Hill Descent Control (HDC)1

General information on HDC

When you release the accelerator on downhill gradients the car is normally braked by means of the engine striving for low engine idling speeds, so-called engine braking. But the steeper the roadway and the more load in the car, the faster the car coasts despite engine braking - the HDC function compensates for this with automatic brake intervention.

HDC makes it possible to increase or reduce your speed on steep downhill slopes using only the accelerator pedal. The sensitivity of the accelerator pedal decreases and becomes more precise by means of the full actuation of the pedal being restricted to adjusting engine speed within a limited range. The brake system brakes itself and provides the car with a low and even speed, so allowing the driver to fully focus on steering.

HDC is particularly helpful on steep gradients with an uneven road surface and slippery sections. E.g. when launching a boat on a trailer from a ramp.

Function

HDC is activated or deactivated with one of the centre console's switches. The lamp within it illuminates when the function is activated.

When HDC is working normally the combined instrument panel's symbol illuminates combined with the text message Hill descent control ON.

With manual transmission, the function works only on first gear and reverse gear.

With automatic transmission, gear 1 must be selected in manual gearshift mode (+S-), or R must be selected. This is indicated by the number 1 or R in the combined instrument panel, see Automatic gearbox - Geartronic*.

Operation

HDC allows the car to roll at a maximum of 10 km/h (6 mph) forwards with engine braking and 7 km/h (4 mph) backwards. However, any speed within the gear's speed register can be selected using the accelerator pedal. When the accelerator pedal is released, the car is braked quickly to 10 or 7 km/h (6 or 4 mph) respectively, irrespective of the hill's gradient and without the need for the foot brake.

The brake lights come on automatically when the function is operating. The driver can brake or stop the car at any time by using the footbrake.

HDC is deactivated:

This function can be switched off at any time. If the function is switched off while driving on a steep downhill gradient, the braking effect will gradually decrease.