ELECTRONIC CONTROL UNIT (ECU)
ECU computes the acceleration, deceleration and slip factors as generated from rotating wheels via the speed sensors. ECU program has the ability to determine correct response behavior of the system based on these computations. ECU response is sent by electronic signals to the electromagnetically operated valves in the ABS hydraulic unit.
The control unit contains electronic monitoring circuits. These monitoring circuits ensure proper operation of the ABS system. If the control unit detects a defect in the wire harness or any electrical part of the equipment, the monitoring circuit will switch off ABS, permitting normal use of standard brake system. At this time the ANTI-LOCK indicator light will illuminate.
On 318 Series, 325 Series, and M3, the ECU is located on the lower left side of the instrument panel, next to the steering column. On 525i, 535i and M5, the ECU is located behind the glove box. On 735 Series, the ECU is located in the right rear corner of the engine compartment.
HYDRAULIC UNIT
Note. Hydraulic units are model specific and are not interchangeable
To control the braking system pressure, the brake hydraulic unit has a 3-way valve which permits 3 separate brake pressure conditions. Pressure build-up, pressure holding and pressure drop are the 3 pressure phases that adapt to the vehicle braking requirements. The control procedures are as follows
As soon as wheel lock-up is indicated, brake fluid pressure is reduced. If the wheel still tends to lock, pressure will be further reduced until wheel acceleration or the slip limit is detected. The pressure is then raised and the control phases repeated. These pressure changes occur in milliseconds.
The return delivery pump returns brake fluid from the wheel brake cylinder, while dropping the pressure to the appropriate brake circuit. The pump is a 2-piston pump design so circuits of a dual brake circuit system remain separated.
Two relays are located under the cover of the hydraulic control unit: an engine relay and a valve relay. The engine relay is the larger of the two.
SPEED SENSORS
Speed sensors are installed at each wheel. Speed sensors measure the rate at which teeth on a pulse wheel rotate. The sensors then relay a signal to the ECU, which computes the actual wheel speed. The pulse wheels, which run past the permanently magnetized edge of the speed sensor, are attached to the hub.
SERVICING
Brake fluid should be replaced every two years. The brake fluid tank has a vent hole. Brake fluid will retain moisture from the air and could cause brake fluid boiling point to drop from 464°F (240°C) to 320°F (160°C). A diminished boiling point can change braking ability of the vehicle.
BLEEDING BRAKE SYSTEM
See appropriate DISC article.
CONNECTING ADAPTER LEAD
Switch off all electrical components. Turn ignition off before disconnecting plug connectors. For TESTS 6 and 7, connect ABS adapter between "T" plug and control unit. Note all connections and switch position for each test step.
REPAIR PRECAUTIONS
Always turn off ignition and remove connectors from ECU when using an electric arc welder on the vehicle. If the vehicle is painted, the electronic control unit is capable of withstanding booth temperature of 185°F (85°C) for about 2 hours. Manufacturer recommends testing the ABS with a BMW Service Tester whenever any major repairs are performed on the brake system.
CONTROL UNIT
Note. On 318 Series, 325 Series and M3, ECU is located on lower left side of instrument panel, next to steering column. On 525i, 535i and M5, ECU is located behind glove box. On 735 Series, ECU is located in right rear corner of engine compartment.