T61 screen don't dimm when running on battery

[Pascal_TTH]

I use thinkpad since 10 years, so I'am common with them. I got a T61 14" and it seems something goes wrong. When you unlplug the power, the screen must be less bright. The option in the bios is set to normal so it mst act this way. But it doesn't !

I change the value to high and then got back to normal, nothing change. With my T60p, my R61 or any other Thinkpad, plug and unplug the power change the screen brightness. No need to reboot or so, it works regardless from the OS.

Does someone have this issue ?
My T61 is 7664-18G with last bios aviable (all drivers are up to date).

[Pascal_TTH]

Screen was already at lower brightness so it did not change.