AF issues can someone please help

I've worked on a number of lenses from this line of Canon lens including some with AF issues. Unfortunately I can't give you the best advice as AF issues can be complex and it's something I'm still learning on how to fix. I'll try to help you the best I can though. Here's a couple things you can try to further diagnose the issue:

• If you haven't already, try the lens on a different camera or try a different lens on that camera, to rule out the possibility that the camera is the problem (it likely isn't though, but worth checking).
• Check the AF at different focal lengths. Basically you want to try to see if there are certain focal lengths (or ranges of focal lengths) that the AF does work normally at.
  • If there are, let me know (as there could then be a very specific issue causing the AF problems)

Otherwise, the problem likely lies with the autofocus assembly and some of the issues that can occur include:

• The USM motor is no longer good
  • Overtime they can become corroded and the resistance becomes too much for them to function properly.
• Improper motor ring tension
  • There's a ring that provides tension to the USM motor and can lose tension over time.
• Damaged flex cables
• Damaged/blocked AF position sensor
  • This sensor detects the position of the focus ring and can either become damaged or sometimes clogged with excess grease.

These individual issues can potentially be diagnosed and repaired, but without the knowledge on how to do so, it would probably be best to replace the entire autofocus assembly. Below is an image of an autofocus assembly that's very similar to the one in this lens.

Your main two options here is either to just use the lens as a manual focus lens or attempt to replace the autofocus assembly, which a replacement AF assembly would cost you as low as $35 (based on what I've found on eBay).

If you do want to try replacing the AF assembly, go into it with the hope that you'll be able to fix your lens, but the expectation that you won't. I say this because (assuming you haven't messed around with lenses before) there is a learning curve to working on lenses and while starting out there is the risk of accidentally damaging something while trying to repair it. Also, according to service manuals, you're also supposed to make some adjustments to ensure the AF assembly is working perfectly (which is a pretty advanced thing to do). The replacement AF assembly might work perfectly fine without this adjustment though, or potentially would only have a minor impact on its performance. I don't have enough experience with replacing AF assemblies to say for sure though.

More information if you do decide to attempt a repair: While there isn't a disassembly guide for this specific lens, there is one for the EF-S 17-85mm lens, which should still help since the lens construction is very similar. I've also added a parts catalog to the device page for the lens which provides an exploded view of all the parts and can aid with disassembly and reassembly of the lens. You may also be able to find YouTube videos of people disassembly this specific lens.

Update:

As to what's going on after you changed out the USM motor, I don't think it would be an issue with the motor. It seems more like the lens has an intermittent connection with the camera.

My guess would be the connector on the lens isn't making complete contact with the camera, the cable that attaches the connector to the PCB board is damaged, or that the main PCB board itself is damaged in some way. Check to make sure the lens connector is fully seated as it should be in the lens mount and check to make sure it's cable isn't damaged in any way.