Function of Individual Components
COMPONENT LOCATIONS
1. Control unit (ABS 5 and ABS 5 / ABD)
Processes the incoming signals and triggers the solenoid vales and/or the return pump of the hydraulic unit if wheel slip is excessive (also refer to system description). Switches ABS or ABS/ABD off if a system fault is detected, activates the warning lamp and stores the fault in a non-volatile memory.
Note: The control units are identified by their Part Nos. and by a self-adhesive sticker with a color edge.
ABS 5 - Sticker with black edge.
ABS 5 / ABD to end of MY '94 - Sticker with yellow edge.
ABS 5 / ABD from MY '95 (Rear-wheel drive and four-wheel drive models) - Sticker with red edge.
To keep the plugging cycles of the control unit connector to the minimum possible, the control unit was located in a suitable position.
2. Relays
The solenoid relay (R68) is triggered (closed) by the control unit as soon as the control unit is supplied with voltage from the alternator (D+ terminal 16). If the relay has closed (picked up), positive (+) battery voltage (terminal 30) is available at the coils of all solenoids (in the hydraulic unit).
As soon as the ABS or ABD starts the regulating cycle, the control unit connects the corresponding solenoid coil of the solenoid to be controlled on the respective wheel to negative.
The return pump relay (R65) is triggered by the control unit (negative) if a return feed operation is required and is then closed. When the relay has closed (picked up), battery + (terminal 30) is present at the return pump, causing the pump to run.
Note: Both relays are identical 50 A power relays. The R 65 and R 68 identifications are indicated on the power relay cover. R 68 is the rearmost relay (further away from the control unit than relay R 65). To remove the relay cover, remove the relay bracket, pushing put the (center) body-bound rivet.
3. Fuses
3a. The 60 A Maxifuse (arrow) is used to protect the return pump and the solenoids.
Note: The second 60 A Maxifuse is used to protect the DME.
3b. The 15 Amp fuse (Nr. 16) on the Central Electrical System protects the control unit power supply (ABS 5 and ABS 5 / ABD).
4. Speed sensor
The speed sensors (rpm sensors) supply wheel speed information (speed information for each individual wheel) to the control unit. The Speed sensors operate according to the inductive principle and, corresponding to the number of teeth of a pulse wheel, generate sinusoidal alternating voltages, the frequency of which is an indicator of the wheel speed.
Note: The front and rear speed sensors are different.
Identification: Part No. on speed sensor wire and position of mounting hole relative to the edge position (see page 45-23).
5. Warning and information lamps
- System readiness display (warning lamps).
- Fault display (warning lamps).
- Control display (ABD information lamp).
Note
No.7 = ABS warning lamp
No.3 = ABD warning lamp
No.2 = ABD information lamp
6. Hydraulic unit
The main components of the hydraulic unit consist essentially of several fast-switching solenoids and a return pump. Regardless of the pressure in the brake master cylinder, the hydraulic unit can modify the fluid pressure to the wheel cylinders (pressure holding or pressure reduction). It is not possible, however, to increase the pressure beyond the pressure applied by the master cylinder.
The ABS 5 hydraulic unit has
3 hydraulic outputs (3-channel system) and
6 solenoid valves (3 inlet and 3 outlet solenoids).
The ABS 5 / ABD hydraulic unit has
4 hydraulic outputs (4-channel system) and 10 solenoid valves (4 inlet solenoids / 4 outlet solenoids as well as 1 switch-over and 1 intake solenoid). The intake and switch-over solenoids are required to allow the return pump to perform two duties:
I. Return feed to brake master cylinder (pressure release) for ABS control.
II. Supply feed (pressure buildup) to right or left rear wheel brake cylinder with ABD control.