I’m a longtime lurker who recently joined the stuck trunk club on a road trip with friends.
Many thanks
to those who discovered and documented the black plug procedure. With that, I was quickly able to open the liftgate once we returned home.
I’ve spent some time experimenting and playing around with the relevant mechanisms in an effort to gain insight into how the auto-close and open processes are supposed to work. As such, this posting gets a bit lengthy in an attempt to include relevant details. I hope to advance the cause and increase my, and the community's, understanding of this fault. Based on the experiences documented in this thread, it seems likely that this problem would disappear if I replaced the high-torque cinch motor and/or the latch. My curiosity has taken hold, however, and I would like to dig deeper to gain a fuller understanding of what is going wrong.
November 2016 build date Model S
TL;DR — The latch, cinch motor, and supporting infrastructure appear to be functioning properly when inspected individually. How does one bench test the sensors/switches and actuator/solenoid in the latch? My efforts to find a wiring diagram, pinout specs, or troubleshooting procedure have come up empty.
At this point in my experimentation, I’m wondering how to bench test the latch mechanism. My efforts to find troubleshooting procedures, a wiring diagram, or a pinout schematic for the four-pin connector have failed miserably. I’ve used Tesla and Mercedes part numbers in my searches. Still, no dice. I’m hoping that is a testament to my poor searching skills and someone here can point me in the right direction.
When the liftgate is fully and quickly closed by hand (no cinch motor help required), the latch will release as expected. The trouble starts when the cinch motor is activated to fully close the liftgate from a half-closed state. When the latch then opens, the cinch motor does not release. Because the cinch is still drawn tight, the latch only releases part way. The liftgate is then in a partially open state where it can neither be fully opened nor fully closed. The emergency release cable has no hope against the grip the cinch motor has on the latch mechanism.
When the liftgate is in this stuck-in-the-middle state, the black plug method will release the cinch motor. That seems to indicate the cinch motor and associated wiring are intact and functioning as designed.
So, that presents me with a quandary. Why doesn’t the lift gate controller release the cinch motor? Is a sensor, presumably in the latch, failing to indicate the partially opened state? The system seems to know when the latch is partially closed. Otherwise, it wouldn’t have actuated the cinch motor in the first place. Is there maybe something awry within the liftgate controller itself? Maybe the sensor has only partially failed and can indicate when the latch has been half closed from an open state, but cannot indicate when the latch has been half opened from a closed state? A wiring diagram or schematic for the 4-pin connector would be most helpful. Is there a suggested test procedure to further diagnose problems with the latch?
I’ve disassembled, inspected, lubricated, reassembled, and tested the high torque cinching actuator. I found no evidence of malfunction or wear. It readily operates in both directions even with as little as 3 volts from my bench power supply.
I’ve also removed the latch and operated it on the bench. It seems to be working well mechanically with no binding. The emergency release cable opens it up with minimal effort and the cable that runs to the cinch motor operates with no binding or excess friction.