In the lengthy and frustrating process of getting my stuck down BMW E39 bonnet opened, I've established there are two basic faults that can occur.
Here is the story of how to fix this problem with some pictures and videos.
A) Either the bonnet release cable has broken, stuck or is otherwise not operative, or B) the Bonnet locks are not opening/releasing when operated correctly.
There is actually a possible 3rd self-induced fault - where the repaired bonnet sticks down on its first re-closure. (More later.)
You need to establish/understand which problem you are working with.
A broken/stuck release cable is relatively easily resolved, whereas opening a jammed release lock is more complex/challenging.
Some basic information first so that you are aware of the general approach:
1. The majority of 3 Series and 5 series I've looked at use almost the same release lock mechanism (photo later).
2. The differences between cars relate to the cables linking to the locks, and the distance between locks and cable adjusters, and the number of bonnet posts/locks
(either 1 or 2). Finally the safety release catch varies between models.
3. The key point is that it's the same release lock that's on all the cars I've inspected, so if you have a jammed lock (likely because its release spring/spigot has been over
adjusted and will now never open) then the process below will be of assistance.
4. The bonnet post(s) secured to the bonnet lid seems to be similar for most 3 and 5 series cars. This is as a threaded post with a 17mm locking nut and coil-spring, and a
slot at the end to turn to post with a flat headed screwdriver. Again this is useful for opening a failed/jammed lock mechanism. See photo later.
5. For the 5 Series E39 (and several others I've seen) the release cable is in 3 parts a) Car cabin lever to the engine bay to an inter-connect plastic box. Linked at the engine
bay to the first release lock. Connected from the first Lock to the second lock. i.e. It's a series of links.
6. The safety release catch (stops the bonnet flying up if opened whilst driving) is simple mechanically and self evident to operate/fix so not discussed here.
Some pictures: (videos at end.)
The bonnet catch arrangement from above:
Demo of reason for lock failing to open.jpg
Forced open lock
Passenger side bonnet lock
Bonnet post showing threaded post, and 17mm locking nut.
Tool made to adjust lock mechanisms:
Bonnet lock - tool to adjust lock.jpg
Some basic stuff before you get scientific:
1. Pull the release cable hard - Do this by removing the release handle and pulling on the wire with pliers. This may work, but if you encounter a hard stop
don't pull any harder - it won't help. This will only over-come a stick cable problem.
2. Have someone push the bonnet down (one side at a time and start on the easy passenger side - it's at the end of the cable sequence so less to go wrong/stick)
while you pull the cable. Also try moving the bonnet side-ways while pressing down. (This is unlikely to work but its worth an attempt before you get serious.)
A) Stuck or broken cable/Cable not operating the release locks.
To resolve this and get the bonnet to released, each lock must have its release mechanism moved manually. For the 5 Series E39 BMW this is as follows:
1. Extract the kidney grills to give access to the locks. At worst these can be easily replaced if you damage them.
2. Each lock is now visible, the passenger side is fairly obvious and the driver side is harder to see. Both are black plastic boxes.
3. For the passenger side, simply pull the release cable and this will move the release spring and that side opens.
If you can't easily move the passenger side lock, make up a rod with a right angle bend with about 1/2 inch at the end, and use this to move the lock.
With the tool, this activity take perhaps 30 seconds in total. The rod fits into a notch in the lock mechanism - see photo.
4. For the driver side, the cable is not readily visible to manipulate, therefore the lock mechanism must be moved using the tool mentioned above.
This is harder that the passenger side due to visibility, but if you look at the photo of what is involved, and use a mirror to assist, its manageable.
With the tool available this should take perhaps 1 minute to complete, and the driver side catch will release.
5. If you have moved both locks through their range of travel and the bonnet does not release then the issue is with the locks themselves so forget
anything about cables meanwhile, and move onto the alternative process below.
Note: I think that for a car with a broken/sticky bonnet cable as above, from a standing start (having now done this) the above sequence would take me less
than 30 minutes from start to finish including making the rod/tool and replacing the grills and clearing up. Fitting a replacement cable within
the engine bay (where its most likely to fail) is relatively easy (on an E39) and takes perhaps another 60 minutes.
