Introduction: External Boot Release

About: Hi all! I'm an engineer-in-training based in Melbourne, Australia. I love taking stuff apart to see how they work, as well as hacking stuff to make it work even better! Hope you like and enjoy the stuff I make…

Whilst the Subaru Liberty is an awesome car the lack of an external boot release button on the sedan is really annoying. The boot can be opened from the remote via the central locking system, but this requires pressing and holding a button which isn’t the easiest thing to do when your hands are full. So, after a bit of work I was able to add an external boot release button to my car in a way that is both simple enough as well as safe enough to have running all the time and with no additional power draw (except when unlocking).

Step 1: Materials and Tools


· Trim removal tools and other tools to remove the panels of your car (e.g. Socket sets, screwdrivers)

· Multimeter

· Pliers

· Wire cutters

· Wire Strippers

· Drill and drill bits (step bits are useful)

· Soldering iron with solder

· Hot glue gun with hot glue

· Blowtorch/heat gun to shrink heatshrink tubing


· Prototyping board

· Mechanically latching relay (eg. Jaycar Cat. No. SY4060)

· Locking/latching polarised 2 pin connectors (e.g. JST-XH latching PCB mount connectors)

· Wire (in decent lengths)

· Momentary pushbutton switch (eg. Jaycar Cat. No. SP0656) This switch must be weatherproof and must be rated for at least 3 amps, otherwise a lower rated pushbutton can be used to switch an additional SPST relay (eg. Jaycar Cat. No. SY4066)

· Standard diode rated at least 3 amps

· Electrical tape

· Cable ties

· Electronic boot release (if your vehicle doesn't come with one already)

Step 2: Make Front Relay Board

The first thing before even starting this project is to ensure the front door locks behave in a way that lets this project work. Find the door loom wires (often hidden under front trim or even door sill trim) and then find the door lock actuator wires. Its easier to use a wiring diagram, which can normally be found free online, rather than probing wires. Using a multimeter, probe the two wires and ensure that when the car is locked the polarity of the pulse going to the actuator is the opposite polarity of the pulse when the car is unlocked. This is the way most door lock actuators work, so there is a good chance your car is suitable for this modification.

If this is the case we are ready to make the relay board. This is simply to make connecting wires to the relay easier, as well as ensuring the relay can be unplugged later for troubleshooting or to revert your vehicle to stock.

The schematic should be easy to follow, with one connector going straight to one of the relay coils, and the other connector going to the common and normally open contacts on one side of the relay. Finally, the prototyping board can be trimmed to size and then wrapped with heatshrink to insulate it. Make sure you mark the connectors to ensure you remember which one is the relay coil and which is the switched contacts!

Step 3: Connect Relay Board

Cut a short length of wire and crimp on the JST-XH plug for the relay coil. For testing, twist the coil wires onto the door lock actuator wires. Using the car’s key fob, cycle the door locks. This will reset the state of the latching relay.

Next, unlock the car and measure the relay contacts at the connector using a multimeter set to continuity. If these are electrically connected, congratulations you got it right the first time! If not, however, change the polarity of the coil wires where they connect to the door lock actuator wires. Cycle the door locks again and then remeasure the connector. It will now be correctly set with the position of the door locks.

Next, lock the car and measure the contacts again. They should not be electrically connected to each other. The last part is to solder the coil wires to the door lock wires. Ensure you insulate these properly with electrical tape- otherwise your doors will no longer lock or unlock!

Step 4: Wire in Constant 12V

The relay board requires a source of 12V direct from the battery. This is because we want to be able to open the boot even if the key is not in the ignition. A constant on 12V can be run direct form the battery but this is overkill for this project; it is much easier to try and find this behind the internal fuse box.

This can be wired into one of the relay contacts via the remaining connector. A relatively high gauge wire is recommended as the boot release motor draws over 2.8 amps in my Subaru, and a high gauge wire reduces the loss to resistance.

