Hill Descent Control (HDC): Notes

Hill Decent Control (HDC) is designed for off road use to automatically slow the vehicle and maintain a steady speed on steep gradients. This function allows the driver to focus on steering and controlling the vehicle without having to use the brakes to slow the vehicle.

Fig 1: Hill Descent Control (HDC)

Courtesy of JAGUAR LAND ROVER NORTH AMERICA, LLC

1.HDC switch

HDC is activated manually through the switch located in the center console next to the transmission gear selector. HDC can be selected under any conditions while low range is selected, and below the 21 mph (35 km/h) speed threshold when high range is selected. The HDC Lamp in the instrument pack indicates HDC selection status. If enable conditions are not met, system goes in to stand by mode. When activated, the vehicles speed is held to approximately 5 MPH (3 km/h) by the DSC ECU pulsing the brakes to maintain the speed.

The following conditions must be met before the HDC will activate:

HDC switch operated -LED "ON "
Vehicle speed:<21 MPH (35 km/h)
Accelerator pedal pressed <15%
Downhill driving recognized.

If the HDC switch is operated above 21 mph (35 km/h) in high range, "NO HDC - SLOW DOWN" message will be displayed in the instrument pack, and HDC will not be selected.

If the HDC switch is held down for more than 10 seconds, the HDC selection status does not change

If HDC is selected, then vehicle ignition switched off, the HDC selection status remains in the ABS ECU memory for 6 hours. If the vehicle is restarted within this time, HDC remains selected. If vehicle is not started within this time HDC will be automatically de-selected. If the vehicle speed exceeds 37 mph (60 km/h) while high range is selected, HDC is automatically de-selected.

HILL DESCENT CONTROL ENABLE CRITERIA

Gear Detected	Neutral	Auto Transmission D, 1-5, R
Throttle less than 5%	Enabled	Enabled
5% to 20%	Stand by	Enabled
Throttle more than 20%	Stand by	Stand by

The target speed used by the HDC varies in response to driver inputs. Current target speed will lie in the range between Maximum Target Speed and Minimum Target Speed.

HILL DESCENT CONTROL TARGET SPEEDS

Low

High

Gear selector position

D, 1, 2, 3, 4, 5

Maximum target speed

35 km/h (22 mph)

Standard target speed

6 km/h (4 mph)

3.5 km/h (2 mph)

6 km/h (4 mph)

3.5 km/h (2 mph)

6 km/h (4 mph)

10 km/h (6 mph)

6 km/h (4 mph)

10 km/h (6 mph)

Minimum target speed

3.5 km/h (2 mph)

6 km/h (4 mph)

When HDC is first activated, if the driver does not press the brake or accelerator pedals, then current target speed is set to default target speed. If HDC is activated and the driver presses the brake pedal, the vehicle brakes operate as normal. While the brake pedal is pressed:

If vehicle speed is equal to or more than current target speed, current target speed is unchanged
If vehicle speed is less than current target speed, current target speed is set to vehicle speed

When the brake pedal is released:

If vehicle speed is equal to or more than minimum target speed, then current target speed is unchanged
If vehicle speed is less than minimum target speed, then Current Target Speed is set to minimum target speed.

If HDC is activated and the driver presses the accelerator pedal, current target speed is defined by a characteristic as follows:

If current target speed is less then default target speed, then the target speed is set to current speed.

When the accelerator pedal is released:

If current target speed is greater than default target speed and vehicle speed is equal to or more than default target speed, then current target speed is set to default target speed
If current target speed is more than default target speed and vehicle speed is less than default target speed, then current target speed is set to vehicle speed
If current target speed is less than or equal to default target speed, and vehicle speed is less than current target speed, then current target speed is set to vehicle speed
If current target speed is less than or equal to default target speed, and vehicle speed is equal to or more than current target speed, then current target speed is unchanged.

To ensure that the vehicle is in an appropriate gear during HDC activity the ABS ECU communicates with the EAT ECU. A unique gearbox map is activated when HDC braking activity begins. The characteristics of the map are that lower gears are selected sooner than normal, and higher gears are selected later than normal. This reduces the work done by the foundation brakes, and improves the response time of HDC.