All Season Traction (AST)
All Season Traction (AST) - Marketing Designation
Internal Designations Include: ABS/ASC - ABS/ASC+T - ABS/DSC
BMW uses two different Traction Control Systems. One manufactured by Bosch, and the other manufactured by Teves.
Functions of AST include: Antilock Brakes - ABS
Traction Control Lateral Stability - AST+T
Traction Control Dynamic Stability - DSC
The Antilock Braking System is the basis of all AST systems. The Traction Control functions work in conjunction with ABS braking. Control electronics are integrated into the ABS control module. Traction control is always On-Line when the key is switched on.
The ABS system constantly monitors and compares the wheel speed signals from individual [1][2]speed sensors monitoring each wheel. If the control module detects a decrease in the frequency (rate of deceleration) of one or more of the individual signals it perceives this as possible wheel lock.
The control module activates solenoids in the ABS hydraulic unit to block any additional hydraulic pressure from being applied to the wheels which will prevent the wheel from locking. As it's name implies it only prevents the wheels from locking up during braking.
ASC+T Systems provide lateral stability by reducing the amount of power applied to the drive wheels or by momentarily applying braking force to the rear wheels. This prevents the rear wheels from spinning, thus improving the traction.
The DSC System provides additional stability while driving by helping to control over-steer and under-steer while cornering. This is also carried out by regulating the applied power and brake force to the rear wheels.
All of the AST systems are electronic control systems that monitor the wheel speeds and all of the signals as shown in the IPO. The control module activates solenoid valves in the AST hydraulic unit which control the hydraulic flow of the brake fluid pressure for the purpose of braking and or traction control.
Two indicator lamps are used in the cluster for ABS and AST. The ABS warning lamp will illuminate when a fault is present with the ABS side of the system.
The AST lamp serves two functions. It will flash when AST is regulating. It will also come on permanently when the system is faulted or switched off by the driver.
ABS Hydraulic Operation
The AST control module activates the solenoids in the hydraulic unit. The solenoids open or close hydraulic circuits that block or relieve hydraulic pressure in the brake calipers.
Depending on the AST system. the hydraulic unit has various types of solenoids. The ABS hydraulic unit has fewer solenoids since it is only responsible for braking control. ASC+T systems have additional solenoids that achieve the additional function of traction control.
The illustration below shows a single wheel circuit for an ABS hydraulic unit in the normal braking condition.
ASC+T Hydraulic Operation
The illustration below shows a complete ASC+T hydraulic unit. The rear brake control is accomplished by the control module activating the solenoids and pump as follows:
- Wheel slip is detected at the drive wheels.
- The switching valve is closed isolating the rear brake circuits diverting the the brake fluid flow form the reservoir to the intake valve. The intake valve is opened, the pump is on.
- The pump pushes the brake fluid through the open inlet valve and causes the caliper to apply brake pressure. The inlet valve is energized and closes the the inlet port.
- The outlet valve is energized and opened to relieve the brake fluid back to the intake side of the pump where it is introduced back to the inlet side of the inlet valve again.
This sequence continues until the wheel has restored traction.