I have had this problem since I picked up my LR3 a couple years ago. The Nav screen brightness will fluctuate sometimes or not dim. On a cold start it often is at full brightness but will eventually dim after a bit of driving. Adjusting dash brightness when stuck has no effect. Other times it will fluctuate wildly for no reason even after its been on for hours. Not sure what to make of it other than a bad illumination input or bad Nav display unit. On the vehicle input screen I noticed there is never a voltage reading for the ambient light sensor. Does anyone have a value other than 0.00V for the light sensor? For the other readings I do get values, with the dash dial all the way down I get blacklist dimming duty at 40% and graphic illumination at 17% - this was the reading when I cold started and the display was at full brightness. With the dial all the way up I had 46% and 100% respectively, again withe ht screen stuck at full the numbers changed but brightness did not.

 

Maybe a bad connector on rear of screen. Possibly the earth connection.

 

Bad connection or screen is on its way out.

 

Interesting thread. I've been driving my LR3 while back in Colorado, and the other day it was cold and upon starting I turned on all four seat heaters. I noticed the left side of the nav screen was darker than the other half. Never had that happen before, even when the battery was on its last leg. Alternator is a factory new unit. LR3 visits the dealership this week, will see if they have heard of this.