After long time of inactivity the time came to deal with a common problem of the Golf 7 users: the trunk lock failure.
Symptom 1
The problem started to manifest itself with a strange boot light behaviour: the boot light sometimes was not working at all or it was switching on and off randomly. More than 95% of the times the boot light was useless.
My initial thought was that maybe something is wrong with the BCM that can not handle the LED boot light for some reason. But the LED light in the glovebox which is powered by the same BCM pin (pin C12) is working perfectly
My second thought is that water ingress from the 3rd brake light in the past has corroded some electrical contacts in the trunk lock. To eliminate this possibility I had to remove the tailgate trim.
After removing the tailgate trim I unplugged the connector of the lock and it was obvious that the problem was not in wiring as everything was in a perfect condition without any traces of corrosion. So I plugged again the connector on the lock
Then by having the tailgate open I tried to manually lock the trunk lock and unlock it via the rear emblem numerous times. It was then obvious that by unlocking via the rear emblem 2 distinct "click" sounds were heard with the second click approximately 3-4 seconds after the first click. The first click was heard when unlocking the mechanism (but there was no boot light at all) while the second click 3-4 seconds later made the boot light switch on.
To me the 2 distinct clicks meant that something was broken inside the trunk lock. There should be only one sound and at the same time the tailgate should be unlocked and the boot light should be on. The second click sound showed that the a microswitch responsible for switching on the boot light was delaying its function.
Then came the Symptom 2 (for which I had read in Drive2 in the past): the infamous "drrrr sound"
You can gain insight into this issue by reading this, this and this. A very interesting article is also this
Having pinpointed the problem, instead of trying solutions to replace the plastic gear from eBay or Aliexpress I went straight to ETKA to see if there were any updated trunk locks. To cut the long story short my factory installed lock was 5G6827505, which later became 5G6827505 A then again changed to 5G6827505 C and lastly 5G6827505 B
I started reading the various members of this forum who faced the same situation. I found out that the use of Torx 40 is suggested in order to remove the bolts that hold the lock on the tailgate. Without paying attention to the bolts I tried to use it only to find that the Torx 40 is inappropriate. The bolts are actually triple square M8 splines and using the Torx 40 destroyed the bolt head. I stopped there in order not to destroy the second bolt.
The next day I bought a M8 triple square spline for 2.70€ and went straight to the delearship to order my new 5G6827505B 9B9 trunk lock for 81.58€ (incl VAT) and a N 909 316 02 bolt (2.05€ incl VAT)
The final result was pleasing (not financially though). The new trunk lock performs its functions perfectly. Both locking the tailgate and switching on the boot light work as they should.
Now that I solved the boot light issue I will proceed in upgrading the boot light in the following weeks. A small glimpse of what is coming is shown in the next photo:
Until then stay safe!