Seat Belt Systems: Description and Operation

Automatic Safety Belts / Key Cylinder Light

The purpose of the Automatic Safety Belt is to allow the car's doors to be opened without restriction by the Safety Belts. Without this feature the Retractor Lock Pendulums would lock the belts as they are pulled out of the retractors making it difficult to open the door. To overcome the difficulty, the safety belt locking mechanism is made less sensitive to movement when the doors are being opened. Higher sensitivity is restored when the doors are closed or when the car is moving. Higher sensitivity is required to protect the safety belt wearer in the event of an accident.


The Passive Restraint Control Module controls the operation of the Safety Belt Retractor Solenoids. The Passive Restraint Control Module receives power from the BELT Fuse. When a door handle switch is operated or a door is open, the associated Door Latch Switch will close. The Module will close its output switch and energize all the Safety Belt Retractor Release Solenoids for both doors. The door can then be opened without interference from the safety belts. The Module will also turn on the Key Cylinder Light to illuminate the LH Front Door Lock Cylinder. When the door is closed, the Latch Switch opens again and the solenoids are de-energized after about three seconds. The safety belts are now ready to protect the wearer and the Key Cylinder Light will go out. To ensure that the solenoids are released, a vehicle speed signal is also supplied to the Module. The Module will release the solenoids any time the car is moving, even if a Latch Switch is closed. If a Latch Switch fails to open and the speed signal causes the solenoids to release, the solenoids cannot be energized again until the switch opens to resets the Module.

Note: A shorted Latch Switch will keep the Module from resetting, so both Safety Belts will interfere with door opening. An open Latch Switch will still allow the system to operate properly when the other door is used.