This page originally started out as a forum entry to help explain the BMW stability systems. It got so big, I just wrote
a whole page dedicated to the subject. I hope I can help clarify things...
1999-2000 E46s have ASC or DSC
2001+ E46s have DSC
ASC & DSC work with the ABS system to provide enhanced vehicle stability.
ASC - Automatic Stability Control
ASC uses speed sensors at each wheel and throttle position to improve traction by applying the rear brakes if the rear wheels spin faster than the front. It can also shut off individual fuel injectors and close the throttle valve to help maximize traction. ASC also acts as a virtual limited slip differential by detecting a spinning wheel and applying braking force to just that wheel until traction is regained. AWD models (the xi) use ASC-X which monitors all 4 wheels to provide limited slip differential effect. The ASC system also provides traction control by modulating throttle, ignition timing and braking force to maintain traction.
DSC - Dynamic Stability Control
DSC builds on the features of the ASC system. While ASC functions only during acceleration and braking, DSC functions during all driving conditions such as cornering and emergency maneuvers. Additional sensors are added to acheive this level of stability such as lateral, yaw, brake pedal, and steering angle sensors. In DSC, each of the three subsystems: ABS, ASC, and DSC have jobs to do, and these are:
ABS: Cornering Brake Control, Electronic Brake Proportioning
ASC: Brake Intervention (or ABD-Automatic Braking Differential), Drag/Drive Torque Reduction
DSC: Dynamic Brake Control, Maximum Brake Control
Let's dig deeper, shall we?
Cornering Brake Control keeps the inside rear wheel from locking during hard cornering when it may be lifted from the road surface brought on by the car's reaction to centrifugal forces a.k.a. body roll.
Electronic Brake Proportioning uses the wheel speed sensors to maximize braking ability by controlling rear braking force bias depending on vehicle's front/rear weight load balance.
Brake Intervention acts to keep any single wheel from spinning faster than the rest. It can apply brake pressure to the affected wheel to control wheelspin. This is the virtual limited slip differential effect.
Drag Torque Reduction evaluates rear wheel lockup caused by hard decceleration or engine braking on low traction surfaces. If the rear wheels spin slower than the front, this function can open the throttle to suspend coasting and regain traction. Also, another feature of this subsystem is Drive Torque Reduction. Drive Torque Reduction acts to limit the engine's torque output during low traction situations such as snow, sand, or mud. It can reduce the throttle valve angle, retard ignition timing, and cut individual fuel injectors to do so.
Dynamic Brake Control will boost brake pressure beyond what your foot can do in emergency situations. During emergency braking, when you mash your brake pedal the floor, DBC kicks in and increases the brake pressure right to the ABS limit to produce maximum braking force and minimum stopping distance.
Maximum Brake Control functions to improve braking efficency by increasing rear brake pressure when the front wheels are at lockup and are being regulated by the ABS system.
ASC/DSC CONTROL BUTTON
The control button on the center console is used to deactivate the ASC/DSC systems.
NOTE: On all systems, ABS IS ACTIVE AT ALL TIMES.
A indicator lamp between the tachometer and speedometer is used to show ASC/DSC status.
A flashing indicator lamp indicates the system has activated.
A steady indicator lamp indicates the system is turned off or inoperative.
On the ASC system:
Press the ASC button to disable ACS while leaving ABD active.
On rear-wheel drive DSC systems:
Press the DSC button to disable DSC while leaving ABD active.
Press and hold the DSC button for 3+ seconds and the "BRAKE" instrument panel lamp will light yellow and disable all stability/traction systems.
On all-wheel drive (xi) DSC systems:
Press the DSC button to disable DSC while leaving ADB active in maximum output mode.
On ALL systems:
Pressing the ASC/DSC button again will reactive the system. If the indicator lamp does not go out or if it should come
on and stay on during driving, a component has failed and should be serviced.
WARNING: If the ABS, ASC/DSC, and BRAKE lamp should all come on during driving, do not drive the car and have it serviced immediately.
These different systems all work in unison to provide a very safe and stable vehicle under almost all circumstances. Remember, physics does still apply and even our BMWs have their limits. On numerous occasions, however, I have felt the DSC system kick in on slippery surfaces and jerk the car back straight without any input from me. It is quite impressive to say the least. While some may feel that ASC/DSC limits their BMW's performance, the payoff in safety on the road is too great to gamble with. At the track... well, that's a different story...