B) The bonnet locks are not opening when moved through their full range of operation manually/using a angled rod/tool.
This is the more serious problem and likely to have occurred when the lock spigot/spring has been over-bent/adjusted during a service.
Operating the lock will not release the bonnet post(s) ever...
You can check this by watching the release cable move in/out in the car cabin while operating the lock arms using the angled rod/tool.
If the cable is moving, then the locks will also be moving but not releasing.
To resolve this you have two options, one more destructive than the other.
C) Non-Destructive locking nut approach
1. Noting that the bonnet posts are threaded posts with a locking nut and a large coiled spring, and that if the locking nut can be released the post can be
simply unscrewed from below this is the first option to consider.
2. Again, remove the kidney grills and attempt to adjust the locking nut at the top of the bonnet post.
If you can move the 17mm nut (angled open spanner?) the post can be simply unscrewed, however access to the nut is extremely difficult.
On the passenger side it's easy to cut away some of the plastic to make more space available to see what you are doing.
(I've not tried this option with the bonnet down, since when I had my problem I didn't know about the locking nut possibility.)
3. If you can move the locking nut, next remove the front bumper to give you access and with a long screw driver unscrew the bonnet post from below.
It may be possible to unscrew this post with an air driven tool with the locking nut in-situ, but with a manual screwdriver (long) and
lever is wasn't moving at all for me. Also, testing on the bench in a vice afterwards it was absolutely solid and needed the nut moved first.
4. Start on the passenger side since it's easy access and uncomplicated by the safety catch getting in the way (E39 model)
5. The driver side is similar but access from below with a long screwdriver is a bit harder.
6. See the photo of the post/structure and an opened lock to understand how this works.
D) Forced mechanical access into the lock mechanism
1. If you cannot release the post by the locking nut approach detailed above then the alternative is relatively easy.
This approach simply involves smashing through the plastic lock cases with a sharp screwdriver/cold chisel and operating the release spring/spigot.
This approach will require replacement locks and probably release cables - however these are very low cost from a scrap yard (£5 UK, or $10 US)
See the video - info below.
2. Simply cut into the lock plastic box mechanism until you can see the internal spring, then operate the spring with a long screwdriver.
This may require several attempts to hit the mark.
Watch the video for the driver side to see the outcome and give you an idea of the approach.
Each catch should take about 10 minutes to release once you are prepared and ready to start hammering.
3. The bonnet lifts up immediately the spring is operated correctly releasing the post and no force is involved once you have broken into the lock mechanism.
Each side pops up independently on release by an inch or so so its obviously when you have released the lock.
4. Now fit the replacement locks/cables and if required adjust the Bonnet post lengths to ensure they engage with the locks correctly.
Read the re-close approach below.
E) Re-closure of the bonnet
1. Finally, with the bonnet open you will thereafter face the issue of repairing the problem and closing the bonnet again.
There is a risk that you will again have a jammed bonnet which I suspect would slightly frustrate the individual involved.
To mitigate this risk there are two steps. Acquire another bonnet post from a scrap yard and use this to prove that all is operating correctly,
or alternatively remove a bonnet post (being careful to note its exact position for replacing later) and use this for testing.
Secondly and crucially, release the locking nuts on both bonnet posts before shutting and make sure you can rotate the bonnet posts with a screwdriver.
At worst if it won't re-open you simply have to unscrew the posts from below the car (with the bumper removed) and try again.
Finally, with everything tested and working correctly, you can tighten the bonnet locking nuts.
I've written this note after I spent many hours trawling the web/forums and never actually acquiring got a complete or correct answer for this
So that's it, I hope this is useful to someone out there.
Photos above and You-Tube videos show the detail:
1 of 3: Reason for bonnet not opening - over adjusted release spring.
2 of 3: Cable routing for cable and non-destructuve opening :
3 of 3: Destructive opening of bonnet lock- howto.