Contents: ABS control unit (Fig. 8.1) ↡ Hydraulic block ↡ Wheel speed sensors ↡ Relay ↡ Malfunction of the ABS system ↡
Fig. 8.1. Diagram of individual elements of the ABS system: 1 – control unit; 2 – front wheel brake mechanism with speed sensor; 3 – hydraulic block; 4 – main brake cylinder with booster; 5 – parking brake mechanism; 6 – rear wheel brake mechanism with speed sensor
Fig. 8.2. Installing speed sensors on the front wheels: 1 – brake disc; 2 – front wheel hub; 3 – shield; 4 – Allen bolt, 25 Nm; 5 – rotation speed sensor; 6 – steering knuckle
Fig. 8.3. Measuring wheel speed: 1 – rotation speed sensor; 2 – coil; 3 – magnetic core; 4 – toothed rotor; 5 – coaxial wire
ABS control unit (Fig. 8.1)
The information from the wheel speed sensors is constantly processed and compared with the programmed values. If the difference in values indicates a risk of locking when braking one or more wheels, the control unit activates the hydraulic unit, which reduces the operating pressure in the brake system of the corresponding wheel until it has free play and is braked again. Depending on the road surface conditions, this cycle takes place in a matter of milliseconds during one braking operation.
Hydraulic block
This is the main unit of the ABS system, installed in the front left part of the engine compartment. It includes an electric pump and a valve block with magnetic valves. When you press the brake pedal, the main brake cylinder under pressure supplies brake fluid through the valve block to the wheels. At the same time, the valve block regulates the pressure in each of the hydraulic drives of the system, connecting the front wheel and the rear wheel located diagonally from it. When ABS is turned on, the control unit sends a command in the form of an electrical signal: "Reduce the pressure of the working fluid". In this case, the brake fluid in a small circle through the valve block goes back to the reservoir. The operation of the pump can be noticed by a slight pulsation of the brake pedal.
Wheel speed sensors
Each sensor 5 (Fig. 8.2) is installed on its own wheel, at a small distance from the toothed disk (pulse rotor), fixed directly to the hub.
The toothed projections of the disk 4 (Fig. 8.3), depending on the rotation frequency of the wheel (speed), pass faster or slower past the magnetic core 3 of the sensor 1. Each tooth of the disk, when passing, induces a short increase in voltage in the coil 2. Its frequency depends on the rotation frequency of the rotor and, accordingly, the wheel. The electrical signal is sent to the control unit via the coaxial wire 5.
(Material based on data from the portal: MERCEDESMAN)
Thus, information about the wheel speed is transmitted as an electrical signal to the control unit.
Relay
The operation of the magnetic valves and the electric pump is controlled by the corresponding relays. They are mounted on the holder under the ABS control unit.
Malfunction of the ABS system
The ABS system indicator light comes on when the ignition is switched on and goes out if the engine is running, no later than 2 seconds later. If the light remains on, there is a fault in the ABS system. However, you can continue driving - the brakes will function as in a car without ABS. To fix the fault, contact a Mercedes service center, where they will diagnose the system control unit using a special device.