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Electronic Stability Program (ESP) function

The "Electronic Stability Program" (ESP) is an active safety system

which improves vehicle stability in all driving situations.

It operates by actuating the brakes individually on one or more

wheels on the front or rear axle. ESP stabilizes the vehicle when

cornering, braking, or during non-driven coasting to keep it on the

road and in the desired lane.

ESP complements the familiar functions of the anti-lock brake

system (ABS), acceleration slip regulation (ASR) and engine braking

regulation (EBR).

The stability regulation function is superordinate to the ABS and ASR

control systems. As well as actively intervening with the brakes, ESP

also has an influence on engine / transmission management.

The Electronic Stability Program (ESP) includes the following system


ABS prevents the wheels from locking up during braking and thus

maintains the steerability and directional control of the vehicle during


ASR prevents the drive wheels from spinning while driving. It also

improves directional control with better traction across the entire

speed range.

EBR reduces brake slip at the drive wheels during deceleration and

ensures directional control.

ESP prevents the vehicle from breaking away when it is oversteered

or understeered. In all situations it ensures that the vehicle does not

deviate from the course specified by the driver (within the bounds of

physical limits). Brake forces are produced selectively at the

individual wheels to correct this.

System interplay

The engine electronics (ME) is used to adjust the drive torque. This

system controls the engine by regulating the throttle valve and the

firing point.

Information on the currently engaged gear as reported by the

Electronic Gear Selection (EGS) system also flows into the ESP

unit's torque modulation calculations.

The ESP system's electronic circuits also employ closed-loop control

of braking to administer the ABS, ASR and MSR functions.

The closed-loop control processes that govern ABS, ASR and ESP

are carried out in the SBC hydraulic unit

The electronic accelerator pedal (EFP) and cruise-control functions

are integrated within the engine-management ECU.

A CAN data wire supports multilateral communications between the

ESP control unit, the engine-management ECU and the transmission

control unit.

A second CAN wire allows the ESP control unit to communicate with

the ESP and SBC control units

Advantages of ESP

  Improves moving-off and acceleration capabilities by increasing

traction; especially useful on road surfaces with different levels

of grip and when cornering.

  Improves active dynamic safety, since only a wheel which is not

spinning can provide optimum traction with no loss of lateral


Automatically adapts the engine torque to suit the ability of the

wheels to transmit this to the road when the driver applies too

much throttle.

Reduces the danger of traction loss under all road conditions

by automatically stabilizing the vehicle during braking,

acceleration and in spins.

Significantly improves the directional stability of the vehicle

when cornering - up to the limit range.

Shortens the stopping distance in corners or on slippery roads.

A flashing warning lamp within the speedometer alerts the

driver that ABS or ESP is in active closed-loop operation, and

that the vehicle is thus approaching the physical limits of


The ESP OFF switch is available to allow deactivation of ESP

and ASR; deactivation is signalled by a continuously illuminated

warning lamp in the instrument cluster. This can provide better

traction (grinding effect) in deep snow or when snow chains are


Function overview

Essentially, all the forces acting on a vehicle from outside attempt to

rotate the vehicle about its center of gravity, regardless of whether

these are one-sided braking or drive forces or lateral forces.

Example A: understeering vehicle

(cornering to the left)

The vehicle pushes outwards over the front


A precisely calculated braking action is

performed on the left rear wheel.

Example B: oversteering vehicle

(cornering to the left)

The tail of the vehicle breaks away.

A precisely calculated braking action is

performed on the right front wheel.

a Desired travel direction

b Braked wheel

c Corrective moment produced in vehicle

d Understeering vehicle motion

e Oversteering vehicle motion

ESP operates:

- when cornering (vehicle understeering or oversteering)

-When driving straight ahead (vehicle deviates off course due

to uneven road conditions)

The standard array of ASR sensors must be expanded to support

this type of closed-loop control intervention.

A distinction is made between:

1.) Sensors which recognize the driver requirement

Steering angle sensor

 A ccelerator pedal position (throttle valve actuator)

2.) Sensors which measure the actual vehicle behavior

Yaw rate sensor

Lateral acceleration sensor

Brake pressure sensors

  Wheel speed sensor

The ESP control unit (N47-5) monitors and proceses data on

wheelspeeds, steering angle, yaw rate, lateral acceleration and

braking pressure at the individual wheels.

The ESP control module (N47-5) is linked to the control modules of

the engine/transmission management system over a CAN data bus.

This digital link permits fast data exchange between the ESP control

module, engine control unit,

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 attempting to rotate the vehicle about its center of gravity

are detected via the yaw rate and lateral acceleration sensors.

The longitudinal and lateral forces acting on the wheels can be

calculated by this data acquisition.

When these data rise beyond predefined thresholds, the ESP (N47-

5) and SBC (A7/3n1) control units respond by triggering selected

control valves to achieve a precisely programmed braking pressure at

one or several wheels.

At the same time, commands are sent to the engine and

transmission control module via the CAN data bus. The system

prescribes a specific torque output level to the ME control unit, while

also intervening at the transmission's EGS ECU to inhibit downshifts

when necessary.

The precise and accurately proportioned intervention is completed

within a few fractions of a second.

Active brake intervention and drive torque reduction by the ESP

ensure optimum vehicle stability.

Auto Diagnostic