August 29, 2011
2003 Mercedes SBC brake system
A technician's best tool is information on how the components work
By: Information provided by Identifix Inc.
Figure 1
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.
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