There have been a lot of discussions around similar issues, but nothing exactly on point.

We have a 1995 Volvo 850 wagon, with no remote locking fob or any of that -- straight-up manual key locking.

When using the key on the tailgate lock, it will lock and unlock the entire central locking system... all doors. So far, so good.

When using the key on the front (driver/passenger) doors, the central locking system will lock and unlock all the four doors, but not the tailgate. In order to lock/unlock the tailgate, I need to manually go actuate the tailgate lock.

Sound familiar?

 

If you pull the key out with the lock in the horizonal position, it will not work with the rest of the system. If you pull the key out with it in the vertical position, it will.

https://www.customers.volvocars.com/..._9.html#pg2.13

 

Mine does the same thing, manual only. Just tried the vertical key thing this evening and it doesn't work.

My tailgate lock will rotate about 60 degrees up to lock and the same down from horizontal to open all the door locks electrically but unless I return the key to horizontal it will not come out of the lock.
I tried the drivers door too, same thing. Am I missing something or can you actually get your tailgate lock vertical (90 degrees) and remove the key ??

I had always thought that my rear door lock actuator was shot as it's never worked with my remote either.

 

Can you hear the actuator trying to lock/unlock?

Mine was trying but just couldnt do it so I took the back panel off and added an aftermarket actuator in parallel to the oem actuator. Its rock solid now.

The question you need answered is if the OEM actuator is still working, dead or the wiring had broken. Its not common but the wiring can break over time where the hing is up by the shock. Highly unlikely but a possibility.

 

I'm sure on mine the actuator isn't even trying to move, no sounds at all.

As for my locking problem I haven't checked for power at the actuator. It's one of the things that is way, way back on a burner on the things to do list but since I came across this thread and I still have the trim off the door from rebuilding the rear wiper motor transmission I'm going to go check the connector for power.
I had that same idea when I saw the price of a new lock and actuator assembly for close to 300 $$$. I have a new chevy door lock actuator laying around I was going to see about wiring in. Nice to know I'm not alone in the "creative" repairs department.