Hydraulics Scheme - Function Description Opel Corsa B

Hydraulics Scheme

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1 Brake master cylinder

2 Brake servo

3 Front brakes

4 Hydraulic modulator

5 Rear brakes

6 Brake servo

Construction of Hydraulic Unit

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1 Brake master cylinder 8 Spring

2 Brake servo 9 Shutter

3 Sound-proofing chamber 10 Brake

4 Return valve 11 Volume governor (hydraulic valve)

5 Dual circuit return pump 12 Shutter

6 Reservoir chamber 13 Regulator corner

7 Solenoid valve

Function Description of Hydraulic Unit

Normal braking

During normal braking, up to start of regulation, the solenoid valve (7) is without current and closed, the volume governor (11) is at rest, due to the spring force (8).

As large hydraulic cross sections are open with the volume governor (11) in this position, there is free passage between the brake master cylinder (1) and the brake (10), i.e. there is no throttling effect (see p-t diagram, phase 1).

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p-t - diagram:

p pressure

t time

Phase 1 Normal braking

Phase 2 Pressure reduction

Phase 3 Pressure increase

______ Brake master cylinder pressure

-------- Brake cylinder pressure

Phase 1 - normal braking

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Pressure reduction phase

If the danger of locking is recognized in one wheel while brakes are engaged, the relevant solenoid valve (7) opens, which leads to a pressure difference at the shutter (12).

This pressure difference pushes the volume governor (11) against the spring force (8), interrupting the direct connection from the brake master cylinder (1) to the brake (10) and producing a connection between the brake (10) and the reservoir chamber (6).

The superfluous brake fluid (15) can thus escape from the brake (10) via the shutter (9) into the reservoir chamber (6). This results in a reduction in pressure in the relevant brake (10) (see p-t diagram, phase 2).

At the same time as the solenoid valve (7) is opened, the return pump (5) starts and pumps the brake fluid temporarily in the reservoir chamber (6) back to the brake master cylinder (1).

Pressure reduction phase (continued)

As long as there is a pressure difference between the brake master cylinder (1) and the brake itself (10), i.e. between volume regulator input and output, it will remain in the governing position and sets a constant volume flow via the regulator corner (13).

This constant volume flow (16) (governed volume) becomes set because the volume governer (11) is balanced between the

a) force resulting from the pressure difference in front of

and behind the shutter (12) and the

b) spring force.

Regulation is thus determined exclusively by the proportional force of the pressure difference at the shutter (12) and the spring force (8), so that it is independent of the pressure in the brake master cylinder (1) or the pressure in the brake itself (10).

As long as the solenoid valve (7) is open, the governed volume will be guided to the reservoir chamber (6) and then pumped back to the brake master cylinder / volume governor input (regulator corner 13).

Phase 2 - pressure reduction

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Pressure increase phase

When wheel slip has fallen below the critical point (no more risk of locking), the solenoid valve (7) closes.

The volume governor (11) remains in the governing position as long as there is a pressure difference between the brake master cylinder (1) and the brake (10), the constant volume stream (16) (regulated volume) now flows to the brake (10), resulting in a proportioned increase in pressure suitable for the brake (see p-t diagram, phase 3).

The pressure increases until a risk of locking is again recognized at the wheel and a further reduction in pressure is required.

The volume governer (11) only returns to its rest position when there is no more difference in pressure between the brake master cylinder (1) and the brake (10), i.e. when the pressure at the brake is the same as the pressure in the brake master cylinder (1) produced by the driver.

As there is then no longer a volume stream through the shutter (12) and thus no pressure difference at the shutter (12), the spring (8) pushes the volume governor (11) back to its rest position.

Te direct connection between the brake master cylinder (1) and the brake (10) is intact again (see "normal braking").

Phase 3 - pressure increase

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Electronic Control Unit

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The electronic control unit is located behind the panelling in the lower left footwell, in a plastic bracket.

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The main components of the electronic control unit are the the two microcomputers, both of which run the same program independently of one another. This doubling-up (redundancy) provides a high degree

of safety against any faults that may occur.

The most important information received by the two microcomputers is the signals from the four wheel speed sensors, which are processed in a special unit (integrated circuit) for the requirements of the microcomputers.

The input information is then converted to

- acceleration velocity

- deceleration velocity

- amount of slippage.

The logic function installed in the two microcomputers decides which actuators (solenoid valves, return pump motor and valve relay) should be activated. The output control commands are also delivered to output transistors by special units (integrated circuits). In addition, the two microcomputers have a monitor logic, which constantly checks the status of all electronic components.

In addition, each time the vehicle is driven off after ignition ON, a function test is carried out on the wheel speed sensors and actuators when a speed of 7 km/h (5 mph) is attained.

In case of a malfunction, the system switches off and a trouble code is set, providing information about the cause. As mentioned above, the function of the conventional brake system remains unaffected. Any cutout of the ABS is displayed by illumination of the telltale.

The microcomputers have a memory which retains its contents even after the battery has been disconnected. The stored trouble codes can be read out with TECH 1.

The control unit's tasks can be summarized as follows:

- Measurement and filtering of wheel speeds

- Calculation of reference speeds

- Calculation of slippage and wheel deceleration

- Determination of nominal values for current regulation

- Monitoring of inputs and outputs

- Performance of function check

- Display and storage of trouble codes

- Actuation of telltale

- Output of stored faults (self-diagnosis)

Wheel Speed Sensor

Each wheel speed sensor basically consists of a magnetic core and a coil.

The tip of the terminal is surrounded by a magnetic field. The teeth on the pulse gear move through this magnetic field when the wheel turns.

This changes the magnetic flux and an alternating voltage is induced in the wheel speed sensor coil. The frequency of the alternating voltage varies depending on wheel speed, i.e. is proportional to the wheel speed.

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1 Front wheel speed sensor

2 Rear wheel speed sensor

The pulse pick-ups have 29 teeth. The pulse pick-ups on the front axle are located on the outer joints of the drive shafts. On the rear axle, there is one pulse pick-up punched out of sheet metal in each brake drum on the wheel hub.

The front axle wheel speed sensors are fastened to the steering knuckle with brackets and do not require adjustment. When correctly installed, there is a reference gap of 0.2 - 1.3 mm/0.0079 - 0.0512 in. between the tip of the wheel speed sensor and the pulse pick-up tooth.

The rear axle wheel speed sensors are fastened to the anchor plate of the drum brake. Installation and adjustment are described overleaf.

The wheel speed sensors are connected to the wiring harness by plug connections.

The wheel speed sensor plug connections are fastened to the front frame side members and vehicle underbody by appropriate brackets.

Installation of rear wheel speed sensor

The wheel speed sensor (3) is pushed into the pulse pick-up (1) up to stop and fastened in this position with a bolt (5).

By then turning the wheel speed sensor (3) until it engages in the coupling (4), it moves out from the anchor plate by a certain dimension which sets a defined reference gap between pulse pick-up (1) and the wheel speed sensor (3).

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1 Pulse pick-up 4 Coupling

2 Reference gap 5 Bolt

3 Wheel speed sensor

ABS Telltale

The ABS telltale is located in the instrument housing and informs the driver about malfunctions in the ABS.

Immediately after ignition on, the telltale illuminates for approx. 4 seconds and then extinguishes again if the system is intact.

Illustration E 2484 shows the allocation of the ABS telltale in the instrument housing in vehicles without tachometer.

Illustration E 2485 shows the allocation of the ABS telltale in the the instrument housing in vehicles with tachometer.