ESP braking torque control circuit, function - GF42.45-P-2000G

MODEL 463.243 /244 /246 /247 /249 /250 /254 ## as of 127357 as of 1.9.01, 463.270 /271, 463.309 /322 /323 /332 /333 ## as of 127357 as of 1.9.01 with CODE (BB3) Electronic stability program (ESP) 

MODEL 463.248 ## as of 127357 as of 1.9.01 with CODE (BB3) Electronic stability program (ESP) except CODE (979) Special protected version 

"Pressure build-up" function Shown on right front wheel brake 

Fig 1: Identifying ETS Braking Torque Control Circuit Diagram

Courtesy of MERCEDES-BENZ USA

"Hold pressure" function Shown on right front wheel brake 

Fig 2: Identifying ETS Braking Torque Control Circuit Diagram

Courtesy of MERCEDES-BENZ USA

"Pressure reduction" function Shown on right front wheel brake 

Fig 3: Identifying ETS Braking Torque Control Circuit Diagram

Courtesy of MERCEDES-BENZ USA

1. Capture and process sensor signals and status of switches 

   The ESP and BAS control module (N47-5) reads in and evaluates electric input signals from the following system components:

   1. 1.1 Wheel rpm sensors (L6/1 to L6/4) 

      The conditioned wheel speed signals are constantly compared both with each other and with the specified slip thresholds on the front and rear wheels in relation to the speed of the vehicle.

      • Calculating the following values:

        Vehicle speed, acceleration, deceleration, wheel slip, drive slip, decel slip.

   2. 1.2 Stop lamp switch (S9/1) 

      Detect and evaluate the position of the dual contact switch (NO contact and NC contact) when the driver applies the brake.

   3. 1.3 BAS diaphragm travel sensor (A7/7b1) 

      Calculate the pedal speed of the brake pedal.

   4. 1.4 ESP brake pressure sensor (B34) 
      • Register actuation of the brake pedal by the driver and record the brake pressure.
      • Calculate the wheel brake forces (longitudinal forces).
      • Calculate the lateral stability (lateral forces) from the existing wheel brake forces (longitudinal forces).

        Only if

        ESP control is required during brake application.

   5. 1.5 Yaw rate sensor (B24/9) 
      • Capture the yaw rate of the vehicle about the vertical axis e.g. when the vehicle skids.
      • This information is required to calculate the lateral forces that occur when cornering.
      • Detect oversteering or understeering.
   6. 1.6 BAS release switch (A7/7s1) 

      If the BAS release switch (A7/7s1) is not actuated, the BAS solenoid valve is active (A7/7y1) and the driver does not press the brake pedal, the ESP and BAS control module (N47-5) prevents the stop lamps from coming on.

   7. 1.7 Steering angle sensor (B24/8) 
      • Register a change in direction or cornering by evaluating the steering angle and the wheel rpm.
      • Register the driver's requirements together with the vehicle speed.
   8. 1.8 ESP OFF switch (S76/6) 
      • If the driver presses the ESP OFF switch (S76/6)
        • the ASR drive torque control
        • and the ESP braking torque control

          are switched off when the vehicle is accelerating and rolling. When the driver brakes the ESP braking torque control remains available up to a vehicle speed of approx. 60 km/h.

2. Activating and deactivating ESP braking torque control 
   • After evaluating the information it has read in, the ESP and BAS control module (N47-5) automatically activates the ESP braking torque control if a driving situation occurs in which the vehicle deviates from the course specified by the driver, e.g. the vehicle is oversteered or understeered.
   • The ESP braking torque control is deactivated again once the vehicle behavior is stable and the vehicle adheres to the specified course.
3. Calculating the brake pressures and actuating the traction system hydraulic unit (A7/3) 

   The ESP and BAS control module (N47-5)

   • calculates the brake pressures for the wheel brakes (braking torque control variables) for each control interval according to the relevant reference variables (specified values)
   • and uses electric output signals to actuate both the solenoid valves and the high pressure and return pumps of the traction system hydraulic unit (A7/3), which functions as an electronic, mechanical actuator in the braking torque control circuit.
4. Brake pressure build-up 

   The solenoid valves of the traction system hydraulic unit (A7/3) are controlled differently during the three control phases (pressure build-up, hold pressure and pressure reduction) according to the brake pressure calculated by the ESP and BAS control module (N47-5).

   • Pressure build-up 

     During the "pressure build-up" control phase, the brake pressure is increased until the calculated brake pressure is achieved.

   • Hold pressure 

     The "hold pressure" control phase begins when the pressure build-up phase has been completed provided that the brake pressure does not need to be increased further. This control phase remains active unless deviating values for the brake pressure are calculated.

   • Pressure reduction 

     If the brake pressure available during the "hold pressure" phase is too high, the "pressure reduction" control phase begins.