Step 5: Running Wire

To actuate the boot release a wire needs to go from the remaining relay contact at the front of the car to the boot itself. The neatest way of doing this it to run the wire under either the carpet or the door sills- the layout of my Subaru meant it was much simpler to run it under the sills with the rest of the loom.

To get the wire into the back of the boot depends a lot on the vehicle you are installing this into- in my case I had to remove half of the boot liner as well as all the trim on the boot lid itself. The wire can then be run into the boot under the hinge trim alongside the rest of the boot lid wiring.

Step 6: Mounting Switch

The pushbutton switch will be used to force the boot release actuator to open the boot. This means the switch must be able to handle the roughly 2.8 amps that the actuator requires. The only low profile momentary switches that are also weatherproof the I could find could only handle a few hundred milliamps. To prevent the switch contacts from wearing out prematurely over time and possibly causing the boot to open when it is undesired, the switch will instead be used to switch a normal relay whose high current contacts will be used to unlock the boot.

The switch needs to be mounted somewhere where water cannot sit- therefore it makes the most sense to mount the switch with the button facing down. The best place for this in the Subaru is the plate which the number plate illumination lights mount to.

In order to mount the switch the plate needs to be removed. In the Subaru this required the external boot trim, along with the number plate, to be removed, then the plate could be easily pulled out. Next, mark out a suitable position and then drill the mounting hole. Measure out a length of wire that can easily make the distance between the switch itself and a suitable mounting position for the relay, as well as enough length to allow for cable management, then solder this to the switch. Insulate the switch terminals with heatshrink if there is space for it, otherwise use hot glue and electrical tape to ensure the switch can’t short to the metal boot lid.

Reassemble the panel and reinstall any external trim pieces.

Step 7: Wire in Rear Relay

Now that the switch is installed we can wire it to the relay. The schematic again is very simple to follow and I did not bother with any prototyping board here as the connections are really simple. The wire from the front relay board gets connected to both one terminal of the relay coil and to the common terminal of the relay contacts.

Next, find a good quality ground connection. Either the number plate illumination or existing ground of the boot release actuator are a good point to track it down from. Don’t use the metal of the boot lid if you are adding a new electronic boot release as the boot lid is only connected to the chassis through a greased hinge which will not be perfectly electrically conductive.

Wire one of the switch terminals to ground, and the other switch terminal to the remaining connection point of the relay coil. Making sure the car is unlocked, test by pushing the switch which should cause the relay to activate while the button is pressed.

The diode between the relay coil isn't necessarily required, however it quenches the high voltage spike you get from turning off the inductive load of the relay coil. This spike can damage the switch contacts over time. Whilst not shown I did add it before heat shrinking in the next step.

Step 8: Wire to Boot Release

Following the schematic for the rear relay again, it can now be wired up to the boot release actuator. Firstly, cut the wire that goes for the central locking unit to the boot release. Most boot releases will have 3 wires if the boot has internal lighting- a ground, a source and a switch which will turn on the light. To determine which is which a wiring diagram is useful, but failing that use your multimeter, ground one probe and test each wire by activating the boot release.

Connect the boot release actuator's source wire to the normally open contact of the rear relay. Ensuring the car is unlocked, test again by pushing the button. You should hear the boot release actuator fire.

To retain the original boot release functionality the original wire from the central locking unit can be connected back to the boot release through a diode. The diode prevents power from being sent back into the central locking unit when the button is pressed.

After everything is wired up properly and insulated the boot release button can be tested. Firstly, use the original boot release to ensure the boot will still unlock. Then, with the boot still open if possible, lock the car and test the button. The boot release should not fire. Then, unlock the car and test again. The boot release should now open.

Step 9: Reassembly

The final step is to reassemble all the interior trims. Use cable ties to get the new wires out of the way of any moving parts and ensure there are no bare wires left anywhere. Pay attention above the boot lid liner ensuring the relay has been wrapped in electrical tape and heatshrink and that there are no exposed connections anywhere.

When everything is reassembled enjoy your new external boot release!