F. Location of hydraulic components - RA4200140B800X

Models 124/201

Fig 1: Identifying Hydraulic Components Location - Models 124/201

Shown on model 124

Model 126

Fig 2: Identifying Hydraulic Components Location - Model 126

Model 129

Fig 3: Identifying Hydraulic Components Location - Model 129

Hydraulics function

The function of the hydraulics is divided up basically into 3 modes of operation, which are effective in the ASR hydraulic unit (A7/3).

Normal mode
ABS control mode
ASR control mode

Normal mode

All solenoid valves are in the original position (position 0).

The tandem master cylinder (4) is connected directly to the front axle brake calipers via the connection "V", the left front (A7/3y1) and right front (A7/3y2) solenoid valves. The brake line to the rear axle brake circuit is connected at connection "HZ".

The rear axle brake calipers are controlled via the switchover solenoid valve (A7/3y5) and both the left rear axle (A7/3y3) and right rear axle (A7/3y4) solenoid valves.

Fig 4: Identifying Solenoid Valves Positions

Accumulator charging

If the pressure accumulator (13) is empty or at a pressure < approx. 160 bar, the pressure switch (A7/3s1) transmits the signal for accumulator charging to the ASR control unit (N30/1).

The ASR pressurizing pump (M15) is controlled and conveys brake fluid at a pilot pressure of approx. 10 bar to the return pump/charge pump (A7/3m1) in the ASR hydraulic unit (A7/3) via the connection "BA".

The return pump/charge pump (A7/3m1) is controlled at the same time with the ASR pressurizing pump (M15). Together the pumps supply the pressure for accumulator charging.

At a pressure of approx. 180 bar, the pressure switch (A7/3s1) transmits the signal to terminate accumulator charging to the ABS/ASR control unit (N30/1). The time for accumulator charging is approx. 30-50 seconds.

ABS control mode

The following changes have been made relative to the ABS without ASR:

The brake pressure for the rear axle circuit is led from the tandem master cylinder via the switchover solenoid valve (A7/3y5) to the two left or right rear axle solenoid valves (A7/3y3 or A7/3y4).

During ABS control mode (pressure build-up, pressure holding, pressure reduction) the left or right rear axle solenoid valves (A7/3y3 or A7/3y4) are controlled synchronously together for the rear axle circuit.

ASR control mode

The control mode requires a full pressure accumulator. The pressure switch (A7/3s1) continuously monitors the accumulator pressure.

The brake pressure for acceleration skid control is provided solely from the pressure accumulator. If the ASR control unit (N30/1) recognizes that a wheel is tending to skid, the switchover solenoid valve (A7/3y5) is switched to position "2" (and remains in this position until the end of the control process). Thus the full accumulator pressure is applied to the left or right rear axle solenoid valves (A7/3y3 or A7/3y4).

Depending how much and which wheel now skids, the three pressure phases: Pressure buildup - position "0", pressure holding - "1", and pressure reduction - position "2" are switched at the left or right rear axle solenoid valves (A7/3y3 or A7/3y4).

The 3 pressure phases during a control process are explained in the example of a left rear wheel (refer to hydraulic diagram).

Normal operation mode

(with accumulator charging)

Fig 5: Hydraulic Brake Circuit Diagram - Normal Operation Mode

Pressure build-up

Fig 6: Hydraulic Brake Circuit Diagram - Pressure Build-Up

The left rear axle solenoid valve (A7/3y3) is in position "0". Now the brake fluid which is at accumulator pressure directly reaches the left rear brake caliper (6b).

Pressure holding

Fig 7: Hydraulic Brake Circuit Diagram - Pressure Holding

The left rear axle solenoid valve (A7/3y3) is switched into position "1". All connections at the solenoid valve are blocked.

The brake pressure in the left rear brake caliper (6b) remains constant.

Pressure reduction

Fig 8: Hydraulic Brake Circuit Diagram - Pressure Reduction

The left rear axle solenoid valve (A7/3y3) is switched into position "2". The pressure connection is blocked. The brake caliper (6b) is connected to the return pump (m1) via the left rear axle solenoid valve (A7/3y3). The brake fluid flows out of the brake caliper via the low pressure accumulator (11), the pump inlet valve (8) and the silencer (10) back into the pressure circuit upstream of the solenoid valves (A7/3y3, A7/3y3) and back into the pressure accumulator (13) via the switchover valve (A7/3y5). In this way the control mode can be repeated in an unrestricted manner without exhausting the accumulator volume.

G. Function of hydraulic components

Pressurizing pump (M15)

The pressurizing pump is located on the brake booster under the tandem master cylinder. It is designed as a gear driven pump. The pressurizing pump can produce a maximum pressure of 8-12 bar.

