I also have this a4 tech 835mj, black screen on ym

Web Camera's have to be setup on the program they are to be used with. Just installing the camera on your computer does not set it up to broadcast on messengers or Skype.
Each program has Options that have to be setup in order for the Audio and Video to function properly.

A. Skype Setup:
After downloading and installing Skype (http://www.skype.com/intl/en-gb/download/skype/windows/beta/downloading/ )
set it up as follows:
Go to Tools, then Options, and click on Audio Settings and set the Microphone and Speakers to the computer Audio Card.
Then click on the Video Settings.
The camera, if Installed Properly, it will come up and show a video picture.
Then, on the bottom of the page click save.
The camera and audio should function properly. Your friends and Family will now be able to see and hear you.

B. Yahoo Setup:
After downloading and installing Yahoo Messenger (http://www.soft32.com/download_30.HTML )
set it up as follows:
Go to Messenger at the top left and click on it.
Next go down to Preferences and click on it.
Next "Calling and Devices" on the right, set the Microphone and Speakers to the computer Audio Card or headphones/mike.
Next go to the Test Audio section below where you just setup the microphone & speaker and (written in blue) click on "Open the call setup assistant"
and the "Yahoo Call Setup Assistant" window will open to the left and follow the the instructions. Click on FINISH and the audio will be setup.
Next go to "Webcam" and click in the circle "Allways allow the following people to view my webcam.
Next Click on "Super Webcam" snd in the circle click "Start Super Webcam mode automatically".
Then click OK and it should work.

C. Windows Live Messenger:
http://www.microsoft.com/downloads/details.aspx?familyid=d78f2ff1-79ea-4066-8ba0-ddbed94864fc&displaylang=en
After you start Windows Live Messenger, expand the view by clicking on the square on top left next to the red X.
Then on the Windows Live messenger window on the right side, click on " "Show Menus." And then click on "Show the Menu Bar".
Next go to the Menu Bar on right side, click on " "Action."
Go to "TOOLS" and click "Audio and Video Setting" and follow the instructions.
NOTE: Your audio and video should work. Enjoy.
When done both the Audio and Video will function properly so you can chat and see family or friends you are connected to.

D. AIM (AOL) Messenger:
http://dashboard.aim.com/aim
On the VIM sign-in window, click on Edit.
Then click on Settings Enhanced IM.
Next click on Enhanced IM.
Then click on Audio Video.
The Audio and Video portion has drop downs so you can select the proper audio card and video camera installed in you computer.
The Audi Video drop downs are
Microphone
Speaker
Camera
The audio and camera should function now.
Additional information to use with VIM settings.
Adjust Your Audio and Video Settings:
In order to adjust your audio and video settings, please follow the steps given below:
1. Click the Audio and Video Settings button directly below the My Buddy or My Camera window. If you are currently
Participating in an AOL Live Video IM session, a message will display: You must disconnect the current AOL Live Video IM
conversation before running setup. Would you like to disconnect now? Click the Yes button.
2. The Audio and Video Tuning Wizard will appear to help you verify that your camera, speakers and microphone are working
Properly. Follow the on-screen prompts.
Notes:
The first time you use AOL Live Video IM, a message will display: Would you like to first check your audio and video settings? Click the Yes button.
The Audio and Video Tuning Wizard will appear to help you verify that your camera, speakers and microphone are working properly.
Follow the on-screen prompts.
Images in AOL Live Video IM may appear reversed, or as a mirror image (text may appear backwards) when you look at your own camera feed or at the
Picture-in-picture (PIP) display.
After making the settings your ready for a chat and video talk with your Family and Friends. Hope this helps. Bud