Auto seatbelts not working

[solanog]

My 1990 Accord came with the auto seat belts, today when I turned the ignition on the driver's side seatbealt didn't move, it is stuck in the "off" or forward position, if I manually force it back and the turn the key off or open the door it goes forward, as expected. But it won't engange when the door is closed and the ignition is on.
What could be wrong, is there a way to diagnose if it is a motor a sensor, the computer or whatever part may be failing?
It happened before with the passenger side and I took the computer off and resolder it and it worked again.