Electronic Stability Program
More commonly known by the generic term Electronic stability control (ESC).
ESC is a computerized technology that improves the safety of a vehicle's handling by detecting and preventing skids. When ESC detects loss of steering control, ESC automatically applies individual brakes to help "steer" the vehicle where the driver wants to go. Braking is automatically applied to individual wheels, such as the inner rear wheel to counter oversteer, or the outer front wheel to counter understeer. Some ESC systems also reduce engine power until control is regained.[1]
Starting with MY2001, all S4s come standard with ESP. A button inside the cabin "disables" ESP's throttle intervention (ASR), but not brake intervention (EDL, EBD) and MSR(?).
Fuse #42 can be pulled from the main Fuse panel between the driver side door and the side of the console. This disables power to the ESP hydraulic pump, which prevents EBD and EDL (but not MSR?) intervention.
Disconnecting the yaw sensor will disable EBD, and ASR/MSR(?). EDL and ABS unaffected(?).
The engine ECU can be remapped[2] to numb or ignore CAN torque intervention signals from the ESP controller to mitigate (or disable) ASR/MSR.
ABS
Antilock Brake System
Selectively pulses braking on wheels that are showing signs of locking up.
Is not disabled via ESP button.
Removing fuse 42 may alter brake biasing!
EBD
Electronic Brake-force Distribution
Selectively applies braking to wheels if the ESP controller detects "unsafe" yaw
Possibly disabled by ESP button?
Possibly disabled by removing fuse 42, removing the yaw sensor, or hardwiring the emergency brake signal input to the ESP/ABS controller to +12V (switched)?
ASR
Anti Slip Regulation (a.k.a. traction control)
Reduces requested torque (slow intervention) and/or cuts timing (fast intervention) if the ESP controller detects wheel spin during acceleration, especially while turning
Can be disabled via ESP button.
Can be overridden via map modifications in the ECU.
EDL
Electronic Differential Locking
Applies braking to a wheel that the ESP control detects is losing traction under acceleration, thus redirecting torque to the opposite wheel (which presumably has more traction) through the open differential. Only active below a certain vehicle speed. The stock S4 threshold is 26mph.
Is not disabled via ESP button.
Can be disabled by removing fuse 42 or hardwiring the emergency brake signal input to the ESP/ABS controller to +12V (switched)
MSR
Motor Slip Regulation (a.k.a. Engine Drag Torque Control)
Increases requested torque if the ESP controller detects wheel slip due to a sudden decrease in engine speed (torque braking), such as in sudden off-throttle conditions. Anti-stall also uses this mechanism.
Possibly not disabled via ESP button?
Can be overridden via map modifications in the ECU