Canon 24-105 f/4 L IS USM Lens Repair

After looking for some information on line about how to repair this lens, I didn’t find a good disassembly guide, so I thought I’d write some instructions.  Apologies for the standard of the photos – if you take some better ones, please share them with me and I’ll update the article.
This lens is designed with a deliberate weak point at the rear of the lens which is designed to break off to prevent more expensive damage.  The one I got from eBay had been bodged after this had happened and was sold as new (scumbag!)  You can see the the 4 plastic lugs around the circuit board in the pictures – these are designed to break off if damaged, and the front of the lens will partly separate from the rear if this happens.   The replacement part is about £90 and I was quoted £120 for fitting.  Since by this point I’d already stripped the thing down, I opted for them to ship me the part.   The replacement fixed barrel assembly includes the switch pack and the zoom ring (see notes at the end on removing the zoom ring if you are going down the ’spares for repair’ route).
Tools needed: #00 philips screwdriver.   Two very small straight screwdrivers
Blunt ended tweezers
Soft plastic pick
A rubber band.  No, really.
Before you start, a word of warning.  Some of these screws will be very small and very tight.  You will need a precision screwdriver and even with that you run the risk of rounding off the heads.  If this starts to happen, you can use this tip to try and get the screws out.  If that doesn’t work, give up, send it off, and know you will pay extra for getting the damaged screws out. 
I also strongly recommend working in some kind of tray, and in bright light.  The tray is in case you drop a screw!
1) Put the lens cap on the front.  You’re not going there, and you don’t want to damage anything.   
2) Before you go anywhere, check the heads of the 4 screws.  If they are damaged at all, order some new screws when you order the assembly.  Extend the lens and pull out the rectangular cover in the centre by hooking a finger behind and pulling.  It will seem to need inordinate force, but does just pull out.  The body contacts will remain in place.  This is shown with the green circle below.
3) Remove the two tiny screws holding the contacts in place.  These really, really are tiny.  
Lens small
Don’t drop them.
4) Undo the 4 screws through the mount and remove the mount, water seal, and rear cover.   These are shown in red on the top image. 
This is what you will see. 
5) Work your way around the connectors.  There are three types:   Flip up catch, pull out catch, and one that’s a push fit, best done with a couple of small screwdrivers.   The flip up catches are highlighted above.   For these gently flip up the coloured piece and the cable will lift clear.   For the ribbon cable top right you will need to push against the tabs on the table to pull it out of the socket.  This is the only one that should have any resistance.  The final ones need you to pull forward the clips at the side very gently to release the cable. 
6) Remove the 1 screw holding the circuit board in place (orange arrow). You can then lift it clear. 
7) There are 4 screws holding the rear fixed barrel to the lens.  Remove all 4 noting positions, and also that one of them holds one of the ribbon connectors in place.  Do not touch the screws on the inner most ring of the assembly. 
8) Carefully slide off the rear barrel making sure you don’t trap any of the ribbon cables.   Note alignment to main lens body.
To reassemble, you will need to line things up – it’s not quite the reverse of disassembly, but isn’t far off. 
1) Gently pull the front of the lens out to 105mm, and set the zoom ring on the new barrel to 105mm.  
2) Check the ribbon cables on the new barrel are routed the same as the old one, then lower into place, lining up the window in the barrel with the focus distance scale. Use a plastic hook of some sort to make sure all eight ribbon cables come up into the rear of the lens. Be gentle. As the barrel drops into place, you need to engage the mechanism to drive the zoom, and with a little juggling of the zoom ring and the lens itself, the barrel should drop fully into place.    Make sure all ribbon cables are clear and not being crushed, then replace the 4 screws holding the barrel in place. 
3) The next trick is to get the circuit board back in the right place. All 8 ribbon cables go around the outside, so if this isn’t the case you’ve routed something wrong.  Look at the image above. 
4) Line up the biggest connector (at the top in the diagram above) and screw the board into place.
5) Refit all the other cables with reference to the fastener mechanisms.  They will pretty naturally fall in the right place apart from the two new ones which will point up more.  This is now a good point to get all the greasy finger prints off the rear element. 
6) Separate the rear cover, mount ring and weather seal.  The rear cover will go on in two orientations, but only one will leave the screw holes in the right place to re-attach the camera connector.   The words “image stabilizer” should line up with the 24 number on the zoom ring with the lens at 24mm (or 105 at full extension).
Lens rear cover
7) Place the weather seal on the rear cover, and sit the mount on top, being careful to line things up so you don’t have to rotate it to get the screws in.  If you rotate them the seal will get caught up and not sit right.
8) Insert the 4 screws holding the cover and the two tiny ones to locate the camera connector.  Make sure all 4 main screws are in tight and not sitting proud or they will foul on the camera mount when you attach the lens.  I’d test at this point.
9) Replace the inner cover (just snaps into place)
Further disassembly of the old barrel:
1) Carefully prise away the ribbon cable leading to the switch unit where it’s held to the inside of the barrel.  Undo the screw on the outside and prise off.
2) Undo the two screws holding on the level that engages in the zoom mechanism.  Twist past the end stop and lift off the zoom ring.  Note alignment for refit.
The zoom ring is hard to get in place without damaging the spring contacts.  I suggest putting a piece of plastic (like flash gel) over the contacts as you slide it back together,  then pull the plastic out.  I’d already damaged the contacts on my old barrel before I figured out how fragile they are. 
Usual caveats apply.  I am not a qualified Canon service engineer and this will definitely void your warranty.  But I figured this out in a couple of hours and saved myself a £120 service cost.
Part Number for the rear barrel is…

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