Central switching unit (Discovery 3)

The Central Junction Box (CJB) is a unit located behind the instrument panel on the front passenger side bulkhead. The CJB contains a set of fuses, relays and microprocessors that together control the power and operation of the vehicle's lighting and systems.

Input signals

The central switching unit receives input signals from the following switches:

• Light switch Position of side lights Position of headlights Automatic position (AUTO) (if any)
  • Position of side lights
  • Position of headlights
  • Automatic position (AUTO) (if any)
• Brake light switch
• The left multifunction stalk controls the direction indicators and high beam/signal lights.
• Hazard warning switch
• Rain/light sensor (lIN signal)

Protection of electrical circuits

The central switching unit provides protection for all outdoor lighting circuits. Most outdoor lighting circuits are protected by conventional fuses, but the following circuits are protected by field effect transistors (FETs).

• Low beam headlights
• High beam headlights
• Cornering lights/steering lights with tilting axis
• Direction indicators
• Trailer Direction Indicators

The circuits listed above are protected by unipolar transistors, which are capable of detecting overloads and short circuits. Unipolar transistors react to the heat generated by the short-circuit current. In a circuit with conventional protection, a short circuit would cause the fuse to blow. The unipolar transistor, reacting to the increased heat generation, switches off the circuit where this occurs. After the fault is eliminated or after the transistor has cooled down, the circuit can function normally again. If the fault is constant, the unipolar transistor will operate cyclically, switching the power supply on and off in the circuit.



The central switching unit records fault codes that can be read with the T4 diagnostic tool. The fault code indicates the location of the fault, which helps in diagnostics.

Alarm system indication

The external lighting system is used to request activation and deactivation of the alarm system.

When the driver locks the vehicle, the hazard warning lights are flashed three times to provide visual confirmation that the locking has been successfully completed. The confirmation signal cycle consists of a 0.5 second flash with a 0.5 second interval between flashes.

When the driver unlocks the vehicle, the hazard warning lights are activated for three seconds to provide visual confirmation that the vehicle has been unlocked successfully.

When the alarm is triggered, the hazard warning lights flash for 30 seconds with a cycle of 0.38/0.38 seconds. (on/off.) and with a 5 second break every 30 seconds. For more information, please refer to Anti-Theft - Active (419-01A Anti-Theft - Active)

Gong warning about lights not turned off

If the ignition key is in the OFF (0) or auxiliary (I) position and the light switch is left in the parking light or headlight position, a warning chime sounds when the driver opens the door. The chime reminds the driver that the exterior lights are left on.

The source of the warning gong is a speaker located in the instrument panel, which is activated when signals about an open door and about the ignition being off are received via the CAN bus from the central switching unit.

Activation of the collision signal

When the SRS controller detects a collision, the CJB activates the hazard warning lights and direction indicator lamps located in the instrument cluster.

The hazard warning lights remain on until the ignition switch is turned to position I or 0 or if the signals from the air bag and safety belt computer are no longer received. For more information, refer to Air Bag and Safety Belt Pretensioner Supplemental Restraint System (SRS) (501-20B)

Headlight timer

The CJB unit functions as a headlight timer, which allows the headlights to remain on for a certain period of time after the driver leaves the vehicle. This function is intended for the driver's convenience: the headlights illuminate the way to the door of the house.

To activate this function, the light switch must be in the headlight or parking light position when the ignition switch is switched from the ignition position to the off position. Then, the light switch must be set to the OFF or AUTO position. (if any). After activating the timer, the low beam headlights will turn on for the set time.

The duration of headlights is set by the driver within 60 seconds in 10-second increments. By default, the duration of headlights is 20 seconds.

The default value can be changed by an authorised Land Rover dealer or by the driver using the user settings menu.

To access the menu when the vehicle is stationary and the ignition key is in any position, press the on-board computer button (located on the end of the left multifunction steering column switch lever). The menu options can be "scrolled" using the search keys on the audio system remote control, which is located on the steering wheel. The menu options are displayed on the information center display. Find the headlight timer option and change the duration of their operation using the search keys. Confirm your selection by pressing the trip computer button. For further information, refer to the Information and Message Center (413-08)

NOTE: The headlight timer function is cancelled in the following cases:

• The key in the ignition switch is moved from position I (auxiliary equipment) in position II (ignition on).
• The light switch switches from the off position to the parking light position.

The headlamp timer can be activated by the programmable button on the ignition key fob if the driver has set this feature. For more information, refer to Handles, Locks, Latches and Entry Systems (501-14)