Failure diagnostics Rear axle modulator’s inlet and discharge valve The rear axle’s inlet and discharge valves are located inside the axle modulator. The solenoid cables are not accessible from outside. Error recognition functions are used to avoid the effects of system failures and/or to inform the driver about functional problems. The error recognition principles correspond partly to those of a conventional ABS device (monitoring the ABS valves, the wheel speed sensors, the computer hardware). The EBS monitors braking pressure control. The electrically controlled braking pressure and the pneumatically redundant pressure are also monitored.
On the other hand, a big part of the surveillance functions concerns EBS-specific functions (EBS-specific sensor analysis, EBS-specific solenoid control, braking pressure control, data transmission via CAN bus). Too low front axle braking pressure The availability of minimum braking pressure (on the front axle at a certain solenoid current supply level) is checked. In addition to the wheel speed sensor signals, the EBS evaluates many other sensor signals and checks that these signals are error-free.
Too high rear axle pressure deviation (from left to right) In normal braking processes (neither ABS nor traction control system – TCS -controls) the measured braking pressure on the left and right sides of the rear axle must almost be equal. If the braking pressure deviation exceeds the admissible value, an error is reported. Nominal value sensing (sensors and switches) The brake signal transmitter provides two sensor and two switch signals. The (pulse-width modulated) sensor signals are checked to see whether they conform with the authorised values, and for mutual deviations. The correctness of the (digital) switch signals are then tested.
Rear axle redundancy cannot be switched off Normally, pneumatic redundancy pressure control in the rear axle is prevented by the redundancy valve. If an error makes this deactivation impossible, the rear axle braking pressure can no longer be reduced during ABS control, etc. (because there is inlet of non-ABS- compatible rear axle redundancy pressure into the rear axle brake cylinder). Error recognition takes place in this case. Braking pressure sensing (on the front and rear axles) The pressure sensor’s (analogue) signals in the pressure control circuits are checked to see whether they correspond to the authorised values. Note:
The cabling for the two rear axle sensors is not accessible from outside, since it is an internal axle modulator cabling. EBS monitors data transmission Wear sensing (on the front and rear axles) The (analogue) signals from the wear sensors are checked to see whether they correspond to the admissible values. between the EBS central module and the axle modulator (System bus) between the EBS and other system control devices (vehicle bus) The EBS monitors EBS-specific solenoid valve control. If communication is impossible or is suddenly cut, an error is reported.
Front axle proportional relay valve The frequent solenoid (pressure proportional to the solenoid current) of the front axle proportional relay valve is checked to see whether control is carried out correctly. Rear axle redundancy valve The rear axle redundancy valve’s solenoid switch is monitored to see that control takes place correctly. Error recognition and display 5. EBS 18