ABS, Function of Individual Components

Function of Individual Components
1. Control module (ABS 5.3 and ABS / TC 5.3)
This unit processes incoming signals and actuates the solenoid valves and/or the return pump in the hydraulic unit when wheel slip is excessive. If the ABS or ABS/TC: switches off when detecting a system fault, it actuates the warning light and stores the fault in a non-volatile memory.

Note: Control module and relay are not supplied individually. In the event of replacement, the complete hydraulic unit must be replaced.

2. Relay
The solenoid valve relay is actuated by the control module (closes) as soon as the control module receives voltage from the generator (D+ /terminal 61). As soon as the relay has closed (picked up), battery + (terminal 30) is applied to the valve coils of all solenoid valves of the hydraulic unit.
If the ABS or the TC (ABD) begins controlling, the control module applies negative potential to the appropriate valve coil of the valve to be controlled at the appropriate wheel.
The return pump relay is actuated (with a negative potential) by the control module if required, and closes. As soon as the relay has closed / picked up, battery + (terminal 30) is applied to the return pump, which then runs.

Note: The relays cannot be exchanged. When replacement is necessary, the complete hydraulic unit must be replaced.

3. Fuses
3a. A 50-A fuse in the current distributor (F1 / arrow) protects the return pump and the solenoid valves. The current distributor is located under the instrument panel. The fastening points are accessible from the luggage-compartment side.
3b. The 15-A fuse B 9 (fuse holder B /fuse No. 9) on the central electrical board protects the control module's electrical supply (ABS and ABS/TC). The arrow points to fuse holder B (field 2).
On vehicles with Traction Control (TC), the 15-A fuse B 2/1 (fuse holder B / fuse No. 1) on the central electrical board protects the supply to the "TC OFF" rocker switch (switch for switching driving-stability control on and off).

4. Speed sensors
The speed sensors provide wheel speed information (speed information of each wheel) to the control module. These speed sensors operate according to the inductive principle, in which sinusoidal alternating voltages are generated in accordance with the number of teeth on a pulse wheel and at a frequency that is a measure of the wheel speed.

5. Warning and information lights

Note:
B = TC information light
A = TC warning light
G = ABS warning light

Information light for TC (B)
- Lights for a lamp check when ignition is switched on
- Indicates control process (even when driving stability control is switched off).

Warning light for TC (A)
- Lights in combination with the indicator light in the rocker switch (TC OFF) for a lamp check when the ignition is switched on.
- In combination with the indicator light in the rocker switch, indicates that driving stability control is off
- Indicates defect: TC is out of order
- Lights together with the ABS warning light if there is an ABS fault.

Warning light for ABS (G)
- Lights for a lamp check when ignition is switched on
- If the ABS warning light lights up with the engine running, the ABS has switched off due to a fault.

6. TC (OFF) rocker switch with light
For switching driving-stability control on and off. The rocker switch has one contact for switching off and another for switching on.
To switch off, press the "TC OFF" logo on the rocker switch. This applies a positive voltage to the control module while the switch is operated. This causes driving-stability control to be switched off. Driving-stability control cannot be switched off during a TC control process (information light on).
When driving-stability control is off, the TC warning light in the instrument panel and the indicator light in the rocker switch are lit.
To switch driving-stability control on, press the indicator light in the rocker switch. This applies a positive voltage to the control module while the switch is operated. This causes driving stability control to be switched on. Driving-stability control cannot be switched back on during a TC control process (information light on).
Driving-stability control is ready for operation, i.e. is switched on automatically, whenever the engine is started.
System description.

7. Hydraulic unit
The hydraulic unit essentially consists of fast switching electromagnetic valves and a return pump. Regardless of the pressure in the brake master cylinder, the hydraulic unit can change the fluid pressure to the wheel brake cylinders (holding or reducing pressure). Increasing the pressure above that of the brake master cylinder is not possible, however.
The ABS 5.3 hydraulic unit has three hydraulic outputs (3-channel system) and six electromagnetic valves (three inlet and three outlet valves).
The ABS / TC 5.3 hydraulic unit has four hydraulic outputs (4-channel system) and ten electromagnetic valves (four inlet valves / four outlet valves, one switch-over valve and one intake valve).
The intake and switch-over valves are required so that the return pump can perform two tasks:
I. Return flow to the brake master cylinder (pressure reduction) for ABS control.
II. Supply (pressure increase) to the right or left rear-wheel brake cylinder for TC control (ABD control).
In the case of ABS control at the rear axle, the solenoid valves for both rear wheels are actuated in parallel Point control as in Solo ABS control (3-channel system)).