Design and function of anti-lock braking system - RA4200124B700X(42-0700)
D. Overall function
The ABS is operational when the ignition is switched on and a speed of 5-7 km/h has been attained. All braking operations in the locking range are controlled starting at the so-called control speed of 8 km/h (from 12 km/h on 1st version up to 02/84).
The following describes the control cycle on one wheel. The control sequence on the other wheels is the same. The wheel speed measured by the wheel speed sensor provides the wheel deceleration and the wheel acceleration signals for the electronic control unit. Linking of the individual wheel speeds provides the so-called reference speed, which is the approximate vehicle speed. A comparison of the wheel speed with the reference speed supplies the slip signals.
If, as a result of excessive brake fluid pressure in a brake caliper the wheel shows a tendency toward locking, a condition which is recognized by means of the wheel speed sequence (wheel slip), the fluid pressure will be held constant, that is an additional pressure increase is not possible.
E. Hydraulics
If there is still a tendency toward locking because the constant pressure is still too high, the outlet valve in the solenoid valve is opened to lower the fluid pressure. Simultaneously, the brake fluid still in storage is pumped back into the tandem brake master cylinder by the return pump. If the pressure is so low that the wheel wants to accelerate again, there will be no further pressure reduction and the fluid pressure will again be held constant.
When the acceleration of the wheel again passes a threshold value, the pressure is again increased by opening the input valve in the solenoid valve.
By means of pertinent signals from the ABS control unit, the hydraulic unit can actuate the three following control stages of
pressure build-up
pressure holding and
pressure reduction.
The control sequence is continually repeated under controlled braking, until the brake pedal is released or shortly before the vehicle stops.
Irrespective of the pressure in the tandem brake master cylinder, the hydraulic unit can change the fluid pressure for the brake calipers during regulation.
However, a pressure increase as compared with pressure coming from the master cylinder is not possible.
The hydraulic unit comprises three fast-switching solenoid valves. Of these valves, one each is associated with the left-hand and right-hand front wheel brake and the third with the rear wheel brake.
By activating the valves with varying voltages, the brake fluid pressure of the individual brake calipers can be increased = pressure build-up stage (no current)
held = pressure holding stage (half max. current)
or reduced = pressure reduction stage (max. current).
Pressure build-up stage
In the pressure build-up stage the pressure can be increased via the open input valve in the solenoid valve up to the pressure activated by the tandem brake master cylinder.
Pressure holding stage
In the pressure holding stage which precedes each pressure reduction stage, the fluid pressure from the hydraulic unit to the wheel brakes is held constant because the output and input in the solenoid valve is closed.
Pressure reduction stage
During the pressure reduction stage the brake fluid flows via a reservoir (6) into return pump (7). To maintain the fluid volume of the master cylinder, the return pump returns the brake fluid into the master cylinder against the prevailing pressure. To dampen the delivery noise, each circuit is provided with a silencer (10).
On plug socket (13a) of the hydraulic unit there is a relay (A7K1) for solenoid valves and relay (A7K2) for the return pump. The hydraulic unit is connected to the vehicle ground at the hexagon nut (10) via a grounding cable.
F. Electronics
Wheel speed sensors
Rod-shaped wheel speed sensors or impulse sensors are used for measuring wheel speeds. In the three-channel system with three wheel speed sensors installed in our vehicles (excluding vehicles with ASR), the wheel speed of each wheel is measured separately on the front axle.
The left/right front wheel-speed sensors (L6/1 and L6/2) are installed in the steering knuckles.
The wheel speed sensor (L6) is located on the rear axle casing (67). The drive pinion serves to measure the mean speed of both rear wheels.
The wheel speed sensors sense wheel speeds by way of the rotor teeth. On the front axle, the rotor teeth (69a) are machined into the front wheel hub (69).
The wheel speed sensors (L6/1, L6/2) on the front axle have a diameter of 18 mm.
On the rear axle, the rotor is a toothed wheel (121) and pressed onto the drive pinion gear (120). For each rear axle ratio there is a corresponding gear wheel with a different number of teeth. Refer to Removal and installation of rear wheel-speed sensor for assignment of rotor to rear axle.
Wheel speed sensors consist of a magnetic core and a coil. Rotation of the toothed wheel or rotor, which is located at a given distance in relation to the wheel speed sensor, will change the magnetic field, so that an alternating voltage is induced in the coil. This alternating voltage changes its frequency in accordance with the wheel speed and the number of teeth, i.e. the frequency is proportional to the wheel speed.
