At Mercedes, "A" stands for acceleration, whether on dry pavement or icy road, whether on a straightaway or a curve.
The reason you can move with confidence when others can't is traction control, the next logical progression in the electronics that brought us anti-lock brakes to stop a vehicle in a straight line regardless of road surface by controlling wheel spin.
If a few computer sensors can stop a car without a wheel spinning, add a few more to control those same wheels when one or more hits ice or snow while moving.
Mercedes offers two forms of traction control for the 1991 model year: automatic locking differential (ASD) and automatic slip control (ASR).
The initials don't follow the wording because something is lost in the translation from German, say the folks from Mercedes.
Suffice it to say that both are designed to work on ice or snow. ASD stops the wheels from slipping when you start from the curb, light or stop sign and ASR keeps the wheels from slipping to provide directional stability and traction while you're moving, such as going into or out of a curve, swinging out to pass or crossing a viaduct or bridge.
ASD is simpler and is offered as a $1,050 option for 1991 on the 300D 2.5 turbo, 350SD turbo and 350SDL turbo diesel models, and in the gas driven 300SL, 300E 2.6, 190E 2.3 and 190E 2.6.
ASR is more sophisticated and is a $1,975 option on the 300E, 300CE, 300TE, 300SE, 300SEL, 420SEL, 560SEL, 560SEC, 300SL (automatic only) and 500SL.
ASD automatically equalizes the torque, or pulling power, to each wheel to provide traction.
ASR sensors measure wheel speed so that if a rear wheel slips, the anti-lock brakes are applied to the wheel or wheels to equalize torque. ASR sensors also reduce engine output so the power provided matches available traction.
Because snow and ice were missing, Mercedes demonstrated ASD and ASR with a parking lot "paved" with soap-caked rubber mats.
The task was to propel a 300D 2.5 turbo from a standing start on a surface made to approximate a street after a couple-inch snowfall -- first without ASD, then with it.
The soapy mat was positioned so the right tires would travel over it and the left were on dry pavement. We floored the pedal and the reaction was predictable, the wheels spun, the soap suds splattered fender and rocker panel and the vehicle wavered sideways while making little or no forward progress.
Next, ASD was activated and the same maneuver performed. The sensors determined that the right rear wheel was spinning out of control and the ASD activated so that torque was directed to the slipping wheel -- all in milliseconds -- so the car pulled away in a straight line without the least quiver.
Then came a 560SEL with ASR, this time on a course with alternating soapy mats so that one wheel, then the other, would hit the obstacle. From there it was a soap and water soaked pylon-lined course that turned sharply to the left so we would have to maneuver as if to avoid a car that suddenly pulled into our lane.
With ASR disengaged, the tires couldn't bite the mats. Attempts at acceleration brought only wildly spinning wheels.
With ASR activated, the sensors instantaneously picked up on the wheel spin when accelerating from a standing start. Those sensors applied the anti-lock brakes, and you could feel the pulsing in the pedal. At the same time, it felt as though we were attached by chain to a granite block; the engine went into a lumbering mode to help equalize power output to the traction available. Wheel spin was brought under control and the car pulled away in a straight line, with the driver in command.
Each system has built-in diagnostic capabilities to continuously monitor whether it's working.
"If the safety logic detects an abnormality, a -- light goes on as a warning," said Peter Patrone, Mercedes product planning manager.
ASD and ASR have been offered in Mercedes cars sold in Europe the last two years.
"We weren't going to introduce such systems without some real-world proven performance first," Mr. Patrone said.