Light Check Module (LCM) (page 4 of 5)

LIGHTING

DESCRIPTION AND OPERATION 86-2-11

Circuit Protection
Operation of the lamps is performed using overload proof Metal Oxide Semiconductor Field Effect Transistors 
(MOSFETs). The MOSFETs have a diagnostic output for bulb monitoring and can detect overload, load interruption 
with the lamps switched on and short circuit to positive with the lamps switched off.

The MOSFETs are protected against short circuits, removing the requirement for the lamps circuits to be protected 
by fuses. The MOSFETs respond to heat generated by increased current flow caused by a short circuit. Normally this 
would cause the fuse to blow. The MOSFETs react to the heat increase and cut the supply to the affected circuit. Once 
the fault has been rectified or the MOSFET has cooled, the MOSFET will automatically reset and operate the circuit 
normally.

If an overload occurs, the current flow is dependant on the temperature of the related MOSFET and can be up to 20 
times the rated current of the lamp. The MOSFET heats up and deactivates the load applied to the circuit. When the 
MOSFET cools the circuit is once again reactivated. This thermal cycling occurs continuously in the event of an 
overload occurring.

The brake switch is also monitored by the LCM. If the LCM detects a short circuit to ground in the switch circuit it 
activates messages to the driver in the message centre. These messages will display switch defective and LH and 
RH stop lamp defective. The brake switch is also monitored when the ignition is in position II. The LCM checks the 
acceleration speed of the vehicle (via bus messages). If the vehicle is accelerating and the brake switch is still active 
after 10 seconds, the LCM starts a timer. If, after 2 minutes, the brake switch is still active and the vehicle is moving, 
the LCM activates defective switch messages in the instrument pack message centre.

18

Front LH side lamp

Output

19

Hazard warning switch lamp

Output

20 to 23

Not used

–

24

Engine coolant level sensor

Input

25

Brake switch

Input

26

Reverse gear selected

Output

27

Headlamp levelling vertical aim control

Output

28

Reverse gear selected

Output

29

Illumination dimmer switch supply

Output

30

Steering column lighting switch (left and right turn selections)

Input

31

Ignition power supply

Input

32

Not used

–

33

Lighting switch headlamp selection

Input

34

Not used

–

35

Trailer module serial interface

Input/Output

36

I bus

Input/Output

37 and 38

Not used

–

39

Reverse gear selected

Output

40

Not used

–

41

Brake fluid level switch

Input

42 to 44

Not used

–

45

Rear fog guard lamp switch

Input

46

Not used

–

47

HDC active signal

Input

48 and 49

Not used

–

50

Steering column lighting switch, main/dipped beam and flash 
selection

Input

51

Lighting switch side lamp selection

Input

52

Lighting switch ground

–

53 and 54

Not used

–

Pin No.

Description

Input/Output