Remote Trunk Lid Release Design and Construction

[calw]

Over the Holidays, I found myself with some spare time but a very cold workshop. So what could I do INSTEAD of actually doing hard work on the 1600 restoration? How about an accessory… A Remote Trunk Lid Release. This very long post describes the effort.

Over time I have accumulated a collection of motorized actuators, mostly from BMW’s- trunk lids, doors, and fuel doors, I think(the exact origin of a few is) obscure. In the back of my mind, the idea was to build a remote trunk lid release. I have found it to be a very useful function in other years, and I remember always fumbling for the “other” key during the time the 1600 was on the road prior to the restoration start.

An objective of this project was that there would be NO modification to the car's sheet metal or other original parts, and it should be easily removable in case something went wrong with the design. Another goal of the project was to stay with BMW OEM parts if possible. Finally, it had to be CHEAP- as much as possible from my existing parts collection or the junkyard.

Well, almost all of the above turned out to be possible. The one necessary modification to original parts is that the latch spring has to be replaced with a slightly weaker spring in order that the available actuator has enough force to function. Since the original spring is teutonically strong, this does not seem to be a problem. I say "seem" as my car is not yet on the road. I can confirm that on the workbench, no (non-destructive) amount of banging in any direction with a rubber mallet will cause the latch to open. The design of the trunk latch is such that a failure of the spring would result in the trunk opening. I cannot think of a problem where it would be stuck shut if something went wrong with the added mechanism- the original lock mechanism remains intact.

Just in case, I plan on carrying the original spring and a 10mm wrench in my tool kit once the car is on the road. Feedback from anyone who tries this project would be appreciated.

The Design:

Basically, an additional way of opening the trunk is created, by the addition of an electro-mechanical actuator mechanism alongside the original trunk latch mechanism on the back wall of the trunk. Nothing is changed on the trunk lid itself, or the lock mechanism, other than the spring. I took home a small selection of springs from the local hardware store’s little drawers and used one what worked for the actuator but still was hard to actuate by hand.

The steel bracket that holds this mechanism is designed to mount using the three original bolts used to mount the trunk latch mechanism. It is just a piece of 20 gauge mild steel 4” x 8” with one 90 degree bend. I didn’t include dimensions here because I never took detailed mesasurements. I just worked my way through a few tries until it worked ! Installation takes a bit of dexterity, but so does installing the trunk latch on a freshly painted car without scratches!

The actuator is attached to the original lock mechanism with a small key ring style part, also from the hardware store. It is about ½” in diameter but the dimensions are not critical. Note that it attaches around the latch below the spring so that the spring keeps it in place.

The Actuator:

You can pay the best part of $100 on Ebay, or more than by buying a new part from BMW, but I got my actuator from the local pull-it-yourself Junk Yard. You can plan on paying $ 5-15 if price ranges are the same in your area. You may need to make a few trips or a few stops depending on how many BMW’s you find in a normal trip to the junkyard. Bring along a Torx T-20 driver and other regular small hand tools, and a pair of wire cutters so that you can get the matching connector to splice into your wiring when installing.

There are at least two different BMW trunk lid actuators which will work with this design as shown here. Both were obtained from E36 models with a conventional trunk lid- a two door 325 and a 4 door 318. The two examples shown in the photo were made by VDO and KIEKERT, and the part numbers stamped on them are different from what you see in REALOEM. (Use the photo to understand what’s needed, and plan on drilling the mounting holes after finding the actual part you will use). On an E36, the actuator is in the trunk lid and is held in with two small screws with T-20 Torx heads- bring your Torx driver to the junkyard.

The design of these actuators is such that they parts inside are bidirectional, (pulling or pushing depending on the polarity of the 12 volt power applied). This design uses the actuator in the "factory standard" mode of pulling. The actuators have a second connector which is wired internally to a set of position-sensing contacts, but I have not used that feature here. Since the contacts sense the position of the actuator and NOT the latch or trunk lid, you should probably forget about using them to turn on a trunk light or other gizmos.

Side note: I have also seen references that the E36 door lock actuators and fuel door lock actuators are similar. I have not looked at those in person yet so this is not confirmed. If the little plastic end used in the trunk lid actuator is interchangeable with the end used in these other applications (and actually available), and the actuator is of the same size and shape, I'd guess that those parts would work also. I have also seen E34 trunk lid actuators made by HELLA which are of a different style but look like they could fit dimensionally with just mounting changes. I have not tested to see if their pulling power is sufficient in this application. That same basic design was used in E30's, for the fuel door lock, but with a different end on the actuator rod for the mechanical connection. Again, the business end of the actuator matters as you have to hook it to the trunk lid mechanism. So far, with the exception of the design used in the E23, every recent BMW design that I have had the opportunity to check has had a motor-based trunk lid actuator. Although these are more complicated than the solenoid used in some cars, I prefer them because the action seems to be smoother- a solenoid is an instant-on device by design. While roaming the junkyard, I noted that there are some AUDI and VW parts that look very similar and from the same German manufacturers. Odds are that you could make one of these work, but I have not tried. Japanese and American vehicles that I examined have completely different actuator shapes, and some are obviously solenoid based.

