im3crazy
im3crazy

Remote Trunk Release Using Off-the-shelf parts

I reviewed a number of articles on the forum and the web to see how others approached releasing a “bear claw” latch. There were many fine approaches. There seem to be two reasonable ways to release the trunk latch: (1) using a motor actuator, or (2) a brute-force solenoid.

Although it used a lot of power, the solenoid seemed more reliable in the long term than the motor. I had just installed motors in the doors, and I had my doubts on their work-life. Using the solenoid approach also made sense because I already had 30 amp capacity in the trunk feeding my stereo amplifiers. I used a heavy duty 30 amp double pole relay to release the solenoid. A #8 wire feeds the trunk from a 30 amp central fuse box directly from the battery for upgraded accessories, and a #10 wire fused at 20 amps serves the release relay and is fused at 20 amp slo-blow- absolutely necessary for the solenoid draw.

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Before I purchased the solenoid I used a spring scale attached to the trunk release lever to determine how many foot-pounds would be necessary to release the trunk. If I recall correctly it was approximately 45 pounds, quite a shock.

I found a supplier online from which I purchased my first solenoid that had this rating. I quickly found out that the rating was cited for the end of the stroke, and not the start to stroke- major difference! I located another provider that had a solenoid that appeared better mechanically and electrically more capable and I purchased the device for about $45.

One of the first challenges you have with a solenoid is both the start pull capacity as well as the stroke length. I determined that I needed a minimum of a three-quarter inch stroke at the 45-50 lb rating. My second candidate worked well.

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I also determined that two other important factors in making this work: (1) securely mounting the solenoid to assure there was no slack or lost effort, (2) providing A fine adjustment along with a way to absorb the massive shock of the instantaneous pull of this big solenoid.

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To accomplish this I used a “10 lb” 4” spring, a rubber grommet, and a turnbuckle. This allowed me to make the adjustments necessary to fine-tune the assembly, and also to absorb unnecessary shock to the release lever. Please check out the pictures and the accompanying text.

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The tension spring is attached at the existing trunk release lever base just below the latch return spring. This thing really needs a lot of energy to release!

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A long-throw alarm switch was installed so it could be easily adjusted to determine if the trunk lid was attacked and pried open.

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I hope this approach might be useful to someone contemplating the same.

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User Feedback


The Ebay unit is mine.  It's fully described in earlier posts in the forum, and still available on request here.  It is fully reversible- no holes need to be drilled in your car's sheet metal.

 

Based on my tinkering to get the kit's design completed, I agree with everything the im3crazy says about the design parameters.  The design looks well thought out, and should work fine. In particular, the inclusion of the 4" spring is a very good idea.  An instantaneous 45 lb. pull is a force to be reckoned with.

 

My design is simply a different approach, as I set a groundrule of NO holes in the car's sheetmetal, and I chose the motorized method because it turned out to be more compact. Because motorized actuators generate less force than a similarly sized solenoid and because I wanted to use an available BMW actuator, I include a new (softer) spring to install in your latch mechanism.  BMW's original '02 latch spring is overdone in my opinion, and so far there have been no complaints about trunk lids abruptly opening on their own with the kit installed.

 

Based on the readily available parts, it seems to boil down to:

 

Solenoid= fewer moving parts, more actuation power required, longer overall mechanism 

Motor= more moving parts, less power required, shorter overall mechanism 

 

The motor draws about 3 amps.  So far, I've found very few bad motor actuators in my salvage yard scavenging, and have had no complaints from kit buyers about the motor (or any other part, for that matter).  BMW uses German built motor actuators from both VDO and Kiekert, and they are functionally equivalent for this application and both are well made.  I provide both types interchangeably in the kit, and again, there have been no complaints from users.

 

The Alarm sensor switch mentioned in the article for monitoring trunk lid opening could be used with my unit by drilling the same hole in it, instead of in the car's sheetmetal.

 

One tiny hole is optionally drilled for my kit if you want to...   My kit includes a little emergency exit pull handle thingie which is not really part of the trunk opener.  It is highly recommended, however, if you plan on being kidnapped in the trunk of your own car.   If you want to install the emergency exit feature you will need to drill one tiny hole in your latch mechanism while it's out of the car being de-gunked.

 

De-gunked?  No matter what method you choose, it is highly recommended that you take the latch mechanism out and give it a thorough cleaning and light oiling prior to automating your opening.  The force needed to work the latch is definitely less if your latch is working correctly without gunk and grime fouling up the works.

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