LR3/Disco 3
vehicle stability.
HDC SWITCH
The HDC switch controls the selection of the HDC function.
The HDC switch is a non-latching switch installed on the center console, to the rear of the gear shift lever. Pressing and
releasing the HDC switch momentarily connects an ignition power feed to the ABS module. With the first press and
release of the HDC switch, the ABS module enables operation of the HDC function. When the HDC switch is pressed and
released again, the ABS module disables operation of the HDC function.
To guard against incorrect operation or a broken switch, if the switch is pressed for more than 10 seconds no change of
state occurs. If the input from the HDC switch is held high for more than one minute, a failure is stored in the ABS module.
HDC RELAY
The HDC relay is used to illuminate the stoplamps when the brakes are activated during HDC operation and during
dynamic application of the parking brake.
The HDC relay is a non-serviceable, solid state relay on the circuit board of the Central Junction Box (CJB). Operation of
the HDC relay is controlled by the ABS module switching the coil to ground. The ABS module monitors brake system
hydraulic pressure and energizes the HDC relay during active braking. A pressure threshold and time filter prevent the
stoplamps from flickering when HDC is braking.
STOPLAMP SWITCH
The stoplamp switch is mounted in the brake pedal bracket and operated by the brake pedal. The stoplamp switch is a
two pole switch: The Brake Switch (BS) pole supplies a brake pedal status signal to the ABS module; the Brake Lamp
Switch (BLS) pole operates the stoplamps and also supplies a brake pedal status signal to the ABS module and to the
Engine Control Module (ECM).
While the brake pedal is released:
The BS contacts are closed, and connect an ignition power feed from the Central Junction Box (CJB) to the ABS
module.
The BLS contacts are open.
When the brake pedal is pressed:
The BS contacts open.
The BLS contacts close, and connect an ignition power feed from the CJB to the three stoplamps, the ABS
module and the ECM.
The ABS module monitors the status inputs from the stoplamp switch and broadcasts the brake pedal status and an
associated quality factor on the high speed Controller Area Network (CAN) bus.
WHEEL SPEED SENSORS