Emergency Escape Handle:

Although I don’t expect to be kidnapped in the trunk of my own car, it was easy to add in the feature. If you ever get locked in the trunk while adjusting the trunk hinges from inside, just move the little glow-in-the-dark handle and the trunk pops open. Beginning with 2002 models, all cars sold in the US have had this feature, why not add it to your '02? The problem here is that the only suitable handle that I could find in the local junkyards was from a Mitsubishi. So the design also includes a JDM part, sorry! This same part was seen in several models. I don’t keep up on Mitsubishi models and can’t recall, but remember 2002 or newer Mitsubishi with a regular trunk when you go to the junkyard, and it will probably be there. The handle is used in those cars with a small cable held in by a pin, all of which I removed before drilling the pivot hole in the plastic that fits over the 10/32 pivot bolt seen in the photos. Under both the emergency handle and the actuator is a piece of HDPE plastic that I had in the parts bin, about ¼” thick. There is no reason that some other permanent material wouldn’t work as this spacer, though. The pivot bolt is fully threaded (what I had in the drawer), but the number to times it will be used in real life that the threads won’t have much chance to wear away at the plastic of the handle. The handle is mounted on the shaft with a nut below and stack of washers to get it to the right height, and a self locking nut to hold it all together.

I made my emergency handle a little tight via the self locking nut, to avoid rattles while driving. Again, you won’t be using this every day, so the additional turning force is not important.

Construction:

All of the parts needed for this project are shown in the photo. On the right side of the photo are the original latch, mounting bolts with washers, and the original spring from the latch mechanism, which will not be used but should be saved just in case… In the center bottom are the parts that make up the Emergency Escape Handle. Depending on their thickness, the number of small washers can vary (four are shown in the photo). Near the washers are the bolt, nuts, and lock washers that make up the rest of the handle pivot. The handle, in the center, was modified before the photo. On the white plastic end of the actuator is a ½” diameter key ring which is hard to see. In the lower left corner are the two small Torx headed screws which came with the actuator from the junkyard. Not shown is the wiring connector or any of the wiring that you will do later.

Most of the construction work is in the sheet metal bracket. Note the curved cutout that matches the round hole in the car’s sheet metal in front of the latch mechanism. I assumed it had a purpose and mimicked it when I made the bracket. To make the bracket, you can cut the slot and drill the holes first if you have the equipment to do the one sheet metal bend. If not, you can get the sheet metal cut to size and bent by your metal supplier or a local machine shop and do the slot and holes later. I used a jigsaw to make the cuts and a drill press to make the holes.

The HDPE part came to me as scrap from a plastic supply house, but you could use a scrap of various sorts of plastic, or perhaps aluminum. Steel would also work fine but would be heavier and a lot more work in a home shop like mine.

Get the latch mounted on the bracket before working on the other parts. Next position the actuator with the key ring in place, getting it in line with the point on the latch mechanism that it will be acting on as well as possible. Drill the two holes and mount it, using the two screws you got with the actuator at the junkyard to hold the actuator and spacer to the bracket. My screws were long enough to still retain the actuator with the additional spacer between.

Now, take the new spring from the hardware store and replace the original spring with it (SAVE the old spring just in case…). Test by applying power briefly to the actuator. When you first power it up, don’t worry about the polarity. If it does not pull, just reverse the two connections.

If your spring guess was good the latch will open quickly when you add power to the actuator. If it moves and stalls, the spring is still too strong. Try to find a spring that is as strong as possible but which will still allow the actuator to work EVERY time. Plan on testing this many times before the final installation to ensure it’s right.

Next, it’s time to add the Emergency Escape Handle. Drive out the tiny pin that secures the cable and remove the cable. Discard both. Drill a hole in the handle for the pivot point in the area which previously held the lead slug at the end of the Mitsubishi’s release cable. I used a 10/32 bolt as my pivot point, and added washers around it so that it was in line vertically with the end of the actuator rod. When you rotate the lever, it should push the actuator enough to unlock the latch. I was fortunate to get the position of the pivot bolt right the first time, and in fact when I tried to improve it a bit with a second hole, it turned out to be worse. I cut off part of the handle after it was mounted so that it didn’t look like a handle that you just pull on, and later I should change the Arrow painted on it to show the direction to move it in when you wanna escape.

