Relay Operation

a) ABS-relay (C) has overvoltage protection in form of a Zener diode. When reaching an overvoltage of 22 V (e. g. defective alternator), this diode will be conductive and prevents further increasing in voltage. This protects control unit against damage from excessive voltage.

b) Solenoid relay (d) closes when ABS control unit receives voltage from alternator. This causes ground connection of relay term. 85 via the control unit. Relay switches over and supplies battery + (30) to solenoid valve coils. If ABS begins to regulate, control unit provides a ground to the solenoid coil of the wheel with regulated pressure.

c) Pump relay (b) closes when ground is provided through the control unit.

Warning System

Diode (e) is installed between terminals 4 and 7 of hydraulic unit connection. If electrical system of ABS control unit has a shortcoming or control unit plug is disconnected, its terminal 29 will be ground. This potential reaches the central informer, which then switches on ABS indicator lamp and central warning lamp. Diode prevents short to terminal 30 of solenoid valve relay.

If solenoid valve relay is malfunctioning, e. g. does not switch over, ground potential also reches terminal 29 of ABS control unit and central informer via relay connections 87a — 30 and through the diode {open for negative current), through which the indicator/warning lamps will be switched on.

The function of ABS warning lamps is tested with other lamps by turning on the ignition.


— ABS control unit


— Speed sensor


— Term. 15


— Pump motor


- ABS hydraulic unit


- Plug next to CEB


- D +/61


- Pump relay


- ABS-relay XVII


— Stop-lamp switch


— Solenoid valve



— Central informer


— Term. 15


— Solenoid valve



— Cable-connector


- MP V


— Solenoid valve



— Instrument cluster


- B +


— Valve relay

dj - Pljjg connection - black MP III

g3 - Speed sensor LR MP VIII

— Groundpoint on wheelhouse wall

— Groundpoint above steering coulumn

— Groundpoint above CEB

— Groundpoint on crancase bell-housing, left


Conform with following points when working on cars with ABS.


Pull off plug on electronic control unit when welding with an arc welder.


Electronic control unit can be subjected briefly to heat of max. 95 °C or longer (about 2 hours} max. 85 °C when painting car.

Charging Battery

Disconnect battery for charging with a fast charger.

Installing Battery

If battery had been removed, make sure wire terminals are absolutely tight on both connections after installation of battery.

Outside Starting

Never use a fast charger to start the engine.

Multiple Pin Plug on Electronic Control Unit

Never disconnect or connect multiple pin plug on electronic control unit while ignition is switched on.

Checking Function

A simple function check is sufficient after completion of wbrk on brake system, which did not directly concern ABS components. In other words indicator lamp in instrument cluster must go out after starting engine, when ABS is okay. This work would include, for example, replacing brake pads, brake hoses, brake discs, brake booster, tandem master cylinder, brake cables and parts of parking brake as well as brake lines, which are not connected on hydraulic unit.

If work is performed on hydraulic unit (*), electronic control unit (*), speed sensors and wire harness or parts/equipment are replaced because of damage through accident, function must be tested with an ABS tester on a braking test stand.

Hydraulic unit and electronic control unit may not be repaired or disassembled.

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