Hi aekhong
Thanks for your reply, from previous post this is what i've done
1. remove the gmappromTH110.img (gmapprom.img), JCV110TH.jcv
2. reset (power on + bottom right screen)

I then retry downgrade the unit again, following this procedure
1. Delete Gupdate.gcd from your Nuvi 1490 US.
2. Backup your GPX.
3. Copy th_TH.kbd , th_TH.glx and 006-D2225-00.bin to their folder. ===> .kbd to \KeyBoards // ==> 006-D2225-00 to .System\ExtData ??
4. Downgrade your FW Nuvi 1490 US to 3.00 US by 1490 300US.rgn with Updater.
4.1 For this step i discover that inorder to use udater.exe the unit have to be in diagnose mode (press battery icon until it enter the dianose mode then connect the usb cable)
4.2 drag "1490 300US.rgn" to updater the screen shows upgrade progress
4.3 upon completion uplug the usb and the unit reboots and stop at screen saying somthing is mismatch (unfortunately i didn't capture it) and suggest using webupdater to update
4.4 Finally i update the unit using webupdate and i managed to boot up the unit again
5. my unit system software is 4.90

any suggestions

Thank you