I have alenovo t420 laptop that brightens and dims every second or so . tried diablind the automactic brighten button under intel but this didn't work . am at a lost and need help .

Do you have a dimmer for interior lamps? Like a rheostat, a little wheel that you turn up or down to brighten or dim interior lamps. Usually the instrument cluster lamps brighten and dim the same way.
Can you brighten and dim the other interior lamps?

The problem is probably at your TFT/LCD panel. Find to know by connecting your Notebook to another monitor is it dimming or not

Lenovo typically uses F11 (dim) and F12 (Brighten). Or, if you are running Windows 8, move the cursor to the lower right and wait for the menu. Select SETTINGS, and you will see BRIGHTNESS button on the SETTINGS screen. Click it and adjust it.

PLEASE RATE ME!
go to the search icon under start. search brightness. it will tell you step by step what to do.
Every laptop is different, so adjusting the brightness or contrast on your laptop may vary from the steps provided on this page. However, understanding the process of how to adjust the brightness or contrast will help you change your settings.
Most laptops adjust their brightness and other settings using the function key in conjunction with another key. For example, pressing and holding the function key (abbreviated as "Fn") and pressing the up or down arrow keys as shown in the picture below, increases and decreases the brightness.

have you tried holding Fn and pressing the button with thr brightening symbol. on mine its Fn and F5 for brighten and Fn and F4 for dim.. buttons may vary but this should work if not you may have a fault with the sreen sensor that shuts the screen down when you close the notebook

Just hold down the light button on the remote. It'll go dim then will brighten back up then dim again.

The Back-light on your LCD screen is bad, your crystals aren't illuminating, so it is dark.... very dark.

You need a new LCD screen.

Sorry.

when you press the keys did you press also the Fn key (usually sets beside the window key.)

it should be Fn + bright positive key. you need to press both...hold fn then tap the bright positve key.

or right click on desktop
go to properties (if you have an intel chipset or other VGA chipset) you will see a Tab with the title of your VGA (like Intel Graphic Media ....) click on the tab, from there look for a button in which you can change the graphics settings...it will give a a new window which you can change color, brightnes, gamma etc..

hopes this helps, please rate.

Backlight or inverter failure sorry to say. No telling which without substitution. Backlights are mostly built into the top of the screen and are tricky to replace. Inverters are generally sited at the bottom of the screen under the facia panel. Look around ebay for replacements