The auto focus will not work on my EOS 400D lense. ,I dropped the camera. The tele-scope lense works fine. I cleaned the contacts as suggested with swab & alcohol

Sorry I only have a such manual, but on the drawing it says Focusinstellingsknop (pag. 30).
That switch, on the lens, should on AF and not on MF. Did you check that one?
All EOS lenses do have such a switch.

Your 100mm lens autofocus not working.it may be due to autofocus flex cable inside the lens damaged. It is replaceable part. You may have to contact service technician. However when you switch to manual mode it should focus & click. Try it.

You have not supplied enough information, however with the Sigma lenses here are a few things that could be a problem.

Is the switch from auto to manual focus set to auto focus.

Is the lens the proper mount for the camera it is installed on (meaning have you used an adapter to go from a Nikon to a Canon mount?) if so then there will be no electronic communication and the lens will only function in manual focus mode.

Is the lens compatible with the camera model? (meaning some newer cameras set the lens aperture in the camera while some older lenses set the aperture on the lens.)

Part 1 of 2 Have you installed an older "film" lens on a newer version digital camera?

Part 2 of 2 Have you installed a digital full frame lens on a Canon "S" mount with the APS size sensor with the 1.6 focal factor?

Have you changed the camera model to a newer Canon EOS

Have you updated the camera firmware recently?

Have you lightly polished (cleaned) the contacts between the camera and lens?
Have you received an error code 99?
Cleaning the gold contacts on the lens and camera body can be done with a clean pencil eraser and lightly polish the matting surfaces making sure no dust gets into the back of the lens or the camera body. Hold the camera lens mount down while lightly polishing the contacts same with the lens. Do not wipe clean with a facial tissue or use any type of liquid cleaner.

Also third party lenses like Sigma often do not have the compatible electronic "chip" to properly communicate with all Canon EOS cameras. Meaning if you had the lens on a Canon EOS 20D and then put it on a Canon 50D chances are it won't work.

Is the lens properly locked onto the camera body dismount and remount the lens generally solves this problem.

If non of the above isn't the problem then a trip to a repair center is needed.

Try cleaning the contacts on the camera body and lens where they meet with a cotton swab lightly dampened with rubbing alcohol.

Have you actually cleaned the contacts or are you just looking at them and they "look" clean? Clean them with a cotton swab lightly dampened in rubbing alcohol.

Try cleaning the contacts on the camera body and the lens where they meet with a cotton swab lightly dampened with rubbing alcohol.

Sounds like your camera and your lens aren't communicating. Have you tried cleaning the contacts on the camera and the lens mount?

Talked to canon support, they suggested to send lense in for review/repair. They said it could be the motor, I think it was a gear problem based on the noises and jerkiness of the focusing action while in manual focus (manual focus gave out as well). Cost TBD - "would definately be more than $50. I shopped around and elected to buy a new lense rather than try to hassle with the repair, shipping, insurance, and the wait, etc. Besides, I have a deadline that I need the lense for, so repair at Canon really became a non-option. I think the cost would've been pretty much the same in the end as the new lense, since this wasn't one of the costlier of lenses to begin with.

if it is an auto focus lens that works on one eos camera, it should work on almost all of them. the only known exception is that some high end extenders will not fit the eos1dmarkII as it protrudes into the opening too far.
but a good rule is that if it fits without any obstruction issues, it should work. try cleaning contacts on the lens and camera body. if that doesn't fix it, talk to Tamron. you can use it manually, but will not be as easy and you would get less predictable results.

my suggestion is to always go with lens from camera maker.

sounds like there is an issue with the tamron lens. not the camera.
good luck
mark

This is a camera body contacts problem Canon Customer support will advise what to do next