ESP braking moment control circuit, function

CAN data bus

ESP/BAS control module

(N47-5)

Transfer case control module (N78)

Engine control module

Transmission control module

Left front wheel speed sensor (L6/1)

Right front wheelspeed sensor (L6/2)

Left rear wheelspeed sensor (L6/3)

Right rear wheelspeed sensor (L6/4)

ESP brake pressure sensor (B34)

Parking brake monitor switch (S12)

ESP OFF switch (S76/6)

Steering angle sensor (N49)

Power supply Terminals 15, 30, 31

ESP brake light supression relay (F1k6)

Hydraulic unit for traction-control systems (A7/3)

High pressure and return pump (A7/3m1)

Solenoid valves

(A7/3y6 through A7/3y23

High-pressure/return pump relay (F1k25)

Instrument cluster (A1)

ABS indicator lamp (A1e17)

ETS indicator lamp (A1e35)

ESP warning lamp (A1e41)

BAS/ESP indicator lamp (A1e47)

Diagnostic data link connectorX11/4

BAS brake booster
(A7/7)

BAS solenoid valve (A7/7y1)

BAS release switch (A7/7s1)

BAS diaphragm travel sensor (A7/7b1)

Yaw rate and lateral acceleration sensor (B24/15)

The functions of the braking moment control circuit are controlled using the components of the electric/electronic system. This process employs selective application of specifically modulated fluid-pressure levels within the wheel cylinders for precise deceleration of individual wheels.

The ESP control unit (N47-5) recieves electrical power from the fuse and relay module (F1).

To provide closed-loop control of all functions based on braking intervention, the ESP control unit (N47-5) relies on signals from the following components:

 4 wheel speed sensors

 Steering angle sensor

 Yaw rate and
lateral acceleration sensor via CAN data line

 ESP brake pressure sensor

 Parking brake switch

 ME-SFI control module over CAN data line

 ETC control module over CAN data line

 ESP OFF switch

 BAS release switch

 BAS diaphragm travel sensor

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The incoming signals monitored by the ESP control unit (N47-5) are processed to generate output signals for transmission to the following components:

 BAS solenoid valve

 Solenoid valves in the hydraulic unit

 High pressure and return pump

 Engine control module over CAN data line

 ETC control module over CAN data line

 Warning and indicator lamps in instrument cluster via CAN data wire:

ABS malfunction indicator lamp (A1e17)

 ETS indicator lamp (A1e35)

 ESP warning lamp (A1e41)

 BAS/ESP indicator lamp (A1e47)

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The ESP control unit (N47-5) determines which of the 7 operational modes to activate based on the input signals:

 Normal operation

 ABS control mode

 ETS/ASR closed-loop control activated

 EBR control mode (see drive moment control circuit)

 ESP control mode

 BAS mode

 Electronic brake force distribution

 ESP OFF mode

Normal operation

No control intervention is required during normal vehicle operation. All of the solenoid valves in the ESP hydraulic unit (A7/3) are in their passive, no-current state. The system is ready for braking. Drive torque control is not activated.

The system also assumes this operational status in response to malfunctions in the BAS, ABS or ESP.

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ABS control mode

The ESP control unit (N47-5) recognizes the presence of the criteria that render closed-loop ABS intervention necessary based on the signals from the wheelspeed sensors. It responds by triggering selected solenoid valves in the hydraulic unit.

The solenoids for front and rear wheels in the ESP hydraulic unit (A7/3) are actuated individually (4-channel control).

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ETS/ASR closed-loop control activated

The ABS control module (N47-2) registers and processes the rotation rates of all wheels. These data serveas the basis for calculating a vehicle reference velocity.

When the wheelspeed signals indicate to the ESP control unit (N47-5) that one or several drive wheels are about to start slipping, the system assumes active ETS/ASR closed-loop control status. At extremely low traction coefficients simultaneous brake-system pressurization at all four wheels is possible.

A distinction is made between:

A Closed-loop control on the highway (comfort-oriented)

B Closed-loop control in offroad use (traction-oriented)

A ETS operation with highway ratios engaged

The drive wheel(s) displaying a tendency to slip is (are) braked by the appropriate wheelbrake(s) until the calculated drive slip (approximately 5 kph above the reference velocity) has been reached. The system allows only the limited amount of slip required to obtain optimal traction at each wheel.

Slipping wheels are decelerated by a pressurization system in the hydraulic unit which supplies pressurized brake fluid to the brake caliper (pressurization phase).

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Solenoid valves within the ESP hydraulic unit (A7/3) furnish closed-loop control of braking force with selective activation of one of three operating modes: pressurization, pressure maintenance and pressure discharge.

With activation conditions present, this ETS control status is available only within a speed range extending from 0-60 kph.

If the activation conditions remain present during acceleration to speeds above the 60 kph threshold, closed-loop ETS/ASR control is active to approximately 80 kph.

B ETS operation with off-road ratio selected (LOW RANGE)

Basic operation as with highway ratio. However, starting at 5 kph the reference slip rate is reduced to

2 kph, if major traction loss is present and the vehicle speed is < 20 kph. This condition also affects pressure modulation.

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ESP control mode

The ESP control unit's (N47-5) integrated status controller monitors and processes data on vehicle wheelspeeds, steering angle, yaw rate and lateral acceleration as well as the braking pressure on the control rod and hydraulic piston sides of the brake ciruit.

After processing the monitored signals the unit triggers the BAS solenoid valve (A7/7y1), the high-pressure and return pump (A7/3m1) and selected valves in the traction-control systems hydraulic unit (A7/3) to modulate the braking pressure at the individual wheel brakes.

BAS mode

The ESP control unit (N47-5) monitors the rate at which the brake pedal is depressed to recognize panic stops, to which it responds by activating the BAS mode (refer to BAS function).

Electronic brake force distribution

During every brake application the wheelspeed is used for precise slip monitoring at the rear wheels. Specific solenoid valves within the hydraulic unit are triggered to prevent the acceptable rear-wheel slip rates from being exceeded.

ESP OFF mode

Activation of the ESP OFF switch (S76/6) deactivates the drive torque control and the engine drag torque (MSR) control.

This also deactivates the ESP stability-maintenance function during acceleration and coasting. ESP continues to enhance stability when the driver initiates braking maneuvers.