Electronic stability program (ESP), function - GF42.45-P-0001V
MODEL 210.08 /28, 220.08 /18 with CODE (472a) Electronic Stability Program (ESP)
| 15.03.07 | The document has been updated to this date and will no longer be subject to revision. |
The "E lectronic S tability P rogram" (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 at the front or rear axle. ESP stabilizes the vehicle when cornering, while braking or when coasting without power and holds it safely on course.
ESP complements the familiar functions of the antilock brake system (ABS), the acceleration slip regulation (ASR), the engine braking regulation (EBR), and the electronic traction system (ETS).
The function of the stability control overrides 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) comprises the following system correlations.
ABS prevents the wheels from locking up during braking and thus maintains the steerability, directional stability and road adhesion of the vehicle during deceleration.
ASR and ETS (4MATIC mode) prevents all 4 drive wheels from spinning while driving. It also improves directional stability and road adhesion and increases traction potential across the entire vehicle speed range. The effect of up to 3 differential locks can be simulated.
EBR reduces brake slip at the drive wheels during deceleration and ensures directional stability and road adhesion.
ESP prevents the vehicle from breaking away when the vehicle 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 correlations
The engine electronics (ME) are used to adjust the drive torque. This system controls the engine by regulating the throttle valve and the firing point.
The gear setting is also evaluated by the electronic transmission control (ETC) to calculate the ESP drive torque control.
The ABS, ASR, EBR, ETS and BAS functions are included in the ESP electronics of the brake control circuit.
The engine control module includes the functions for the electronic accelerator (EA) and the cruise control system.
Data is exchanged between the ESP control module, the engine control module and the transmission control module via a CAN data line.
Benefits 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 stability.
- Automatically adapts the engine torque to the respective wheel-to-road-surface traction possibilities if the driver accelerates too heavily.
- Reduces the risk of skidding under all road conditions by automatically stabilizing the vehicle when braking, accelerating or coasting smoothly.
- Significantly improves the directional stability of the vehicle when cornering - up to the limit range.
- Shortens the stopping distance when cornering or on slippery roads.
- A flashing warning lamp in the speedometer notifies the driver of ESP control and informs him that his vehicle is approaching the physical limits.
- ESP or ASR can be shut down using the ESP OFF switch, which is indicated by the warning lamp in the speedometer shining constantly. Deactivation can help to improve traction ("cutting" effect) in deep snow or when snow chains are fitted.
Function survey
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 forces, or drive forces or lateral forces.
The electronic stability program (ESP) analyzes the vehicle behavior and applies specific braking force to individual wheels to correct any instability.
Example A: understeered vehicle (cornering left)
The vehicle pushes outwards over the front wheels.
A precisely calculated braking action is performed on the left rear wheel.
Example B: oversteered vehicle (cornering left)
The tail of the vehicle breaks away. A precisely calculated braking action is performed on the right front wheel.
ESP operates:
- when cornering (vehicle understeering or oversteering)
- 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:
- sensors which register driver requirements
- Steering angle sensor
- Accelerator pedal position (throttle valve actuator)
- Sensors which measure the actual vehicle behavior
- Yaw rate sensor
- Lateral acceleration sensor
- Brake pressure sensor
- Wheel speed sensor
The ESP control module (N47-5) records and processes the wheel speeds, the steering angle, the yaw rate of the vehicle, the lateral acceleration and the brake pressure at the front axle.
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 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 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 means of the data acquisition described.
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 produce a defined brake pressure selectively at one or more wheels.
At the same time commands are passed on to the engine and transmission control modules over the CAN data bus. To reduce the drive torque, calculated values for the 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 torque reduction by the ESP ensure optimum vehicle stability.
The following control functions are performed during this process:
ESP braking torque control circuit ABS control
If, for example, a wheel has a tendency to lock, the brake pressure in this wheel brake is regulated. The pressure in the ESP hydraulic unit (A7/3) is regulated via the solenoid valves by the pressure build-up, pressure hold and pressure reduction control phases.
ESP drive torque control circuit
ASR control
In order to reduce an excessive drive torque and thereby obtain optimum traction, the drive torque is reduced over the CAN data bus between the ESP control module and the engine control module. The ESP control module continuously checks whether the control functions can be canceled owing to, for example, a sudden improvement in road grip so that the drive torque requested by the driver via the gas pedal can then be made available again.
ESP drive torque control circuit
EBR control (except engines with fuel cut-off)
If wheel slip at the drive wheels occurs when the gas pedal is released, this is also detected in the ESP control module. The signal is sent to the engine control module over the CAN data bus. With this information the brake slip is reduced by increasing the drive torque, thus improving the lateral stability of the vehicle. This process occurs without notifying the driver (ESP warning lamp).
ESP braking torque and drive torque control circuit
ESP control
If oversteer or understeer is detected, a calculated braking action is performed at the front or rear axle via the ESP control module and the hydraulic unit. This brake intervention counteracts the undesired driving behavior.
A signal sent to the engine control unit via the CAN data bus triggers the demand-based reduction in drive torque by decreasing engine torque.
ESP braking torque control circuit
ASR and ETS control (4MATIC)
A spinning wheel is braked by a pressure system in the hydraulic unit which directs brake pressure into the brake caliper on the rear axle (pressure build-up).
The other wheel can then transmit optimum driving power (locking differential effect).
Solenoid valves in the ESP hydraulic unit (A7/3) regulate the braking torque with pressure build-up, pressure hold and pressure reduction phases.
ETS control (4MATIC)
The ETS is activated according to the vehicle speed and according to the priority of traction, directional stability and road adhesion.
- Individual brake application at all 4 wheels (simulation of 3 locked wheels) in the lowest vehicle speed range. Maximum traction is thus achieved.
- Limited brake pressure increase at the 2nd rear wheel if one rear wheel spins and simultaneous pressure increase if both rear wheels spin (operating in part without rear axle lock effect) in the middle vehicle speed range. This ensures improved directional stability and road adhesion with slightly reduced traction.
- Simultaneous pressure increase at both rear wheels if one rear wheels spins and no pressure increase at the front axle (longitudinal lock effect only on one side) in the upper vehicle speed range. The best possible drive torque control is achieved with respect to directional stability and road adhesion.
| Electronic traction system (ETS), notes on towing, testing and repair work | Model 210 | GF42.35-P-0001-05A | |
| Model 220 | GF42.35-P-0001-05PW | ||
| Electronic stability program (ESP), driver information | Model 210 | GF42.45-P-0001-04A | |
| Model 220 | GF42.45-P-0001-04B | ||
| Electronic stability program (ESP), location of electrical components | Model 210 | GF42.45-P-0001-03D | |
| Model 220 | GF42.45-P-0001-03F | ||
| Electronic stability program (ESP), location of hydraulic components | GF42.45-P-0001-02C | ||
| ESP braking torque control circuit, function | Model 210 | GF42.45-P-2000VE | |
| Model 220 | GF42.45-P-2000VS | ||
| ASR drive torque control circuit, function | GF42.40-P-2100A |