5. Actuation of the traction system hydraulic unit (A7/3) solenoid valves 

   The following example describes the actuation of the traction system hydraulic unit (A7/3) solenoid valves during the three control phases at the right front axle brake (5a).

   The relevant solenoid valves of the front and rear axle brake circuit would be actuated in the same way at the other wheel brakes.

   1. 5.1 BAS solenoid valve (A7/7y1) 
      • Pressure build-up 

        → The BAS solenoid valve (A7/7y1) is actuated from a vehicle speed of 20 km/h. The open BAS solenoid valve (A7/7y1) controls the air at atmospheric pressure inside the BAS brake booster (A7/7) so that a primary pressure of approx. 5 bar is produced in the brake circuits, which is supplied as primary pressure at the intake side of the high pressure and return pumps (A7/3p1, A7/3p2).

   2. 5.2 Stop lamp suppression relay (K55) 
      • Pressure build-up 

        → The stop lamp supression relay (K55) is actuated at the same time as the BAS solenoid valve (A7/7y1). The stop lamp supression relay then interrupts the voltage supply to the stop lamp switch (S9/1). This prevents the stop lamps from switching on.

   3. 5.3 Front axle switchover solenoid valve (A7/3y18) 

      Same function as rear axle switchover solenoid valve (A7/3y19).

      • Pressure build-up, hold pressure 

        → Actuated, switches to lock position and therefore uncouples the front axle brake circuit from the master brake cylinder. The integrated pressure limiting valve (approx. 170 bar) limits the high pressure.

      • Pressure reduction 

        → Is no longer actuated, switches to the flow position and therefore initiates the pressure reduction phase.

   4. 5.4 Front axle inlet solenoid valve (A7/3y22) 

      Same function as rear axle inlet solenoid valve (A7/3y23).

      • Pressure build-up 

        → Actuated, switches to flow position so that the primary pressure of approx. 5 bar can be built up.

      • Hold pressure, pressure reduction 

        → No longer actuated after successful pressure build-up and switches back to lock position (basic position).

   5. 5.5 Front axle high pressure and return pump (A7/3p1) 

      Same function as rear axle high pressure and return pump (A7/3p2).

      • Pressure build-up 

        → Actuated, draws brake fluid out of the reservoir via the open front axle inlet solenoid valve (A7/3y22) via the master brake cylinder and builds up the high pressure in the system for the front axle brake circuit.

      • Hold pressure 

        → Still in operation after the pressure build-up phase but can no longer draw brake fluid out of the reservoir via the master brake cylinder when the front axle inlet solenoid valve (A7/3y22) is closed and therefore cannot increase the brake pressure any further.

      • Pressure reduction 

        → Remains in operation after the pressure holding phase. The brake fluid flows back to the reservoir via the open front right solenoid valve, hold pressure (A7/3y8), the front axle switchover solenoid valve (A7/3y18), which is also open, and via the master brake cylinder.

   6. 5.6 Left front solenoid valve, hold pressure (A7/3y6) 

      Same function as:

      Right front solenoid valve, hold pressure (A7/3y8)

      • Left rear solenoid valve, hold pressure (A7/3y10)
      • Right rear solenoid valve, hold pressure (A7/3y12)
      • Pressure build-up, hold pressure 

        → Actuated and switches to lock position. High pressure is applied so that if necessary immediate build-up of pressure would also be possible at the left front wheel brake (5b).

      • Pressure reduction 

        → Remains in the lock position after the pressure holding phase. The same reduced pressure is applied as at the right front wheel brake (5a).

   7. 5.7 Right front solenoid valve, hold pressure (A7/3y8) 

      Same function as:

      Left front solenoid valve, hold pressure (A7/3y6)

      • Left rear solenoid valve, hold pressure (A7/3y10)
      • Right rear solenoid valve, hold pressure (A7/3y12)
      • Pressure build-up 

        → Not actuated, remains in flow position and therefore allows pressure build-up in the right front wheel brake (5a).

      • Hold pressure, pressure reduction 

        → Remains in flow position after successful pressure buildup, the check valves in the brake circuit prevent pressure reduction.

   8. 5.8 Right front solenoid valve, pressure reduction (A7/3y9) 

      Same function as:

      • Left front solenoid valve, pressure reduction (A7/3y7) Left rear solenoid valve, pressure reduction (A7/3y11)
      • Right rear solenoid valve, pressure reduction (A7/3y13)
      • Pressure build-up 

        → Not actuated, is in lock position and high pressure is applied.

      • Hold pressure 

        → Remains in the lock position after pressure build-up, brake pressure is applied and pressure reduction in the right front wheel brake (5a) is still not possible, i.e. the brake pressure level remains unchanged.

      • Pressure reduction 

        → Remains in the lock position after the pressure holding phase.