Function of individual components

1. Control unit (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 (see also ABS 5.3 system description  or ABS/TC 5.3 (4-channel system) system description  ). 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 unit and relay are not supplied indi- vidually. In the event of replacement, the complete hydraulic unit must be replaced.
2. Relay 

   The solenoid valve relay is actuated by the control unit (closes) as soon as the control unit receives voltage from the alternator (D+ /terminal 61). As soon as the relay has closed (picked up), battery + (terminal 30) is present at the valve coils of all solenoid valves of the hydraulic unit.

   If the ABS or the TC (ABD) begins controlling, the control unit applies negative potential to the appropriate valve coil of the valve to be con- trolled at the appropriate wheel.

   The return pump relay is actuated (with a negative potential) by the control unit 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 F 2/9 (field 2, No. 9) on the central electrical board protects the control unit's electrical supply (ABS and ABS/TC). The arrow points to field 2. On vehicles with Traction Control (TC), the 15-A fuse F 2/1 (field 2, 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 infor- mation for each wheel to the control unit. 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:

   • A = TC information fight (see Figure )
   • B = TC warning light (see Figure )
   • G = ABS warning light (see Figure )

   Information light for TC (A) 

   • 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 (B) 

   • Lights in conjunction with the indicator light in the rocker switch (TC off) for a lamp check when ignition is switched on
   • In conjunction 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 when 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 unit 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 unit while the button 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, that is, switched on automatically, whenever the engine is turned on.

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

1. Return flow to the brake master cylinder (pressure reduction) for ABS control.
2. 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 (joint control as in Solo ABS control (3-channel system)).