That’s about it for the basic mechanical construction.

Tuning the Mechanism on the Workbench:

One of the stimuli that created this project is the design used for Keith Krieger’s installation (my2002tii.com). I used Keith’s experience to guide mine. I think that Keith attached his solenoid to the trunk latch using the same point near the spring, but was never sure as I had trouble understanding the pictures. To be honest, BMW’s design for the 2002 trunk latch is not optimum for a simple add-on remote actuator that does not take up a lot of the trunk’s usable space. Pulling to the side works but requires more force that if the actuator were to pull forward on the latch, but that would either require a more complicated mechanism or an actuator that projects forward into the trunk.

I don’t have the equipment or skill for a more complicated design, so I paid attention to the things that I could control, and tested everything on the workbench before ever trying it in my car. I used three large flat washers to simulate the thickness of the car’s sheet metal between the latch and the mounting bracket on the workbench. Any 12 Volt power supply that can supply 3 Amps will be sufficient to power the actuator during workbench testing.

Having a smoothly operating trunk latch to minimize friction will help your cause. That means a cleaned, un-rusted, properly lubricated latch. I won’t tell you how to clean yours because the process is just grunt work with cleaning solvents, rags, Q-tips,etc. Time is the biggest ingredient. I cleaned up my spare trunk latch also so that I could test with more than one sample, and could find no difference even though their production dates were 6 years apart.

Next, for the best mechanical advantage, you want the actuator’s moving “shaft” to line up with the pulling point. That was the original reason for the HDPE part in mine. It worked when I just mounted the actuator to the inside of the sheet metal but the angle was wrong and I’m sure the actuator would suffer over time.

Finally, the emergency escape handle needs to press squarely against the actuator shaft face in order to work properly. Drilling the mounting hole for it is best done after you’ve gotten the basic mechanism working on the bench.

Installation:

Once you have the actuator working the latch correctly, time to try it on the car.

IMPORTANT: When doing the installation, you need to be sure that your regular button on the lock and this new mechanism both work correctly BEFORE closing the trunk lid for the first time by working the mechanism with the lid up! To do this, use the shaft of a screwdriver to simulate the lid’s part of the mechanism. Do this several times to make sure the trunk will open!

VERY IMPORTANT: Don’t skip the step above. It’s also in the instructions below, so you were warned. A trunk lid that can’t be opened on a 2002 is a real pain…

To install, take the already assembled mechanism that you tested on the workbench and take it apart. Almost all of the way… Leave the emergency handle installed, as it will hold the spacer in place. Everything else comes apart except for the attachment of the actuator to the latch via the key ring.

Next, get all the parts and tools in the trunk, including all hardware, a 10mm wrench or socket for the latch mounting bolts, and the screwdriver or Torx driver appropriate to the actuator mounting screws.

Now fish the original latch mechanism into it’s normal place. Hold it there while you take the sheet metal mounting bracket and put it in place over the trunk latch mount of the car. Install the three original mounting bolts, leaving them a bit loose until all three are started. Then, tighten all three bolts.

Next, take the dangling actuator (it should be hanging by the key ring at this point) and mount it in place with the two screws.

You are now done with the installation EXCEPT FOR testing. Make sure the original trunk lock is unlocked. Take your screwdriver and simulate the action of the trunk lid closing on the latch. It should click “closed”. Press the trunk lock button and the latch mechanism should pop “open” as normal. So far so good. Now re-latch it with your screwdriver and try the emergency handle. If that works, on to the electromechanical gizmo. Again, re-latch with a screwdriver. Finally, apply power to the actuator and the latch should again pop open.

If that all testing worked, TRY IT AGAIN for safety’s sake.

All’s well? Then go ahead and close the trunk lid. With no small children inside. If the lock button and the new Actuator work correctly, you are done!

Except for taking it all apart and painting the bracket, then reassembling and wiring it up, again retesting with the trunk lid open.

Electrical considerations:

This is not meant to be an all inclusive set of instructions on how to wire your car, just a set of personal comments. Please ensure that you are competent to modify the wiring of your car or get this wiring done by someone who is! First, I recommend using an inline fuse to feed this added circuit. All of the actuators that I tried drew less than 3 AMPS current at 14 V DC, so a 5 AMP inline fuse is recommended. In normal use you would only need to apply power momentarily as the action is very fast, but as a test I applied power to one actuator continuously for 5 minutes. There was no permanent damage, but it did get quite warm, the current dropped by about 1 AMP as the actuator warmed up, and the action was weakened until the actuator cooled down for 15 minutes or so. I plan on using a relay in the trunk to control mine, but with reasonable wiring you could control this with just a pushbutton in series with the actuator, getting power from your choice of an always-on battery connection or the accessory position. For testing in and out of the car, the only important thing is to have a reliable source of 12V power and properly secured connections.

