OK, I have wondered about this for some time. When I exit the car and lock the doors, sometimes the trunk does not unlock. when I relock and then unlock the doors with the door key, the trunk then unlocks. what gives?

I dont have an answer, but I similar issue with my driver's side back door. In its case, it does not always lock. I have to open it up and shut it hard.

Per the manual, this is a sign that the door was not closed or fully closed. It is however. I can only assume something similar with the trunk. There must be a sensor/switch that allows action of the vacuum system to lock/unlock the door/trunk. And when that thing is out of line, there is a problem. At least thats my guess. I have not had time to look into it.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I don't have the car/manual right now, and memory is hazy, but there is a process by which the trunk unlock can be overridden, by inserting the key and turning it the opposite direction. it might be possible your trunk switch is tweaked in that direction, causing an intermittant malfunction - but probably not.

The whole system operates on a central vacuum source, so if the trunk or one of the doors doesn't lock/unlock, it's very likely due to a faulty vacuum actuator, or mechanical resistance/friction in that actuator mechanism.
It works when you lock/unlock it rapidly because it temporarily loosens it up by going forward/backward. You should be able to track it down by:
- Hit the unlock and see which switches unlock first and which unlock last. The slowest lock wiill be the weakest or stickiest mechanism. Check the vacuum line to the trunk lock if it's slow.
- insert the key in the trunk and work the mechanism manually to see how sticky it is. Zap it with WD40 and see if it helps.

DG

On my car all of the lids and doors lock and unlock together.

-Jason

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Thanks, will try this and get back here on the forum with updates.

The trunk lock has 3 settings: Unlock (the whole car), Lock (the whole car), and Bolted (trunk remains locked when car is unlocked).

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

So, how do I change the settings?

Use the key in the button IIRC to the left is unlock, middle is with central locking, and right is lock / bolt. To lock the trunk and leave it on central locking you turn the key to the right then back to the middle.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Never thought of that trunk lock setting being the problem. I never use that function and totally forgot after reading the manual. I always unlock/lock the trunk the same way and never turned the key to the right.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Yeah, I never used that function, so I forgot the details.
Then there's the Valet key, which will not unlock the glovebox or the trunk, but will unlock the doors and start the car.

As Compu said, all the locks SHOULD lock/unlock together, but often some are slower, indicating a possible problem in the making.

DG

Mine never locked together, they go in a quick sequence. Same goes for unlocking. If I unlock the driver's side door but plan on opening the rear, I have to wait about 1 second.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car