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Adding a Remote Trunk Release to a BMW E36 3-Series

ADDING A REMOTE TRUNK RELEASE
TO AN E36 3-SERIES

I have long wanted to add an electric trunk release to my 1996 318is so that I could pop the trunk from the front seat or by remote control. I finally did after helping Ron Stygar add one to his Z3M coupe. With a little planning, I added one to mine without modifying the original sheet metal and interior trim, or disturbing the trunk lock mechanism.
video
My car came with a convenient, console mounted, central locking switch and my remote control had an unused, second-stage unlock feature. Using one of my Mini Automotive (MAC) Controllers, I extended the switch so that it can also pop the trunk, and programmed the second stage unlock to pop the trunk and flash the hazard lights (0.2M video).

What follows is my solution which likely won’t be everyone’s beer. Consider it a series of suggestions and feel free to improve and change things!

POPPING THE TRUNK

actua_installed_bottomThe stock, trunk button ends with a broad, flat tab which the central locking system orients through the long, horizontal rod, and a motor off the left side of the page. The system rotates the tab to six o’clock to enable the trunk button, and to three o’clock to decouple it.

Here, the tab is at six o’clock to allow the trunk button to pop the trunk. Pressing the button moves the tab in the direction of your gut and tips the rocker. The rocker pulls a long, steel rod that runs up to the trunk latch. It is just visible inside the white tubing. The rod tugs on the trunk latch (off the top of the page) and pops the trunk.

I added the black, gun-style, linear actuator at the right, an aluminum catch plate and the blue and white, nylon tubing, to remotely pop the trunk. The actuator pulls down on the catch plate, the plate presses on the outer, blue tubing, the tubing tips the rocker and the rocker tugs on the long rod and trunk latch, popping the trunk. I slipped the catch plate and blue, outer tubing over the white, inner tubing to have metal slide over nylon and reduce friction.

Since it directly tips the rocker, pulsing the actuator always pops the trunk – even when the trunk button has been mechanically decoupled from the trunk latch. To prevent theft, I have the MAC controller watch the trunk motor position switch and only pop the trunk when it is unlocked (the tab is at six o’clock).
actua_installed_side
Here, you can see how the actuator, catch plate and tubing tip the rocker, and how the rocker tugs the trunk latch rod rising off to the right. A more traditional, cable-pull, trunk release might have been simpler, and I considered using one. Unfortunately, the coupe trunk lid has several, internal, stiffening ribs and only about 12 cm (4.7 inches) clearance behind the rocker. I couldn’t find a good spot to mount a solenoid. I am not very mechanically inclined, however!

Note how I slit the white tubing in this picture and the previous one. More anon.

MOUNTING THE ACTUATOR

mounting_holesI used the two empty holes warning labelin the trunk lid sheet metal at 3 and 5 o'clock from the rocker, to mount the actuator and keep it from twisting. The rear, 5 o'clock hole is some distance away, but perhaps that is just as well. The sheet metal below the rear of the actuator falls away and might have been a little trouble to fasten to (probably just a little). The yellow label warns people not to overtighten the rear cap screw.
rear_bridge
To jump the long, vertical rib that runs between the holes, I tied the top, rear of the actuator to the rear hole through a bridge made from 2mm, aluminum angle stock.
actuator+brackets back
I tapped the mounting bridge foot to accept an M6 screw and took off the foot’s outer corner so that I could maneuver the actuator assembly inside the trunk lid. I broke all corners and edges to avoid scratching the inside.

The right edge of the mounting foot ends up next to the long, vertical rib.
actuator+brackets front
I slotted a piece of 2mm angle stock and used it as a catch plate. The slot lets the tubing sink as the actuator pulls at an angle toward the rocker. (See the second picture at the top of the page.) I beveled the slot edges to keep the plate from digging into the tubing.

I cut an open slot and split the inner and outer tubing (as you saw in the first two pictures) to slip the tubing around the trunk latch rod, and the actuator assembly over the tubing, without disturbing the trunk lock mechanism.

ADDING AND WIRING THE ELECTRONICS

controllerI programmed one of my Mini Automotive (MAC) Controllers to service the hazard lights, trunk release actuator, central locking switch and remote control, second stage, unlock output. I arranged that:
Pressing and holding the switch is faster and more convenient than reaching for another, near the hood release, as you do on newer cars.

I installed the controller behind the middle console and plugged it into the switch and the central module (ZKE) through a T-cable.

As you can see, this was not my first project behind the middle console.

The MAC controller draws power, monitors the switch and pulses the ZKE central locking input through the T-cable at the top of the schematic:
schematic
x188 speed signal
The controller monitors the instrument cluster road speed signal (schematic, middle left) and won’t pop the trunk while the car is moving, for example, on the Autobahn. I tapped into the signal at comb terminal junction X188 in the terminal block on the left wall of the driver’s footwell.

To prevent theft, the controller also monitors the trunk unlocked signal (schematic, lower left) and won’t let the central locking switch pop the trunk when it is locked.
trunk unlocked signal
I tapped into the signal near the left, rear tail light as I had already disassembled that corner to replace the trunk to trunk lid cable. I had also laid some spare circuits between the trunk and dash when I last pulled up the carpet. If I hadn’t, I would have probably tapped in behind the glovebox, near ZKE connector X13253, pin 3.x13025

The controller pulses the trunk release actuator (schematic, lower right) over one of the circuits I had laid between the dash and trunk, and a spare, violet wire I had added to the trunk to trunk lid cable.

Finally, the controller flashes the hazard lights through the BMW/Alpine alarm connector, X13025, behind the glovebox (schematic, middle right). I added a comb terminal junction to the nearby terminal block. I tied the controller to the junction and the junction to X13025, pin 8 so that I can splice in other controllers (e.g. my "ZZKE") and also have them flash the hazard lights.

CLOSING THOUGHTS

A remote trunk release is a greater creature comfort than one might expect. It is quite convenient to have a single, central locking switch unlock the car, pop the trunk or unlock the car and pop the trunk. We plan to retrofit this nicety to newer cars with harder to reach trunk releases.
Copyright © 2007 by John Firestone