The secondary mirror is attached to the bottom of the main mirror. Its job is to bounce the IR from the auto focus unit to the object and back into the camera.
When this failed, the first thing I noticed was the auto-focus stopped working. The motors would spin, but it wouldn’t lock. Being on a shoot when it happened, I just switched to manual focus and was good for another 25 frames or so… then it totally failed. (note: this is a different body than the one I had Canon do a shutter replacement on)
The secondary became unable to lift itself out of the way when the primary mirror went up. Consequently, half of my image was black.
I follow the process described at http://www.abo.fi/~jskata/300Drepair/
While the procedure fixed the problem, the tear down lacked a good many steps. Because of this and my lazyness in the disassembly, I forced one of the components out, and tore a ribbon cable.
Canon was really cool about sending me a replacement part ($35). As long as you have the actual canon part number (see link below) I imagine that the parts counter will be helpful to your adventures as well.
If you take the time to disassemble the camera so that the flash power supply (the thing with the big capacitor, full of voltage) can be removed properly without any forcing, Jan-Erik Skata's site is a valuable resourse.
I, didn’t follow my own instructions, didn’t take any pictures and screwed it up. The following is the repair odessy that followed. Conceptually, it may help someone in a similar spot.
