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ESP braking moment control circuit, function - GF42.45-P-2000M

MODEL 163 as of 1.9.02 with CODE (472a) Electronic Stability Program (ESP) 

Fig 1: Flow Chart - ESP Braking Moment Control Circuit, Function
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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.

IMPORTANT The ESP control unit (N47-5) receives 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:

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

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

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.

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).

ETS/ASR closed-loop control activated 

The ABS control module (N47-2) registers and processes the rotation rates of all wheels. These data serve as 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:

  1. Closed-loop control on the highway  (comfort-oriented)
  2. Closed-loop control in off road use  (traction-oriented)
  1. 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).

    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.

  2. 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.

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 circuit.

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.

  ABS closed-loop operation function   GF42.45-P-3000D 
  ASC control mode, function   GF42.45-P-3500D 
  ESP control mode, function   GF42.45-P-3700D 
  Electronic brake pressure distribution, function   GF42.35-P-3600A 
  BAS function   GF42.31-P-0001A 
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