[bueller]

nice work cal! so i just totally sent this thread to you on rmbmw forum. i guess the strep i've been fighting the past couple days has affected my processing faculties. well done though.

matt

ps--my seatbelts went in without a hitch. pun intended. and they function perfectly.

[TobyB]

Nice job!

I just rigged up a cable that drops out underneath the car.

Now you can all steal my crap out of the trunk.

t

[02Les]

An excellent and a thorough write-up.

Going to save this for a rainy day (or two).

[jimk]

Nice job Cal. Remind me to eyeball it next time we get together.

[FunkyLaneO]

If you use a relay you could also wire this into your car alarm on one of the accessory circuits so you could open it with your keychain. Pretty cool!

[2002isGuy]

(nt)

[dsiegel]

Cal

This is a great description. Thanks for taking the time to post it in such detail. I ended up using a length of 2x2 inch right angle aluminum which was fairly easy to cut and the horizontal and vertical faces fit just right.

The problem I am having is finding a suitable replacement spring.

I've gone to the nearby Ace hardware store and rummaged through the drawers of extension springs for anything that looks close, as well as buying a collection of assorted extension springs at Harbor Freight, and getting the ones they sell at Home Depot and I still can't come up with one that pulls hard enough but not too hard and is the right diameter and length. In your posting you said that you bought an assortment at the hardware store, brought them home and found one that works.

Any chance you can remember any details of the one that did the job? Length, outer diameter, guage of wire?

Or, better yet, where you got it? I'm at the point where I would order it from somewhere if it is not a franchise or chain hardware store where I live in order to get a suitable spring.

Thanks for any help you can provide.

David

[yeewiz]

Cal, nicely done. Thanks for a great write-up. Been trying to get my remote trunk opener project underway. This is definitely a great help!

[BillWilliams]

Incredible

[Dev02]

This is great! I plan on putting remotes in my car but didn't know how I'd do the trunk.

[calw]

Sorry I missed the recent updates to this thread. The "notify" function must expire.

First, watch for further developments in the very near future. I want to make it easy to do this job....

David, unfortunately I was unable to find actual specs on the springs, and ended up guessing my way through it. I settled on one that was 5/16" OD, wire diameter of 0.036", plain steel, with a coiled section of about .75" long. With the ends it is about 1.3" long overall (un-stretched). You can use a longer spring with those basic dimensions and shorten it, forming your own ends. Look for as hardware store with those little drawers filling a whole aisle and you'll probably find something that can be made to work. CLEAN your latch mechanism and lightly lubricate it as step 1 of the work, please. If it's rusty, clean it some more or get a new one, they're not too expensive. The gunk in most trunk latches that I've seen add friction that you don't need. With the spring removed, it should operate with little friction.

[dsiegel]

Cal

I found a spring of the correct diameter and wire guage, but a little too long to have enough pulling back power. I shortened it a couple of times and now have something that the actuator can overcome, but the spring is still able to pull back with enough force to set the latch.

I haven't put it back in the car yet, but did you have any problem with the piece attached to the trunk lid with the bar that engages the latch not being able to push in place because of the thickness of the bracket that now surrounds the latch? Just looking at it, I'm wondering if the bracekt in the piece attached to the lid will be interfered with.

I'm assuming it worked out for you, but did you have to file down part of that bracket that is attached to the trunk lid so that it is not hitting on either side (front and back) of the latch?

Hope my question makes sense.

Thanks

David

[calw]

That question makes perfect sense.

I worried about it also, because it looked close, but did not have a problem with mine. I know it's close because the trunk lid's latch part does just touch the bracket with a good hard slam shut. It may come down to the number of layers of paint below it! I'm not sure that it will work with a thicker bracket. X-Ray eyes would be handy in this situation...

My latest idea was to eliminate the possibility by only using one bolt hole on the horizontal surface- ending the bracket to the right of the possible interference, and also by nibbling away the material of the front part of the bracket where it might interfere. I meant to try this last weekend but didn't get around to it yet.

[dlhoovler]

Very clean install and great writeup. Even though I never plan to do this to my '02, it it's always nice to read and learn from someone who not only has skill but also writes about it well to help others.

-Dave