[Android Head-Units] threadreaper:
If you have one of these 8227L units that doesn't have any physical buttons, and you've tried to unlock your bootloader, you may have given up in frustration when the on-screen instructions say, press volume down to continue...

If you have one of the units that does have physical buttons, your process will be similar but much simpler, as you won't have to take your unit apart, so you can follow along with this tutorial after that portion is done.

Disclaimer: If you don't have physical buttons on your unit, you're going to have to dismantle it and potentially do some soldering to get through this. The points in question are tiny and if you permanently short them or apply too much heat and lift a pad, you could get stuck with a unit that's permanently muted or just bricked altogether. You have been warned about the risks, and I am not responsible for any damage you may incur!

Here's how it's done.

Prerequisites:

8227L unit with no physical buttons

Phillips screwdriver

Comfort with soldering on fairly small stuff

(You might get by without having to do any soldering, but if you can solder, you'll have an easier time)

PC of some type, probably a laptop if you want to do this inside a vehicle

adb and fastboot installed - there are plenty of tutorials on this elsewhere on these forums

It is possible to root these things without unlocking the bootloader, so if your device happens to be

rooted already, note that you'll be able to skip the adb portion entirely by installing a reboot utility

from the play store that allows you to just reboot the device to bootloader mode, but you'll still need to

use fastboot over USB.

If you need to bring the unit inside to get to a computer, you'll need a 12v power supply of some type

USB A male to male cable - that's just a cable with a "full size" usb plug on both ends - You can just chop	up two old USB cables and splice them back together by matching the colors of the inner wires to make	your own if need be.

We're going to be disassembling the unit, so obviously you'll want to completely disconnect it first.

On the backside of the unit there is a Phillips head screw at each corner, go ahead and pull those out and put them someplace safe.

Once you remove those screws, the back of the unit is loose, but don't just pull up on it! It's still connected to the board, which is connected to the screen assembly by some fragile ribbon cables, and you don't want to tear those! The cables attach towards the bottom of the screen, so tilt the back of the unit away from the screen top side first, opening it like a book. This will reveal the three ribbon cables we need to disconnect. They connect into three plastic connectors (They're called ZIF or Zero Insertion Force connectors) attached to the board that can also be fragile, so take care with this next step. On the back side of each connector, the side away from the ribbon cable is a little plastic "flap," usually black in color. Take your fingernail or a plastic spudger, or a toothpick or something (non-metallic preferably) and get underneath that and flip it upwards. The flap should stay in the connector and just hinge open. Once this is done, the ribbon cable itself should come out easily. If you have to apply any pressure to get it out, you don't have it unlocked and you will damage something, so make sure you've got it unlocked before you go pulling on the ribbon cables.

With the ribbon cables disconnected, the screen assembly can be set to the side for the time being. We now have access to the front side of the board, but what we need is on the other side, so we'll have to remove it completely from the housing. There will be 4 or 5 more Phillips screws to remove, depending on your model. Once you get those out, you can pull the board free and flip it over. We're looking for 5 small copper pads exposed on the surface of the mainboard, in the area of the CPU, RAM and NAND/EMMC memory chips. I have attached an image that I borrowed from elsewhere on these forums, because I don't have a unit opened up right now and didn't want to take one apart just to take some pictures.

In the image I attached, you're looking for the points marked 1 and 2. The caption on the image suggests they connect to Volume Up, but on every unit I've seen these points connect to Volume Down. Your mileage may vary. The layout of these points can vary from model to model, but they're always in this area. On all the models I have seen, two of these points are slightly larger than the rest, and those are the two we're concerned with for now. You may here this referred to as the "test point," but in fact these points simply expose access to Home, Back, Volume Up and Volume Down. Of the two bigger points, one of them will be ground. (If you know your way around a multimeter you can verify which one by checking for continuity to ground elsewhere on the board, but it's really not important for our current purposes which is which.) The next part will be much, much easier if you can handle soldering on stuff this size. We need to have access to momentarily short the two bigger points. The one that isn't ground is volume down, and making a connection between the two sends a Volume Down key press. (If you have identified which is ground you can access Home, Back and Volume Up by shorting it to the three smaller points) At first, I thought I'd just be able to touch the two points with a

screwdriver to accomplish this, but I never ended up getting that to work. If your unit is a bit different than mine, you may have more luck with that. But in my case, I needed to tin both points (that simply means to apply fresh solder to them) and attach a small wire to one of them. (The jumper wires that come with an Arduino work great for this if you have any of those laying around, otherwise just use a really small wire of some kind.) This jumper wire is going to function as our "volume down" button for the next part. These are also the same points you will need to access if you ever manage to brick your unit and need to get access to the "backup" pre-loader.

Now that we have our "button" in place, we're going to need to connect the ribbon cables for the screen back to the board, power the unit up and get connected via adb. There are two different usb cables that come with the unit. One of them has a 6 pin connector and one has a 4 pin connector. You want to use the one with the 4 pins. At this point you've got to handle the unit with the board exposed, power running through it, and those ribbon cables connected. It should go without saying that you need to be very careful. Don't tear any ribbon cables, and don't let your jumper wire accidentally short out against anything as you turn the unit over to use the screen.

If your unit has a physical volume down button, you can follow along from here and just ignore all the horrific parts about running the unit with the guts hanging out and jumper wires and all that scary stuff. ;)

Once the unit powers on, you'll probably notice that it's not detected by the PC right away. That's because these units default to usb host mode, so they can mount your flash drive or whatever you may store music on and plug into them. So we're going to have to go to the "factory" settings menu. This is the one that asks for a password when you try to go into it. For most units it's 8888, but I have seen a few where the password is 1111. Once you're into the factory settings menu, towards the bottom you should see USB options. Open that menu, and click on "Other" towards the bottom left. On the next screen, tap the "usb mode" option, and you should get a popup where you can toggle between "host" and "device" mode. Device mode may be already selected, but go ahead and click it anyway. After this, you should be able to get a connection over adb. Try entering the command "adb devices" from your command line, and you should see your unit identifed by it's serial number. If it says "offline", go back to the factory settings menu, toggle it to host mode once and then back to device. If you're seeing "unauthorized" there are some pre-requisite steps you're missing. They are covered extensively on these forums and elsewhere, so if you need to find them immediately just search these forums or google for "adb tutorial mediatek device" and you'll find about a dozen walkthroughs. All we need from here is to be able to ender the command:

adb reboot bootloader

Once you get that done, your device should reboot to the boot logo screen, but it will stay on that screen and display the words "FASTBOOT MODE" Back to the command line of your PC, you can enter the command:

fastboot devices

to verify that you have a connection. If you're still with me at this point and you're able to communicate with the bootloader through fastboot, enter the command:

fastboot oem unlock

At this point, the instructions on the screen of your unit will warn you about how Santa Claus won't bring you any presents if you unlock your bootloader, and prompt you to press Volume Down if you really want to proceed. At this point you'll need to use that jumper wire to make the connection we prepared previously to execute the Volume Down entry. You may have to hold it in place for a couple of seconds, but eventually you should get a confirmation that your bootloader has been unlocked! Now, power the unit off, disconnect your jumper wire, and reverse the disassembly process to put it back together. One thing to keep in mind, if you're attempting to flash a new firmware and it comes with a locked bootloader, you'll have to repeat this proces, so if you can get away with it, don't flash the LK partition.