (SBC) is a type of brake system used by Mercedes, similar to the throttle-by-wire systems that most technicians are familiar with. SBC has several components that, visually, appear no different from any other ABS-equipped car. SBC has a hydraulic pump/control unit, normal brake calipers, and what looks like a master cylinder with fluid reservoir. Although it may look like other ABS systems, there are more brake lines, hoses and wiring; no brake booster; and the pump runs a lot more. This is just what you see on the outside.
What you don’t see is that the complex operation of the SBC has the capability to perform all traction control and ABS operations that are needed at each wheel, separately from one another, without any feedback at the brake pedal. Not only does the SBC apply the brakes before you even get into the car but it can apply the brakes when the wipers are turned on so that the brake pads stay dry. The control unit for the SBC is in constant communication with all of the vehicle’s engine, transmission and body systems.
Several components located in the engine compartment are special to the SBC (see Figure 1). The Brake Operating Unit or BOU is what most would call a “master cylinder.” The BOU contains a pedal sensor so the control unit knows how fast and how far the brake pedal is being depressed. It also has a brake pressure simulator that incorporates a couple of springs and a floating piston so that the brake pedal feels like a normal master cylinder to the driver. The simulator is needed since there is no direct hydraulic contact between the brake pedal and the brake calipers under normal operation.
The SBC hydraulic unit itself contains the high pressure pump, pressure reservoir and the SBC control module. Like other Mercedes control units, this module is programmed to the car using factory tools.
A factory type scan tool is needed to correctly test and service the brakes on this system.
For example, if the brakes need to be flushed, a factory scan tool performs activations of the pump and solenoids in the hydraulic unit to remove all air, along with prompting the technician as to what bleeder screw to open and the pressures to set the pressure bleeder.
This takes us back to the beginning of this story. The customer did not deactivate the SBC before working on the brakes. It is necessary to deactivate the SBC to prevent buildup of brake pressure to the brake calipers. If any work being done on the car involves removal of the calipers, the SBC must be deactivated or the brakes may apply themselves during the repair.
All it takes for the system to apply the brakes is for someone to open a door, operate the locks, open the trunk, depress the brake pedal, and, of course, turn the key on. You can only imagine what can happen if a hand is in the way when there is over 900 psi applied to the calipers.
Like the case of the customer performing his own repairs, you could end up with all four caliper pistons and 1.5 liters of fluid on the garage floor.
The SBC is very expensive, so not every Mercedes is equipped with this system. It is mainly found on the 2003 to 2006 E, CLS and SL (The SL may include newer models). The trick is to know what to look for and how to safely repair these cars. Even if you don’t have a scan tool that does the deactivation and reactivation operations, a series of steps can be performed to manually deactivate and then reactivate the SBC.
Since the process is a bit long, I suggest reading the instructions completely before starting the work.
One other item to be aware of: noise. In order to maintain the proper amount of operating pressure, the hydraulic pump needs to run after a couple of pedal applications. This constant turning on and off of the pump can be questioned by customers as a possible problem. To help determine what is normal, Mercedes has provided some service bulletins on the subject.
Some noises that are not inherent to the design of the SBC system require updated hoses, possible bleeding of air trapped in the system, or a software update of the control unit itself. Some require the factory tool to perform.
Overall, the SBC system works quite well, but in order for it to be maintained properly, the technician’s best tool is information on how it works.
Information provided by Identifix Inc. For more information, call (800) 997-1674 or visit www.identifix.com.