Light Check Module (LCM)
The LCM is located on the RH 'A' pillar, behind the trim panel. The LCM is connected to the vehicle wiring harness with three multiplugs.
The LCM receives two permanent battery power supplies via the Central Junction Box (CJB) and power feeds from the ignition switch positions I (AUX) and II (IGN).
The lighting circuits are not protected by conventional fuses. The control circuitry within the LCM for each individual circuit can detect and isolate a problem circuit.
A monitoring system within the LCM can determine a bulb failure and indicate this to the driver via the instrument cluster message center.
Input Signals for Lamp Control
The LCM receives inputs from the following switches: Lighting control switch for side lamps and headlamps
- Momentary push switches for front and rear fog lamps Left hand steering column multifunction switch for turn signal indicators and high beam/headlamp flash
- Stop switch
- Momentary push switch for hazard warning.
The switches are supplied with a
10 mA supply from the LCM and switch to ground when operated. The LCM detects that a switch has been operated (ON) when its closing resistance is less than
100 Ohm and is detected as OFF when its resistance is more than
10K Ohm .
The LCM also receives ignition switch status via hardwired connections and also on the I and K Bus via the instrument cluster.
A reverse gear engaged signal is also received on the I and K bus systems to enable the LCM to activate the reverse lamps. The trailer module also outputs a signal to inform the LCM that a trailer is or is not connected. If a trailer is connected, the LCM transfers control of the trailer reverse lamps and the rear fog lamp to the trailer module.
Via the bus systems the LCM receives a hazard warning lamp activation message from the restraints control module, via the GEM (generic electronic module), in the event of a crash or from the GEM if the alarm system is triggered.
LCM Harness Connector C0937
LCM Harness Connector C2039
LCM Harness Connector C2040
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 center. These messages will display brake 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 cluster message center.
Bulb Monitoring
Bulb failure monitoring is performed by the LCM processor. The lamps are cold and warm monitored by the MOSFETs in order to detect bulb failure.
The LCM processor provides outputs to each MOSFET. The output switches the MOSFET to supply the required output to power the lamp circuit. The microprocessor evaluates the lamp circuits by detecting the returned signals from the controlling MOSFET.
When the bulb is functioning normally, the output signal voltage from the controlling MOSFET is
0 V . If a bulb in the circuit fails, an open circuit occurs and the MOSFET outputs a signal of
5 V to the processor. The signal is interpreted as a bulb failure and generates a message which is output on the I Bus to the instrument cluster. The instrument cluster displays the applicable bulb failure message in the message center to provide visual warning to the driver.
Warm monitoring is performed continuously when the lamps are switched on by evaluating the diagnostic output of the MOSFET switches. Cold monitoring is performed at
32 second intervals when the lamps are switched off. The MOSFETs briefly switch on the lamps for approximately
1 millisecond (this is insufficient to illuminate the bulb) and checks the bulb as per warm monitoring.
Cold monitoring is not possible for the low/high beam headlamps of vehicles using xenon bulbs. On these vehicles the cold monitoring of the low/high beam headlamps is switched off in the LCM. The LCM detects a failed xenon bulb via a reduction in current flow to the affected headlamp's xenon
control module.
When a xenon bulb fails, the
control module's current consumption falls to 60mA, which the LCM detects as unsuccessful bulb illumination.
Alarm Indications
The LCM communicates on the I and K Buses with the GEM to display alarm visual indications for alarm arm, disarm and triggered conditions.
If the hazard warning lamps are active when a lock or unlock request is made, the hazard warning cycle is interrupted to allow the visual indication of the requested lock cycle. When visual indication is completed, the hazard warning operation will continue.
If the vehicle is involved in crash of a severity for the restraints control module to initiate deployment of the airbags, the
control module outputs a hazard warning lamps on request on the K bus to the instrument cluster and on the I bus from the instrument cluster to the LCM. The hazard warning lamps will be activated and will continue until the restraints control module outputs a message to deactivate the hazard warning lamps or until the hazard warning lamp switch is pressed.
Redundant Data Storage
The LCM stores data relating to the Vehicle Identification Number (VIN), total mileage and service interval indicator. This data is received by the LCM from the instrument cluster and used as a back-up in the event of instrument cluster replacement.
If the LCM is to be replaced, T4 must be connected to the vehicle and the LCM replacement procedure followed to ensure that the stored data is transferred to the new unit.
Low Voltage Operation
If the battery voltage falls below
11.2 V , the LCM operates the minimum lighting to preserve the remaining battery charge.
Crash Signal Activation
In the event of an accident of a severity to activate and deploy the airbags, the restraints control module requests various electrical operations to assist with the crash situation. The restraints control module requests via the bus systems to the LCM to activate the hazard warning lamps and flash the headlamp high beam at the same frequency.
Security System Activation
In the event of the security system being triggered, the GEM requests activation of the hazard warning lamps. In some markets the low beam and/or high beam headlamps can also be activated.
Instrument Panel and Switch Illumination Dimming
The LCM controls the instrument cluster backlighting illumination and also illumination of all instrument panel switches.
The LCM supplies a power output to all switch illumination bulbs at a voltage determined by the position of the manual dimmer rheostat. The switch illumination is activated when the lighting control switch is in the side lamp or headlamp position.
