Originally Posted by dakota

This is probably more than you wanted to know, but...

To understand the "snow chain switch", you need to understand ASR a bit. ASR is a control system that activates the brake on the spinning wheel and depending upon conditions, reduces engine power output until the slip stops. There are several stages in the ASR activation which occur in milliseconds and are transparent to the driver.

In the US, ASR was an option from '91-on, on non-4matic vehicles only. My 300CE, for instance, has ASR, with the /!\ in the speedometer, just like the 4matic. It's tied in with the ABS and has four wheel speed sensors and a connection to the acceleration control system. There are three different control ranges in the ASR control mode using both a drive moment control circuit and a brake moment control circuit. More info is also contained in Brake System job 42-0800.

The acceleration skid control (ASR) will engage at all vehicle speeds, if one or both drive wheels begin to lose traction and spin due to excessive acceleration. While engaged, the yellow function indicator in the speedo lights up.

With the ASR engaged, the brake is applied to the spinning drive wheel until it regains sufficient traction. If both drive wheels lose traction and spin, the brake is applied to both wheels and simultaneously, engine torque is limited.

As traction on the road surface increases, the allowable engine torque also increases again and the brake is no longer applied to the drive wheels.

As I mentioned, there is a brake moment control circuit and a drive moment control circuit. The three control ranges of the ASR are:

A. Control mode with one skidding drive wheel and at wheel speeds < 40 km/h. The allowable slip threshold is higher for the Drive moment control than the Brake moment control, so the brake is applied to the slipping wheel. If it still slips too much, the Drive control comes in and retards the electronic accelerator control.

B: Control mode when both drive wheels skid or at a speed < 40km/h. Drive control has priority and retards accelerator. If wheels still skid, brake control is added.

C. Control mode when cornering at speeds between 20 km/h and 120 km/h. Depending on lateral acceleration, Drive moment control comes in sooner than for mode "B".

The console mounted switch, the "snow chain" switch, when activated, increases the speed threshold of when each mode will start to engage. This "buffers" the ASR a bit and lets some slippage occur when first starting out from a stop.

There is no traditional locking differential; the asymmetrical braking independently applied to the rear wheels provides this function, sort of.