The cable (27) of the wheel speed sensor (L6/1 or L6/2) from the steering knuckle to the coaxial plug in the engine compartment is routed via the bracket (29, 32) in the wheelhouse.
Location - front axle models 124, 201
The wheel speed sensor on the rear axle is connected to the cable harness under the rear seat via the plug connection (L6x).
Location - rear axle models 124, 201
ABS control unit
Model 124 (except 124.034), 129 (except 129.058/063), 201, 202, located on the right side of the engine compartment.
The control unit installed in Models 124.034, 129.058/063, 140 and 202 includes a fault memory. On Models 124, 129 and 140 this is located in the module box. For information on diagnosis and test program, see Chassis Diagnosis Manual, Volume 2, Section 6.
The control unit processes the signals from the wheel speed sensors and controls the valves in the hydraulic unit.
The entire signal conditioning and signal processing is digital.
The control unit is connected to the ABS main cable harness via a 35-pole plug connection. The electronic control unit is functionally divided into:
- the signal conditioning section
- the logic section and
- the safety circuit.
- fault memory (Model 124.034, 129.058/063, 140, 202)
Signal conditioning section
In the signal conditioning section the signals supplied by the wheel speed sensors are converted into a form suitable for use by the logic section.
To prevent problems while measuring the wheel speed, which may be caused by production tolerances and by movements in the steering knuckle, the input signals are filtered prior to use. Deceleration and acceleration signals obtained from the wheel speed signals are processed in the logic section.
Logic section
The logic section of the ABS control unit employs the following input signals for each controlled wheel or the controlled rear wheels:
wheel slip
wheel speed acceleration
wheel deceleration
Output signals from the logic section control the solenoid valves of the hydraulic unit. As a result, the following hydraulic functions can be generated in the brake calipers of the wheel brakes:
pressure build-up
pressure holding
pressure reduction
Safety circuit
The safety circuit has the job of recognizing faulty signals in the ABS control unit and faults outside the ABS control unit in the electrical installation. In addition, the safety circuit intervenes in the control sequence during extreme driving conditions such as aquaplaning. When a fault is recognized, the system should be switched off, a condition which is indicated to the driver by the ABS malfunction indicator lamp coming on.
The safety circuit continuously monitors the battery voltage. If the voltage is below specific requirements (11 V), the system is also switched off until the voltage returns to the specified range.
In addition to monitoring operation, the safety circuit also incorporates an active component, a test cycle with the designation BITE (B uilt I n T est E equipment).
The test cycle begins as soon as the wheel speed in all three speed channels is higher than 5-7 km/h. The cycle is activated by the wheel speed sensor voltage, which is monitored automatically at the same time. The test cycle itself checks parts of the monitoring circuit as well as the logic section. For this purpose, the electronic control unit is fed with the specific test sample signals to check whether the correct output signals are available.
Fault memory
The control unit installed on Models 124.034, 129.058/063, 140 and 202 incorporates a fault memory and is thus suitable for fault diagnosis. Recorded errors remain stored even when the battery cable is disconnected.
Relay with overvoltage protection
To guarantee the function of the ABS system under all operating conditions, the power supply is switched via a relay, which is activated by terminal 15 (ignition lock).
The overvoltage protection, which protects the electronic control units against overvoltage is connected in series between the battery and relay (ABS, CIS-E, ASD or ASR).
The relay and the overvoltage protection form one unit. The overvoltage protection has a replaceable fuse. A 4-pole overvoltage protection was fitted on models 124, 201 up to December 1984. On vehicles as of January 1985, depending on the vehicle model and special equipment, a 5-, 7- or 9-pole overvoltage protection relay has been fitted.
On Models 124.034, 129.058/063 and 140 the electrical current is supplied by the basic module (N16/1). This is located along with the control unit (N30) in the module box.
ABS hydraulic unit (A7)
Two relays are located under the cover of the hydraulic unit. Relay (A7K2) activates the return pump and current flows to the solenoid valves via relay (A7K1).
A diode soldered into the plug socket (13a) controls the ABS malfunction indicator lamp in the instrument cluster when the multipin plug is disconnected from the electronic control unit. Phased in at the beginning of 1986, the diode is no longer located in the plug socket but in the solenoid valve relays. The new relay has 6 contact pins (previously 5).
In connection with the appropriate relay and fixing clamps the revised design of hydraulic unit can also be installed in vehicles produced earlier. It can be recognized on the outside by the inclined cover with the letters ABS (item A).
Electric wiring diagrams
For electric wiring diagrams and information on the layout of wires and plug connectors as well as relay and control unit locations, consult the manuals: "Electric wiring diagrams"