Systems Description

ABS 5.3 (3-channel system) system description

ABS operation

The ABS control module receives a signal from the stoplight switch and the AC voltage signals from the four speed sensors. These signals are converted into digital wheel-speed signals independently of each other by two microprocessors. The wheel slip (approximately proportional to the computed vehicle reference speed) is formed from these wheel-speed signals.

If vehicle deceleration and excess slip at a wheel are detected, the pressure-holding phase is initiated as the first step, i.e. the inlet valve for the relevant wheel is closed to prevent a further increase in pressure. If in spite of the pressure being held constant the wheel tends to continue to lock up, the pressure in the wheel cylinder is reduced. For this purpose, the outlet valve is opened and the brake fluid is pumped back to the master brake cylinder via the return pump (pressure-reduction phase) until the wheel turns again.) Then, depending on the control cycle, further appropriate cycles are initiated.

This function and the input signals are continuously monitored. If a fault is detected, the control module switches the ABS function off, switches on the ABS MIL and stores the fault in a non-volatile memory in the control module.

In addition, whenever a trip is begun and a speed of 6 km/h is exceeded, a test programme is started.) The solenoid valves and the pump motor are electrically actuated and checked. If a fault is detected, the control module switches the ABS function off, the ABS MIL is switched on and the fault is stored.

Function of the individual components. Function of Individual Components

Diagram: ABS 5.3 (3-channel System):

ABS/TC 5.3 (4-channel system) system description


NOTE: Traction Control (TC) represents an extension of the ABS system and is a combination of anti-slip control (ASR) and Automatic Brake Differential (ABD).

TC prevents spinning of the drive wheels when moving off and accelerating. Driving stability and traction are improved over the entire speed range.

Traction Control (TC) is ready for operation whenever the engine is started.

The TC information light in the instrument panel is lit during a control process and warns that the driving style must be matched to road conditions.


Function

Driving-stability control:
If Traction Control (TC) detects that a certain speed difference between the wheels has been exceeded (wheel spin), engine power is automatically reduced.

Brake control:
In addition to reducing the engine power (driving- stability control), the TC (via the ABD) brakes the drive wheel that is spinning.
Since this control requires the drive wheels to be individually controlled, the ABS/TC system is a 4-channel system.

Switching off driving stability control *

Press the "TC OFF" logo of the rocker switch. Driving stability control cannot be switched off during a TC control process (information light on).

One-sided wheel spin on the drive axle is further prevented by brake control up to a speed of 100 km/h.
Driving stability is not monitored, since the drive wheels can spin at the same rpm (slip).

With driving stability control switched off, the TC MIL in the instrument panel and the indicator light in the rocker switch are lit.

It can be advantageous to switch off driving stability control:
- on a loose surface and in deep snow
- when "rocking" the vehicle free
- when using snow chains.

Switching driving stability control back on *

Press the indicator light in the rocker switch.

Driving stability control cannot be switched on again during a TC control process (information light on).

* Press the switch for at least 0.1 second. Then a further 0.3 second will pass until the routine is complete. Only then is driving stability control switched off or on.

Diagram: ABS/TC 5.3 (4-channel system):