It is connected to the ASR hydraulic unit via the line (BA). It is controlled directly from the ASR control unit (N30/1).

Fig 9: Identifying Function Of Hydraulic Components

ASR hydraulic unit (A7/3)

The ASR hydraulic unit is located in the front left of the units compartment on all models. It is designed as a compact hydraulic unit, i.e. the components for the hydraulic functions of ABS and ASR are combined into one housing.

The ASR hydraulic unit has two plastic covers. The smooth cover is clipped in at 4 points (arrows). The cover with lettering is fixed with a screw.

Fig 10: Identifying ASR Hydraulic Unit

In detail the meaning of the connection markings is as follows:

Fig 11: Identifying Left And Right Front Wheel Brake

Fig 12: Identifying Bleed Screw

Return pump/charge pump (A7/3m1)

The return pump/charge pump (A7/3m1) are integrated in the ASR hydraulic unit (A7/3). The return pump/charge pump (A7/3m1) are driven together by the motor of the return pump (known from ABS). Thus the return pump has a direct drive and the charge pump is driven via an intermediate drive gear.

The delivery of the integrated return pump is less than that of the separate charge pump for ASR (1st version). Therefore the time for accumulator charging increases from approx. 10 seconds to approx. 30 seconds.

Pressure accumulator

Models 124 and 201

The pressure accumulator is located behind the window on the plastic cover beneath the left front fender.

Fig 13: Identifying Plastic Cover Beneath - Models 124 And 201

The hydraulic lines from the ASR hydraulic unit to the pressure accumulator are separated to improve assembly by means of a connector piece (arrows) next to the damper strut fixing.

Fig 14: Locating Hydraulic Lines

Model 126

The pressure accumulator is located behind the window on the plastic cover beneath the left front wing.

Fig 15: Identifying Plastic Cover Beneath - Model 126

The hydraulic lines from the ASR hydraulic unit to the pressure accumulator are separated to improve assembly by means of a connector piece (arrow) next to the ASR hydraulic unit.

Fig 16: Locating Hydraulic Lines

Model 129

The pressure accumulator (13) is located behind the left headlamp next to the ASR hydraulic unit (A7/3).

Fig 17: Identifying Left Headlamp Next To ASR Hydraulic Unit - Model 129

A check for leakage at the connections on the pressure accumulator can be performed from the underside of the vehicle through the recess in the wheelhouse panel.

Fig 18: Identifying Wheel House Panel

All models

The pressure accumulator is designed as a piston-type reservoir on all models.

The volume of brake fluid stored is approximately 42 cm ³ at a pressure of 180 bar.

The initial pressure of the gas fill is approx. 100 bar.

The charge pump delivers the brake fluid to the storage chamber through the filter and via the check valve. The brake fluid stored is applied directly to connection "E" of the ASR hydraulic unit via one line.

The purpose of the bore (7) is to convey any leakage at all (either gas from the gas chamber or brake fluid from the storage chamber) outwards into the collection chamber beneath the plastic cover. This prevents mixing of gas and brake fluid.

Fig 19: Identifying Pressure Accumulator Components Location

Pressure accumulator

When bleeding the hydraulic system, the double seal piston must seat positively on the base of the storage chamber. Thus the brake fluid forces any residual air directly to the outlet line "E" via the de-pressurizing bore (9) and from there to the bleed screw "SP" (refer to bleeding the ASR hydraulics). The bleed screw "SP" must be unscrewed at least one turn for bleeding.

Brake fluid reservoir (4a)

As previously, the brake fluid reservoir (4a) is located above the tandem master cylinder. The volume has been increased to 350 cm ³ for vehicles with ASR (300 cm ³ without ASR).

The brake fluid reservoir has an additional line connection, to which the pressurizing pump (M15) is connected.

2 brake fluid level check switches are located on the brake fluid reservoir.

Fig 20: Identifying Brake Fluid Reservoir

Rear wheel brake

On vehicles with ASR the rear wheel brakes have a wear indicator as on the front brakes.

The fixed brake caliper is made from spherulitic cast iron. A fixing eye is cast in for the plug connection of the wear indicator (S10/3).

The linings differ, however, by recesses in the lining back plate, which are used to attach the wear sensor.

The lining quality and the dimensions of the linings are the same as on vehicles without ASR.

Note on model 124

On the T saloon, an uprated rear wheel brake as well as a separate matched brake lining is fitted.

NOTE: The contact sensor lines must be routed as shown in the figure. Only contact sensors in the transparent (temperature-resistant design) may be installed.

Fig 21: Identifying Plug Connection Of Wear Indicator