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Home >> Mercedes Benz >> 2006 >> ML350 >> Repair and Diagnosis >> Brakes >> Mechanical - Hydraulic >> Brakes - 164 Chassis >> Basic Knowledge >> ESP braking moment control circuit, function - GF42.45-P-2000GZ

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

MODEL 164 

Fig 1: ESP Braking Moment Control Circuit Function Diagram
G05258368Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The

Electronic Stability Program (ESP) regulates: 

To be able to perform these control interventions, an expanded system of sensors is required compared with acceleration slip regulation (ASR).

There are general distinctions between:

In the ESP control unit (N47-5) the wheel speeds, steering angle, vehicle yaw speed, lateral acceleration and brake pressure of the individual wheels are recorded and processed.

The ESP control unit is connected to the engine/transmission management control units via the Controller Area Network bus class C (engine compartment) (CAN-C). This digital cable connection enables rapid data exchange between the ESP control unit, ME control unit (N3/10) and the electric control unit (VGS).

The ESP control unit is constantly supplied with the current data on engine torque, accelerator pedal position and transmission ratio. Via the micromechanical yaw rate sensor AY pickup the forces are detected which want to turn the vehicle about the center of gravity.

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 corresponding control valves are actuated by the ESP control unit in order to selectively regulate the brake pressure applied at one or several wheels.

The commands transmitted via CAN-C to the ME control unit and the electric control unit (VGS) relating to the torque requirement initiate a reduction in the drive torque and if necessary a downshift is prevented by the electric control unit (VGS). 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 processes are performed: 

Start-off assist 

If the driver changes from the brake pedal to the accelerator pedal the vehicle can roll away against the engaged gear if the vehicle is positioned on a steep enough gradient or slope. An integrated inclination sensor detects when a defined slope or gradient is exceeded. A booster helps to maintain the brake pressure until the accelerator pedal is pressed.

Antilock brake system (ABS) 

If, for example, a wheel has a tendency to lock when braking, the brake pressure in this wheel brake is regulated. The regulation in the traction system hydraulic unit (A7/3) takes place via the control valves through the control phases of pressure buildup, pressure maintenance and pressure reduction.

Electronic traction system (ETS) 

To brake the spinning wheel (slippery ground) brake pressure is applied to the brake caliper of the spinning wheel (pressure buildup) through the pressure system of the traction system hydraulic unit. This allows the other wheel to continue to transmit drive force (differential lock effect).

Via control valves in the traction system hydraulic unit the braking torque is regulated by pressure buildup, pressure maintenance and pressure reduction.

Acceleration slip regulation (ASR) 

To prevent the drive wheels from spinning as a result of a drive torque which is too great for the surface, the ME control unit reduces the drive torque. The ESP control unit detects this occurrence by evaluating the wheel sensor signals and transmits this to the engine control unit via CAN-C.

The ESP control unit constantly checks whether, for example, the control functions can be canceled due to a sudden improvement in road grip, in order to once again permit the drive torque specified by the driver through the accelerator pedal.

Engine braking regulation control (MSR) 

If wheel slip caused by the engine braking torque occurs at the drive wheels when the throttle is released, this is also detected in the ESP control unit. The signal is sent to the ME control unit via the CAN-C. On the basis of this information, the wheel slip is reduced by increasing the drive torque, and the side stability of the vehicle is thus increased. This process occurs without notifying the driver (ESP warning lamp (A1e41)).

Electronic Stability Program (ESP) 

If oversteer or understeer is detected, a calculated braking action is performed at the front or rear axle via the ESP control unit. This brake intervention counteracts the undesired driving behavior.

A signal sent to the ME control unit via the CAN-C triggers the required reduction in drive torque by decreasing engine torque.

Electronic brake force distribution (EBV) 

A dynamic movement of the center of gravity is caused by braking, the vehicle's center of gravity moves forwards. The rear axle wheels are relieved of pressure and tend towards overbraking. The lack of side stability at the rear wheels would result in unstable driving conditions. This situation is detected by the ESP control unit through the wheel sensors. The ESP control unit then reduces the brake pressure in the rear wheels and prevents the rear wheels from overbraking.

Offroad operation 

Some control parameters of the ESP control unit are changed for offroad operation.

Offroad ABS 

The offroad ABS function was developed for speeds under 30 km/h when the Low Range switch is pressed (N72s31). It permits the wheels to block when the surface is poor to reduce brake travel by up to 50 %.

ASR control in offroad operation 

ASR control detects road surfaces with low friction values. The ASR activation thresholds are developed depending on the tire characteristics to improve traction quality.

Downhill Speed Regulation 

This system can provide considerable safety benefits when driving downhill in offroad mode. This system can be activated at speeds below 30 km/h by pressing the Downhill Speed Regulation switch (N72s30). The slow driving speed specified by the driver (between 6 km/h and 18 km/h) is now maintained. This is achieved by appropriate engine and transmission control and by automatic brake use.

Offroad package 

The Offroad package is a special equipment package with the following additional components:

The two-stage transfer case included in the Offroad package allows the driver to choose between an on-road ratio (1:1) and an off-road ratio (2.93:1).

The high drive torque that is made possible, combined with the locking differentials, considerably improves off-road characteristics.

ESP traction control has been adjusted to take the special requirements into account.

If the differential locks are active, these are immediately opened when brakes are applied by ESP (speed-dependent), or ABS control (speed < 15 km/h) or driver braking (speed > 15 km/h). The signal to open the locks comes from the ESP control unit.

Electronic stability program (ESP), function GF42.45-P-0001GZ 
ASR drive torque control circuit, function GF42.40-P-2100GZ 
Description of components in ESP control unit GF42.45-P-5118GZ 
Wheel rpm sensor, component description GF42.45-P-5134GZ 
Steering angle sensor, component description GF42.45-P-5107GZ 
Yaw rate and lateral acceleration sensor, component description GF42.45-P-5120GZ 
ESP Off switch, component description GF42.45-P-5121GZ 
Traction system hydraulic unit, component description GF42.50-P-4001GZ 
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