General Functions

NOTE: For model name/model number cross reference refer to Model Designation Table. Application and ID

Terminology
- ESP Electronic Stability Program
- ETC Electronic Transmission Control
- BAS Brake Assist System
- MIL Malfunction Indicator Lamp
- ABS Anti-lock Brake System
- ASR Acceleration Slip Regulation
- MSR Engine Braking Regulation
- CAN Controller Area Network
- DTR Distronic Function
- ME-SFI Engine Control Module
- EA Electronic Accelerator
- EBR Engine Braking Regulation
- SPS Speed-sensitive Power Steering

Function Of Electronic Stability Program (ESP)
General





The "Electronic Stability Program" (ESP) is an active safety system for improving vehicle stability in all driving situations.

It works through individual brake intervention at one or several wheels on the front or rear axle. ESP stabilizes the vehicle when cornering, braking or when rolling without drive and it keeps it safely on course.

ESP complements the familiar functions of the Anti-Lock Brake System (ABS). Acceleration Slip Regulation (ASR) and Engine Braking Regulation (EBR).

The function of stability control overrides the ABS and ASR control systems. In addition to the active brake intervention of the ESP it also has an influence on engine/transmission management.

The "Electronic Stability Program" (ESP) is made up of the following systems;

ABS prevents the wheels locking when braking, maintaining steerability and vehicle stability when the vehicle decelerates.

ASR prevents the wheels spinning when driving. It also improves vehicle stability with better traction throughout the entire speed range.

EBR reduces brake slip at the drive wheels on overrun and ensures vehicle stability.

ESP prevents the vehicle breaking away when oversteering or understeering. It ensures that the vehicle does not deviate from the course specified by the driver (within the bounds of physical limits) in all situations. Brake forces are produced selectively at the individual wheels to correct this.

System correlations
The engine electronics is are used to adjust the drive moment. Using this system, the engine is controlled by the throttle valve and ignition timing.

The gear stage is also evaluated by the Electronic Transmission Control (ETC) to calculate the ESP drive moment control.

The functions of ABS and ASR are included in the ESP electronics of the brake control circuit. On the following vehicles the Brake Assist (BAS) function is also included (as at 3/99)
- Model 129 with engine 112 or 113
- Model 202 with engine 111, 112, 113, 605 or 611
- Model 210 with engine 112, 113, 606, 611, 612 or 613
- Models 208, 215 or 220

The ABS and ASR basic components are also combined in the ESP hydraulic unit.

The function for the Electronic Accelerator (EA) and cruise control (Tempomat) are included in the engine control module.

Data is exchanged between the ESP control module, engine control module and. if necessary, with the transmission control module and BAS control module via a CAN data bus.

Advantages Of ESP
- Improves the starting off and acceleration capacity due to increased traction; particularly advantageous on road surfaces with different levels of adhesion and on corners.
- Increases active dynamic safety, since only a wheel which is not spinning can provide optimum propulsion without any loss of lateral control.
- Automatically adapts the engine torque to the current ability of the wheels to transmit this to the road when the driver applies too much throttle.
- Reduces the risk of skidding under all road conditions by automatic stabilization when braking, accelerating or rolling away evenly.
- Significantly improves the directional stability of the vehicle when cornering - up to the limit range.
- Reduces the stopping distance on corners or on roads with a slippery surface.
- A flashing warning lamp in the speedometer signals ESP control mode to the driver and informs him that he has approached the physical limits of his vehicle.
- The ESP or ASR can be shut off using the ESP OFF switch which is recognized by the warning lamp in the speedometer coming on permanently. Better traction (milling effect) can therefore be achieved in deep snow or when snow chains are fitted.

Function Survey
Basically all forces, which act on a vehicle externally, want to turn the vehicle about the center of gravity irrespective of whether they are brake or drive forces acting on one side or whether they are side forces.

The electronic stability program (ESP) registers the vehicle's behavior and applies corrective brake forces to individual wheels.





