Instrument Panel Dimming

CIRCUIT OPERATION
The Body control Module (BCM) controls the dimming of the LCD displays and the incandescent backlighting. When the Park Lights are off the BCM uses the ambient light level to calculate dimming the dimming level. If the Park Lights are on the BCM uses ambient light level and the Panel Dimming Switch setting to calculate the dimming level.

The BCM uses the twilight photocell input at terminal 1D3 to monitor ambient light levels. The Twilight Photocell is a variable resistor whose resistance changes as the level of ambient light changes around it. As ambient light decreases the resistance of the Twilight Photocell increases. The voltage level at the BCM twilight photocell input increases as the Twilight Photocell resistance increases.

If the Twilight Photocell, CKT 278 or CKT 736 opens, BCM Code B119 will set. A short to ground in CKT 278 or the Twilight Photocell will also set Code B119. An open in CKT 736 between the BCM and splice S228 will set multiple codes.

The setting of the Panel Dimming Switch in addition to the twilight photocell input is used by the BCM to calculate the dimming level when the Park Lights are on. Park Light status is monitored by the BCM through the lighting data line. The BCM monitors the Panel Dimming Switch at terminal 1C1 (the panel dimming input).

The Panel Dimming Switch is a slider potentiometer (variable resistor) with a limiting resistor in series with the 5 volt supply from the BCM. In the - position, resistance is low and most of the 5 volt supply from the BCM is dropped across the panel dimming input resistor in the BCM. As the Panel Dimming Switch is moved to the + position, the resistance of the switch increases and more of the 5 volt supply is dropped across the switch and less voltage is dropped across the panel dimming input resistor in the BCM.

If the Panel Dimming Switch opens or shorts to ground BCM Code B122 will set. If the 5 volt supply from the BCM or CKT 705 opens or shorts to ground multiple codes will set. An open in CKT 736 from splice S228 to the BCM also will cause multiple codes to set.

In vehicles equipped with DRL, the LCD display and incandescent backlighting dimming will be based only on the ambient light level.

LCD Display Dimming
The Digital IPC LCD display is powered by ignition 3 and is on only when the Ignition Switch is in the RUN position. The Climate Control Driver Information Center (CCDIC) LCD display is powered by switched battery to allow the display of certain messages even when the Ignition Switch is OFF. The Radio LCD display is powered by ignition 2 and will operate when the Ignition Switch is in the ACCY or RUN position and when RAP is energized. When the Ignition Switch is in the ACCY position the Radio LCD will operate at full intensity without the BCM powered up.

To dim the LCD displays the BCM generates a Pulse Width Modulated (PWM) signal at terminal 3C16 (the LCD dimming output). This PWM signal is sent to the LCD displays through CKT 724. The LCD displays dim as the duty cycle (on time) of the PWM signal increases.

When the Park Lights are off the BCM bases the PWM value on the ambient light level sensed at the twilight photocell input. When the Park Lights are on the BCM calculates the PWM value based on the ambient light level and the panel dimming switch input.

Incandescent Backlighting
The BCM generates a PWM signal at terminal 3C15 to control the lighting level of the incandescent backlighting. This PWM signal is sent through CKT 717. The value of the PWM signal determines the brightness of the incandescent backlighting. As the PWM duty cycle (on time) increases the incandescent backlighting gets dimmer. The BCM calculates the PWM value based on the ambient light level and the panel dimming switch input. The incandescent backlighting is turned on only when there is low ambient light and the Park Lights are on or the Ignition Switch is in RUN. If the Park Lights are on and there is low ambient light the brightness of the incandescent lights is determined by the panel dimming switch input.

The Wiper/Washer Switch, Headlight Switch, IPC and CCDIC condition and filter the PWM signal and generate their own PWM signal to drive their incandescent lights.

The CCDIC provides illumination power and dimming for the Ashtray Lamp, Cigarette Lighter Lamp and the Gear Selector Lamp through CKT 8 at terminal A7. The signal is based upon the value of the PWM signal from the BCM through CKT 717.