ABS prevents the wheels from locking under hard braking. This reduces the braking distance, especially when there is little grip on the road surface caused by dampness or snow. The force grip of the wheels with the road in these cases is greater if the wheels are still rotating during braking. In addition, the car remains steerable when braking to a complete stop.
The speed sensor, 2 each for the front and rear wheels, measures the speed of the wheels. Based on the signals from the individual sensors, the electronic control unit calculates an average speed that approximately corresponds to the average vehicle speed. By comparing the speed of an individual wheel and the average speed of all the wheels, the control unit determines the slip state of an individual wheel and thereby determines how close that wheel is to blocking.
As soon as the wheel approaches a lock-up condition, in which the brake fluid pressure in the corresponding caliper becomes too high in relation to the tire's adhesion to the road, the hydraulic system keeps the fluid pressure constant based on signals from the control unit. This means that the pressure in the brake caliper does not increase even when the brake pedal is pressed even harder. If the blocking tendency persists later, the fluid pressure is reduced by opening the outlet valve. However, it is reduced only so much that the wheel receives a slight acceleration, after which the pressure is again maintained unchanged.
If the wheel acceleration exceeds a certain value, the pressure in the hydraulic system starts to increase again, but not higher than the value available in the entire system.
This process is repeated under hard braking for each wheel until the brake pedal is released or until the vehicle comes to near standstill (2—3 km/h). The driver feels the work of the anti-lock device by the pulsation of the brake pedal.
The battery switch in the electronic control unit ensures that, in the event of a defect, (e.g. cable break) or at low operating voltage (battery voltage below 10.5 volts) the ABS system turned itself off. In this case, a warning light will illuminate on the instrument panel while driving that the ABS system is disabled. The conventional braking system remains in working condition. The car behaves when braking in this case as if the ABS system was not installed on it.
If the warning light illuminates while driving, this indicates that the ABS system is disabled.
Stop the car quickly, turn off and start the engine again.
Check battery voltage. If the battery voltage is less than 10.5 volts, charge the battery.
Attention: If the ABS warning light comes on at the beginning of a trip and goes out after a while, this indicates that the battery voltage was initially insufficient, and then increased as a result of charging the battery from the alternator.
Check if the battery terminals are tight enough and have good contact.
Raise the car, remove the front wheels, check the electrical wires of the wheel speed sensors for external damage (grinding).
The rest of the checks must be entrusted to the VAG workshop C 6/88 The ABS control unit has a memory device that, during the operation of the vehicle, recognizes an occurring malfunction and stores its code. In the workshop, the memory device is interrogated and the fault is eliminated in a targeted manner.
ABS with electronic differential lock (EDS)
Depending on the model and version, VW PASSAT vehicles have an electronic differential lock, abbreviated as EDS, in addition to ABS (elektronische DifFerentialsperre). Built on ABS components, an electronic differential lock improves traction from the engine to the vehicle's wheels in adverse road conditions, especially when driving in mountains, accelerating or starting off.
On roads with varying ground grip, one of the drive axle wheels may slip. In the presence of an interwheel differential, this leads to the fact that both wheels of the axle will create the same traction force, the value of which is determined by the friction force of the tire of the slipping wheel on the road surface. Thanks to the mechanical differential lock, traction is improved, as more torque can be transmitted to the wheel, which has higher friction with the road surface. However, conventional mechanically locking differentials cannot be installed between the driven front axle wheels for two reasons: firstly, driving comfort would be severely limited, and secondly, the effect of ABS would be negligible.
Functioning of the EDS: The electronic control unit, thanks to the wheel speed sensors of the ABS system, receives information about the speeds of the drive wheels (in vehicles with both driving axles - about the speed of each wheel) and quickly compares them with each other. If there is a difference in speed over 110 1/min, EDS automatically turns on and brakes the slipping wheel until it reduces the speed to a value close to the speed of the wheel without slip. With this control, the same effect is achieved as with a mechanical differential lock, namely: a wheel with better grip on the road surface can develop increased traction. In vehicles with all-wheel drive, with EDS, there is no need to install a mechanical rear axle differential lock. At vehicle speeds over 40 km/h, as well as when cornering or overheating of the brakes, EDS is automatically deactivated. The system has several degrees of protection against errors during braking, both in the electronic circuit and in the hydraulic circuit.
In the EDS system, many parts and components of the ABS system are used together. The hydraulic system contains as an additional component the valve block of the EDS system, the control unit is supplemented with software for the EDS system.
Attention: Before carrying out welding work with an electric welding machine, the ABS/EDS electronic control unit must be disconnected. Remove the plug only when the ignition is switched off. During painting work, the control unit can be exposed to a maximum temperature of 95°C for a short time and for a long time (in about two hours) — exposure to a maximum temperature of 85°C.
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