Skyray 3xT6 3800 Driver Problem (Only 1 Mode)

Hello everyone.

Before I start, I want to excuse me for my bad english, i´m german.

I have a problem with my Skyray 3xT6 3800 (5 Mode).
After I tried to desolder the driver, the Skyray has only High Mode. If I click the Tailswitch, the Flashlight is turning off, instead of switching to the next mode. I have to turn the light of. After 2 secons, i´m able to turn the light again.
My soldering iron was to weak to melt the solder, because the heat is gone to fast into the heatsink.
Because I have a second Flashlight, I´m sure, that only the driver could be the problem. (I changed the drop in)
I inspected the soldering points, but I did not notice anything unusual.

I have already ordered a new driver. It´s this one: http://kaidomain.com/product/details.S020121

But anyway, I´m looking for the reason of my problem.
I don´t know much about electronic oder drivers, but maybe there is a easy answer to my question.

Which electronical parts are responsible to change between the modes?

I hope you understand my bad english. If you need more information, please let me know.

Thanks in fortune

Christian