I've had the same problem on our 1986 740. Remove the driver's door panel and adjust the limit switch on the rod connecting the button to the lockmechanism. Since you are having trouble with locking, I suspect that your switch needs to be moved up slightly. You'll have to experiment until it works correctly and reliably. Ours works fine now. It is not likely that the switch needs replacement, but if it does, this switch has had several wiring harness and switch changes in the eighties, and you must get the right switch.