I just got a used Canon EOS 6D and I have a Tamron AF 28-75mm F/2.8 Macro Lens. What I have noticed is that when I try to take a picture and have the lens switched to AF mode it won't work if I use the viewfinder. If I switch it to manual then I can use the viewfinder. If I switch the camera from viewfinder to live-view then it will let me shoot with the lens AF turned on and also in manual. Can anyone help me with this?
It may be that you are experiencing what is often the downside of using an older third party lens on a newer camera body: AF incompatibility. Lens makers such as Tamron do not license technology from camera makers such as Canon to insure compatibility. Instead they reverse engineer the technology in order to design compatible lenses. When a manufacturer such as Canon then introduces a newer model that was not available when the third party lens was designed they can make subtle changes in the protocol that renders the older third party lens incompatible.
Sometime lens makers will offer firmware updates to allow their older lens designs to be used with newer cameras. I searched for such an announcement form Tamron regarding the 28-75mm f/2.8 but couldn't find anything. Both Sigma and Tamron have introduced USB lens docks that allow the end user to update a lens' firmware without requiring a trip to the nearest service center to be updated. The Sigma dock has been out a couple of years. The Tamron TAP-in connector has been announced with a projected June 8, 2016 release date. These docks make dealing with updating firmware for third party lenses much easier. You're still taking the risk, though, that the lens manufacturer will continue to support the lens and release firmware updates as needed for new camera models.
I'm assuming it is seeking wildly without ever finding focus, right? Are you sure there's enough contrast for phase-detect autofocus to work reliably? Beyond that, it is possible that the lens is decentered or that it just has issues with the AF system in that particular camera.
Regardless of the underlying cause, it is probably worth telling the 6D to use only the center AF point to see if that fixes the problem. In addition to being the most sensitive in low light, the center point is also the most accurate and least easily confused. I've often seen the 6D's outer points result in the camera failing to focus with certain lenses in certain environments. Telling the 6D to ignore everything but the center point usually fixes that problem. I pretty much leave mine set that way most of the time.