ABS 5
ABS 5: Changes compared with 1993 model year:- Change from 3/3- to 2/2-way control valves with two hydraulic unions and two switching positions.
- Brake force split "black-and-white" in all cases; in other words one circuit to the front wheel brakes, the other to the rear wheel brakes.
Components:
Hydraulic Unit:
The ABS has three control circuits which operate independently. The front wheels are controlled separately, each by way of two inlet and outlet valves, the two rear wheels are controlled together, each by one inlet and outlet valve.
The inlet valves are open when not energized, the outlet valves close when not energized. This ensures that brake applications can still take place in the normal way if the ABS should fail, though of course, without any antilock effect in this event. The return pump delivers the brake fluid released in the brake pressure reduction phase back to the fluid reservoir. The reservoir chamber accepts the volumetric flow during the initial regulating phase. The damping chamber smooths out pulsation caused by the pump and thus reduces feedback to the brake pedal.
The hydraulic line fittings at the hydraulic unit are of two different thread sizes (M10, M12) to minimize the possibility of incorrect attachment.
[1][2]Speed Sensor:
The [1][2]speed sensors have been optimized in their function and to make them easier to manufacture.
The sensor works by the inductive principle: an inductive voltage builds up in a coil if it is moved through a magnetic field. Since the permanent magnet is in a fixed position in the sensor, the magnetic field itself has to change. This change (alternate strengthening and weakening of the field) takes place as the pulse wheel rotates. It induces a regularly varying voltage in the coil; the frequency of the variations are measured as a means of establishing how fast the wheel is rotating.
Control Module:
This regulates the ABS action and monitors the system's components. The key elements in the control module are two microprocessors, which operate independently of each other.
If any faults are detected, the information is stored in an EPROM memory which can be read out using the DIS/BMW Service Tester/MoDiC. The contents of the memory are not lost even if the car's battery is disconnected.
The ABS control module has a self-diagnostic capability and cannot be recoded. In addition to continuous monitoring the system components, it checks the solenoid valves, pump and [1][2]speed sensors each time the car is driven. If a fault is detected, the ABS warning light comes on. However, the brakes themselves still operate normally (only wheelslip control function is lost).
Function:
If the brakes are applied and there is no tendency for a wheel to lock and skid, the braking pressure built up at the brake pedal passes directly through the inlet valves (open when not energized) to the wheel brake cylinders.
If the braking pressure has to be maintained, the control module closes the inlet valve. Pressure between the wheel cylinder and the valve block remains constant. If a tendency for one or more wheels to lock is detected, pressure is reduced in the affected brake circuit by opening the outlet valve and closing the inlet valve. With the aid of the return flow pump, brake fluid can then pass to the reservoir chamber and back to the master cylinder. The driver is made aware of this by pulsation at the brake pedal.
The individual states of the regulating cycle (pressure buildup, retention and reduction) can recur between 4 and 10 times a second. They ensure that maximum braking effect is available and the ability to steer the car is maintained according to the friction coefficient of each wheel and the prevailing road conditions.
Diagnosis:
The ABS 5 control module has self-diagnosis capability. It memorizes any faults which are detected, so that they can be read out with the DIS/BMW Service Tester/MoDiC. This greatly simplifies troubleshooting.
ABS 5 and ABS/ASC 5 diagnosis takes place by way of the DIS/BMW Service Tester/MoDiC, from Diagnostic Software Version 13.0 on. During diagnosis, the relevant repair and testing instructions must always be complied with.
It should be noted, however, that indirect faults such as improperly connected hydraulic lines or wheel speed sensors are not registered by the self-diagnosis procedure. In such cases, a functional check has to be performed by way of the diagnosis interface.
The following diagnosis functions are provided:
- Identification of control module.
- Fault memory readout
- Fault memory erasing
- Solenoid valves and pump activation
- Status check (output of initial information)
- Test procedure (check on [1][2]speed sensors, hydraulic unit, brake light switch)
- Special functions (bleeding routine, static fault test to locate faults that occur sporadically such as poor electrical contracts)
After repairs have been made to electrical or hydraulic brake components, a functional test must be performed. Service Information 34 05 94 (3692) summarizes the extent of diagnosis capability of previous wheelslip control systems.