I came across an article about a.o. our SBC system, written by a German company that repairs automotive electronic units. I believe it shares a well balanced opinion. With MB’s extended SBC warranty much of the financial agony around SBC has gone (for now) but the technical ins and outs remain as interesting. The author mentions 180.000 as the exact number of pedal hits (at last we know) that triggers the unit to come up with the SBC warning.

I used the German text and a less than perfect English (machine-)translation on the same site. I omitted the sections about a BMW motorcycle ABS system, made by FTE.
Source: https://rhelectronics.de/de/qualitaet-und-sicherheit/sicherheit/

Safety and the repair of control devices.

The problems described here relate mainly to ABS, ASR, DSC, ESP and SBC control units.For engine control units, comfort control devices and other such units, this description is true only in part. We do not repair airbag control units as a principle.

Basic principles

Following the highway code, every automotive component must operate safely and without defects or hazards. This is established in the initial check of the vehicle by the Automotive Authorities and in the first traffic worthiness authorization of the particular vehicle. The condition of the vehicle is checked over the years by recurring compulsory security checks (TÜV). Of course, the car experts may not test all vehicle components, only the most important, safety-relevant components and the overall function are scrutinized in this test.

However, what’s installed must function. This goes for additional electronic security systems such as ABS, ASR, DSC, ESP, SBC. Certain performance and safety features operate exclusively through these additional components (e.g. towing a trailer, brake force distribution, elk test, etc.). A gray area are those systems that can be switched off at will, these must proof to be working when switched on.

A non-functioning safety system is reported to the driver (usually in yellow) by a warning lamp or corresponding text message. The driver should go to a workshop as soon as possible and have the vehicle inspected. The operation of the motor vehicle is now at your own risk and without insurance coverage. If the vehicle is presented with a defective system, the TÜV approval will not be issued, due to significant deficiencies.

Repairs to Automotive ECUs

We generally only repair electronic components of the control units. Repair of hydraulic components in the hydraulic block / valve block, etc. are not performed, except by replacing them with original parts.

When the electronics of an electronic auxiliary system fail, or enter an implausible condition, the component is immediately disabled and the driver receives a message (yellow warning light or text message). The hydraulic system is still working, the vehicle can be stopped without the additional electronic system. The hydraulic system is designed with 2-fold safety (2 hydraulic brake circuits front / rear), so in the very improbable failure in one of these brake circuits it would still be possible to brake the vehicle with reduced braking power. The hydraulic part of the brake system is to be checked in the annual safety inspection, mistakes and shortcomings may not occur here. Electronic components in a control unit cannot be verified nor can wear be detected. These components can fail at any time without advance warning.

It is therefore prohibited that an electronic defect in an auxiliary system would greatly impair or completely disable the actual braking function. The prerequisite is that the system has been properly installed and tested according to its manufacturer’s instruction and that it has not been tampered with. Therefore, the removal, installation and subsequent testing is allowed by a specialist only.
In short: an electronic defect or malfunction in auxiliary systems such as ABS, ASR, DSC and ESP is immediately recognized and displayed by the system. An impairment of the basic hydraulic braking function is impossible with professional installation and testing. Electronics can fail at any time, this applies to brand new appliances also, so these additional systems (also called assistance systems) must without exception be repaired by a specialist using high-quality technical means.

Active Brake Assist / High Tech with pitfalls?

SBC brake Mercedes Benz

A special type is built by Bosch and used in Mercedes Benz vehicles with SBC. In this case the additional system adds to the ABS / ESP function by increasing and regulating the brake pressure. If the system fails, the two-circle hydraulic system is still active, however, only in the so-called "residual braking function" without brake booster. The driver is of course alerted, as with all additional electronic systems, by the corresponding warning (warning light yellow or text message). But a severe impairment of the braking performance results. It is very frightening for the driver if the braking power suddenly takes a sharp drop when the system fails during braking. In such situation, the driver has no possibility and above all no time to look for an error message in his displays. Accidents with these systems are known, but the component manufacturers and the automotive industry rely on the road worthiness approval for these vehicles, and that there are no safety concerns if regular maintenance is observed.

We have our concerns about these active systems, because the residual braking situation may occur at any time following an electronic or other defect. Here no maintenance etc. helps, because the electronics are not maintained and no one can check the electronic and electromechanical components in a control unit for wear. A power failure alone, by a weak or defective battery, already triggers the residual braking function. The numerous mechanical components of the units can cause an error at any time. Only a residual braking function remains available.

Brief technical explanation of SBC system (Bosch)

Active brake system having electro-hydraulic brake boosting, using an hydraulic pump and a pressure accumulator. Numerous pressure sensors and monitoring functions in the device. The technical advantage lies in the elimination of a brake booster and in applying the brake shoes against the discs by the system, even before the driver operates the brake. While this reduces the braking distance it of course also means high brake pad wear and cost. Bosch built a counter in the device software which counts the number of brake pedal operations. From 180,000 hits the counter is full and a warning is given in the vehicle and in the diagnosis: "SBC maintenance required" or "SBC exceeded run time". According to the manufacturer, the entire unit should now be replaced because it has reached its originally calculated wear limits. This is a purely preventive measure against system failure and it gives manufacturers and parts distributors the greatest possible revenue.

We have seen lots of SBC units that had reached this warning and inspected the wear on the electronics. We couldn’t tell, because, as mentioned above, wear on electronics can not be measured. Of course, the electronic pump motor in the unit is subject to a certain amount of wear. In all tested units, the motors ran without wear induced problems, with wear of the carbon brushes at 10-15%. Thus, these engines would still be good for 3-4 times their run time. We know cases with other devices with carbon brush DC motors in which the motors failed after only 1,5 years and approximately 5% wear. Therefore, a preventive exchange offers no guarantee for 100% reliability. The rest of the system consists of several sensors, electro-hydraulic valves and an accumulator.

The accumulator, especially its membrane, of course, is subject to a certain wear. However, the storage pressure is constantly monitored. If an error occurs, as with any error, the driver is warned immediately. We recommend replacing the accumulator after 200.000 km or after about 10 years, to prevent failure. The accumulator can be ordered from us separately from resetting the SBC unit. Unfortunately, it is annoying that after removal of the unit the pressure accumulator unit (diaphragm) often relaxes following the drop in pressure, causing an error message after installation. We therefore always recommend the additional exchange of the accumulator. Installing the SBC unit invariably leads to problems, depending on the diagnostic tool used and on the build process. You’ll get your repaired device with installation instructions and relevant tips and tricks.

Conclusion: We consider the SBC Bosch System to be non-mature and technically vulnerable, but we see no reason why they should not be repaired. However, after repair and professional installation, the system is exactly as fit for use as before the defect. A defect may still occur in both a brand-new device, an over 10 years old device or a repaired device. If an error occurs, the system is deactivated and the driver receives an error message as with all systems. However, it is possible that in these active braking systems only the residual braking function is then available and braking in a hazardous situation becomes a problem. This vulnerability is known to the manufacturer of the equipment and to the vehicle manufacturers, and it is concealed from the customer. Systems of this type and design are no longer being developed and installed by manufacturers. This shows that the technical problems were too large, despite the good profit margins they provided.

Do I take it that when the car has been sitting and you hear the pump go when you open the door, the diaphragm is dying? Should it hold pressure for weeks?