Question about the central lock on a 88 240:

It works when I push the lock down from the inside of the car (all 4 doors lock). Doesn't work when I use the key from outside (only driver's door locks).

 

i think maybe you have to turn the key all the way a bit more?

 

Pull the door panel. There is an adjustment there. Most likely over the years it has loosened up. Either that or the connecting rod has simply come loose...I believe it's held on w/ a clip. It's good the electrics are working...