Example A: understeering vehicle (on lefthand bend) The vehicle pushes outwards via the front wheels. An accurately calculated brake intervention takes place at the left rear wheel.

Example B: oversteering vehicle (on lefthand bend) The rear end of the vehicle breaks away. An accurately calculated brake intervention takes place at the right front wheel.


ESP Controls:
- When cornering (vehicle oversteers or understeers)
- When driving straight ahead (vehicle deviates off course due to uneven road conditions)

To be able to perform these extremely precise control interventions. an expanded system of sensors is required compared with ASR. A distinction is made between:
1.) Sensors which recognize the driver requirement
- Steering angle sensor
- Accelerator pedal position (throttle valve actuator)

2.) Sensors which measure the actual vehicle characteristics
- Yaw rate sensor
- Lateral acceleration sensor
- Brake pressure sensor
- Wheel speed sensor

The wheel speeds, steering angle, vehicle yaw rate, lateral acceleration 3nd brake pressure are recorded and processed in the ESP control module (N47-5).
The ESP control module (N47-5) is connected to the control modules of the engine/transmission management system via a CAN data bus. This digital line connection enables rapid exchange of data between the ESP control module, engine control module and transmission control module.

The ESP control module (N47-5) is continuously supplied with current data on engine torque, accelerator pedal position and transmission ratio.

The forces which want to turn the vehicle about the center of gravity are recognized via the yaw rate and lateral acceleration sensor. The longitudinal and lateral forces at the wheels can be calculated by means of the value acquisition which is carried out.
If these values exceed certain control thresholds, the appropriate solenoid valves and the high pressure/return pump in the hydraulic unit are actuated via the ESP control module (N47-5) in order to control a defined brake pressure at one or several wheels. At the same time commands are passed on to the engine control module and transmission control module via the CAN data bus. To reduce the drive moment, calculated values for throttle valve position and ignition timing are specified and a downshift prevented if necessary.

The precise and accurately proportioned intervention is completed within a few fractions of a second.

Active brake intervention and drive moment reduction by the ESP ensure optimum vehicle stability.

The following processes run here:
- ESP Brake Moment Control Circuit
ABS Control
If a wheel tends to lock. for example, the brake pressure in this wheel brake is controlled. Control in the ESP hydraulic unit (A7/3) takes place via the solenoid valves through the pressure build-up, pressure holding and pressure reduction control phases.

- ESP Brake Moment Control Circuit
ASR Control
To brake the spinning wheel, brake pressure is led to the rear axle brake caliper (pressure build-up) via a pressure system in the hydraulic unit. Optimum tractive force can be transmitted by the other wheel as a result (locking differential effect).
The brake moment is controlled by pressure build-up, pressure holding and pressure reduction via solenoid valves in the ESP hydraulic unit (A7/3).

- ESP Drive Moment Control Circuit
ASR Control
To reduce an excessive drive moment whilst achieving optimum traction. the drive moment is reduced via the CAN data bus between the ESP control module and engine control module.
A check is constantly carried out in the ESP control module on whether the control functions can be cancelled e.g. as a result of a sudden improvement in road surface adhesion, so that the drive moment applied by the driver via the accelerator pedal can come back into operation again as soon as possible.

- ESP Drive Moment Control Circuit
EBR Control (Not On Engines With Fuel Shut-off)
If brake slip occurs at the drive wheels when the throttle is closed, this is also recognized in the ESP control module. The signal is reported to the engine control module via the CAN data bus. Using this information brake slip is reduced due to an increase in drive moment and lateral control of the vehicle is thereby increased. This process takes place without informing the driver (ESP warning lamp).

- ESP Brake And Drive Moment Control Circuit
ESP Control
If oversteer or understeer is recognized, the ESP control module and hydraulic unit carry out a calculated brake intervention at the front or rear axle. This brake intervention counteracts undesirable handling characteristics selectively.
The drive moment reduction as per requirements is achieved by reducing the engine torque through a signal via the CAN data bus to the engine control module.