Seat Belt Systems: Description and Operation

INTRODUCTION
A structural seat belt control system is standard factory-installed equipment on all extended cab (club cab and quad cab) versions of this model.

Structural Seat Belt Control System
A structural seat with driver and passenger side integrated seat belt retractors and height adjusters is standard equipment on all extended cab (club cab and quad cab) models. In this system the seat belts are anchored to the seat back frame instead of the vehicle body, providing easier access to the rear seating area of the vehicle.

However, because the structural seat system has the seat belt and retractor mounted on a movable reclining seat back frame, a typical inertia-type mechanism cannot be used to latch the seat belt retractor. Therefore, an electronic structural seat belt control system is used to actuate the seat belt retractor latches. The structural seat belt control system consists of a Seatbelt Control Timer Module (SCTM) and two electric seat belt retractor latch solenoids, one for each front outboard seating position.

The structural seat belt control system also has a test mode feature. This feature allows the seat belt control system to be tested for proper operation while the vehicle is stationary by overriding the normal SCTM control functions. The seat belt control system must be tested for proper operation following the service of any seat belt control system or airbag system component. See Seat Belt Control System Test Mode on the Diagnosis and Testing for more information.

Following are general descriptions of the major components in the structural seat belt control system. Refer to the owner's manual in the vehicle glove box for more information on the features, use and operation of all of the factory-installed passenger restraints, including the structural seat belt system.

SYSTEM OPERATION
Seatbelt Control Timer Module
The Seatbelt Control Timer Module (SCTM) is secured to a bracket underneath the front edge of the front seat center cushion. The SCTM mounting bracket also serves as the support for the slide-out rear seat cup holder unit. The SCTM controls the supply of battery current to both of the front seat belt retractor latch solenoids. The SCTM contains an electromechanical Gravity (G)-sensor and an electronic timer circuit. The SCTM monitors the ignition switch state and both door jamb switches through hard wired inputs. In response to those inputs, the SCTM controls hard wired outputs to both seat belt retractor latch solenoids. The SCTM also sends diagnostic outputs to the Airbag Control Module (ACM) over a hard wired fault circuit.

The SCTM provides battery current to energize the seat belt retractor latch solenoids whenever the ignition switch is in the On or Accessory positions, unless the G-sensor input indicates a vehicle condition that requires the seat belt retractor to be latched. When the seat belt retractor latch solenoids are energized the retractor spools are unlatched, and the seat belt webbing can be extracted from the retractor. When the solenoids are de-energized the retractor spools latch, preventing the seat belt webbing from being extracted further from the retractor. This logic ensures that the seat belts will latch and/or remain latched if battery power is lost during a vehicle collision.

The electromechanical G-sensor within the SCTM monitors the rate of vehicle acceleration and deceleration in any horizontal direction. The G-sensor also responds to the horizontal attitude of the vehicle. If the G-sensor monitors a gravity force of greater than about 0.7G in any horizontal direction, or that the vehicle is tilted ill any direction at an angle of greater than about 45 degrees, the SCTM will sense the input from the G-sensor and de-energize the seat belt retractor latch solenoids, which will cause the retractors to latch.

The SCTM electronic timer circuit provides the vehicle occupants with the ability to extract the seat belt webbing from the retractor spool for a time period of about 30 minutes after the ignition switch is turned to the Off position. The electronic timer circuit also monitors the state of the door jamb switches, and unlatches the seat belt retractors after either door jamb switch cycles from open to closed or from closed to open. Each time the SCTM receives an input indicating a change in the state of a monitored switch has occurred, the 30 minute timer starts again. The timer also is used to de-energize the retractor latch solenoids after about 30 minutes, and prevent the battery from being drained while the vehicle is not being driven.

The hard wired SCTM output to the ACM is used to indicate whether a fault condition is present in the structural seat belt control system. The ACM monitors the input from the SCTM and sends the proper messages to the instrument cluster on the Chrysler Collision Detection (CCD) data bus to turn the seat belt reminder lamp on or off. If the ACM receives a fault input or does not detect any input from the SCTM, it sets a fault code and sends messages to the instrument cluster to turn the lamp on. See Seat Belt Reminder Lamp in the Description and Operation of Instrument Panel Systems for more information.

See Airbag Systems for more information about the ACM. For diagnosis of the CCD data bus, the ACM or the ACM input from the SCTM, the use of a DRB scan tool and the proper Diagnostic Procedures are recommended. The SCTM cannot be repaired. If faulty or damaged, it must be replaced.

Seat Belt Retractor Latch Solenoid
A seat belt retractor latch solenoid is integral to each of the two outboard front seat belt retractors. The solenoid is grounded at all times through its wire harness connector and circuit. The solenoid receives battery current, which is switched by the Seatbelt Control Timer Module (SCTM), through a fuse in the junction block.

When the seat belt retractor latch solenoids are energized the retractor spools are unlatched, and the seat belt webbing can be withdrawn from the retractor. When the solenoids are de-energized the retractor spools latch, preventing the seat belt webbing from being withdrawn any further from the retractor.

The seat belt retractor latch solenoids cannot be repaired. If the solenoid is faulty or damaged, the entire seat belt retractor unit must be replaced.

Door Jamb Switch
The door jamb switches are mounted to the door hinge pillars. The switches close a path to ground for the Seatbelt Control Timer Module (SCTM) when a door is opened, and open the ground path when a door is closed.

The door jamb switches cannot be repaired and, if faulty or damaged, they must be replaced. Refer to Door Jamb Switch in the Replacement of Vehicle Theft Security Systems for the service procedures.