This maybe a liquid damaged board works fine with external monitor. but with the internal display i see image but no back light unfortunately i don't nave a test screen or another board to test on U8400
pin1 and 2 LCDBKLT_SW= .1V
PIN 5 PP5V_SO_BKLT_D =5V
PIN 9 ISNS_LCDBKLT_N=12V
PIN 10 ISNS_LCDBKLT_P=12
PIN 11 BKLT_SD=12
PIN 12 BKLT_PWM_KEYB=0
PIN15 BKLT_SCL=4.5
PIN 16 BKLT_SDA=4.5
PIN 17 BKLT_EN_R=3.3
PIN18 PP5V_SO_BKLT_A=5V
PIN 19 BKLT_SENSE_OUT=0
PIN 21 LCDBKLT_FB=0
pin1 and 2 LCDBKLT_SW= .1V
PIN 5 PP5V_SO_BKLT_D =5V
PIN 9 ISNS_LCDBKLT_N=12V
PIN 10 ISNS_LCDBKLT_P=12
PIN 11 BKLT_SD=12
PIN 12 BKLT_PWM_KEYB=0
PIN15 BKLT_SCL=4.5
PIN 16 BKLT_SDA=4.5
PIN 17 BKLT_EN_R=3.3
PIN18 PP5V_SO_BKLT_A=5V
PIN 19 BKLT_SENSE_OUT=0
PIN 21 LCDBKLT_FB=0