I’ve been using my Garmin Edge 305 for over two years and am pretty happy with it. Unfortunately though, it seems just like with many other electronic gadgets these days, two years is about the time at which things start to fail. One doesn’t have to be entirely paranoid to assume that they may be just designed that way. Anyways, the first symptom was the device randomly shutting off during bumpy sections on my road bike - looked like a case of ‘battery bounce’. This got gradually worse and worse, to the point at which the slightest bump on my (suspended) mountain bike would kill it; it just wasn’t usable anymore. The thing was long past warranty and I didn’t feel like forking out Garmin’s $100 flat rate for repair - so it was time for some surgery. It’s always fun to try fix things yourself.
A quick round of Googling showed that I wasn’t the only one with this problem, and soon I ran into this helpful thread on one of the Motionbased forums, from which I came up with this plan of attack:
Open up the device; the case consists of two halves which are glued together. You basically have to pry open the rear black section from the front section. Important: a rubber strip runs along the side of the device and covers the switches; it has been molded onto the gray front part and is to be permanently attached to it. The seam which has to be pried open is between the rubber strip and the black rear part, NOT between the rubber and the gray front part. You can use your nails or a spatula, see the picture below (all the photos below are linked to higher resolution images btw, click on them to see these).
The adhesive will slowly come off (and make a bit of a mess), a gap will open up and at some point you’ll be able to lift the black cover off. As usual with these things, don’t force it or you may break stuff.
With some patience, you’ll be able to separate the two halves.
The random shutdown problem is most likely caused by the spring connector (the 8 gold coated pins on the bottom left of the top part, which contact gold coated pads on the bottom part, see image below). When the device is closed up, the leads of the battery (in the top half) run through this connector to the GPS board in the bottom half; the other contacts of the connector contact the mini USB port. The little springs (see pic below) only create a good electrical contact if they’re sufficiently compressed. 
And that’s the heart of the problem: the compression of the springs is determined by the gap between the two halfs. The contact pads on the bottom half sit on a small piece of PCB, onto which the external USB port is directly mounted (see pic).
A spacer underneath the small PCB defines the gap (see the profile shot below) and it is the adhesive force of the glue that holds the two parts together.
So, after numerous cycles of plugging in and out a USB cable and applying significant forces on this piece of PCB, it is not hard to imagine that it can get somewhat wedged loose over time and as a result the compression of the springs decreases or fluctuates, something which only will be aggravated when you have the device mounted on your handlebar during a bumpy ride. The intermittent contact then leads to the device shutting down.
Basically, it’s a design error with respect to strain relief and could have been avoided by not having the USB port directly mounted onto the piece with the contact pads for the springs.
In order to fix the problem and make the connector more robust for a hopefully long future use I decided to combine two fixes mentioned in the Motionbased forum thread: hardwire the battery leads to the GPS board, and add a spacer to the small PCB with the USB port.
First though, you want to properly clean all contacting surfaces to make sure there’s no dirt or other contamination creating trouble - you can use for instance DeoxIT contact cleaner for this - check out the macro-photo I took of the connector tips: it’s easy to see that some dirt on those tips can become an issue.
Detach the small PCB to expose the battery leads - it’s kept in place by two screws on the sides.
Next, solder a wire from each battery lead to the GPS board - this requires some care and a steady hand, but it isn’t that hard. A good type of wire to use here is magnet wire - thin, plastically deformable wire that has an insulating coating on it (and is typically used for coil spools). Because it keeps its shape when you deform it and the thin wire is very light, it won’t move around too much inside the device during use after you’ve closed it up again, and the solder joints shouldn’t come under any significant stress.
The picture below shows where I soldered the wires at the battery/USB connector side (and is also a testament of my sub-par but in this case sufficient soldering skills).
Soldering a wire from the battery leads to the board will pretty much eliminate the battery bounce effect during rides. But to ensure the contacts to the USB port (which you need to download data or recharge the battery) remain in good shape, the additional spacer comes in - this will basically compress the little springs a bit more and create a more robust electrical contact. I took a thin piece of rubber with adhesive on one side (the type you can buy to cut out for instance rubber feet to glue on small furniture or equipment) and cut out a piece that is pretty much identical in shape to the original spacer, then placed it on top of it.
Then put both spacers on the small PCB and screw it back in place. The picture below shows how it goes together. It also shows the contacts on the GPS board where you need to solder the other ends of the battery wires to (as always, be careful when doing this - you don’t want to smolder components or splash solder all over the place).
The trick then is to nicely wrap the extra wires you’ve put in there alongside the board in such way that you avoid them touching the spring connector or getting squeezed when you flip the two halves of the device back together. Practice this a few times, because in the final step you’ll need to do it with glue on the case.
When you feel comfortable with this, it’s time to put new adhesive on (of course, you’ve already scraped the old one off as well as you could). I used some ‘Black Max’ Loctite (see picture) that I applied on the edges of the black rear cover - this adhesive works well with rubber and plastic.
Move both parts now gently together, making sure the wires sit nicely in place and out of the way and being extra careful with the area that contains the spring connector. When the two parts are locked back together, put a weight on the device (see picture) and let the adhesive cure. You want to use this weight and apply a uniform force in order to minimize any gap between the two parts (remember this affects the spring compression and also the operation of the Start/Stop and Lap buttons).
Fifteen minutes later, take off the weight and power on the GPS! Check whether the USB port works as well (you could also do this before applying the adhesive by clamping the halves together and gently plugging in the USB cable in the port.
If all went well, it will stay on, including during the roughest bumpiest rides you can find. (If it doesn’t power on, not all is lost: go back to start - the Loctite adhesive is removable just like the original adhesive). I’ve done this fix a few months ago, and my Edge was working almost like new again - and as to date, it still is.
