seat belt tension when under way

[Dieter]

When underway, my seatbelt is slack. I can pull it off me for two or more feet. When the car is braking hard, it's still slack and loose. Should I be worried? Car has 13,000 miles.
Also, the driver's side door key tumbler, where the key goes in, just spins around and doesn't lock or unlock the door. I found broken plastic bits on the sill below the door handle. I lock and unlock the car now through the pass side door. Is this problem common to 1990 300ZX's? Thanks for your time.

[NolimitZ32]

Does the seatbelt retract and tighten up or it just lays there when you pull it out?

[Dieter]

It retracts completely.

[NolimitZ32]

The only time it should lock is in a sudden stop like in a collision, generally you can simulate that by jerking it (the seatbelt). IF yours doesn't lock then it should be replaced

[Dieter]

Thank you NolimitZ32. I'll try that today.