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.

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. In order to fully disable ESP (but retain ABS), 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.

ABS

Antilock Brake System

Selectively pulses braking on wheels that are showing signs of locking up

EBD

Electronic Brake-force Distribution

Selectively applies braking to wheels if the ESP controller detects "unsafe" yaw

Can be disabled by removing fuse 42 or the yaw sensor(?)

ASR

Anti Slip Regulation (a.k.a. traction control)

Reduces requested torque if the ESP controller detects wheel spin during acceleration, especially while turning

Can be overridden in the ECU

EDL

Electronic Differential Locking

Applies braking to a wheel that the ESP control detects is losing traction, thus redirecting torque to the opposite wheel (which presumably has more traction) through the open differential.

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.

Can be overridden in the ECU