Automatic Shoulder Belt System - Information
The Automatic Shoulder Belt System ExplainedThe automatic shoulder belt system is an interesting combination of mechanical and electrical components. While the system is actually pretty simple, diagnosing and repairing it may be confusing if you don't completely understand it. Here's a thorough description of the system's components, their operation, plus some troubleshooting tips.
Description
The seat belt control unit operates the shoulder belt motors, the shoulder belt retractor solenoids, and the seat belt indicator light and beeper. The control unit keeps track of the shoulder belt position, and decides where and when to move it, by monitoring the reference voltage from various switches (see "Reference Voltage Review" in last month's issue). If the control unit detects a belt in the "wrong" position, it turns on the indicator light and beeper.
Each shoulder belt retractor assembly consists of a spring-loaded reel to take up excess slack in the belt, a mechanical lock to lock the belt during hard acceleration or deceleration, or if the car tilts too far in any direction, a solenoid to prevent the retractor from locking, and a sensor switch to detect solenoid operation. The control unit energizes the solenoid before moving the shoulder belt to allow the belt to unwind from the retractor. Otherwise, the shoulder belt motor woutd stall.
NOTE:
The retractor solenoid can only prevent the retractor from locking if it's not already locked; it can't override the mechanical lock once it's locked. (See the first Troubleshooting Tip.)
Each shoulder belt track assembly consists of a shoulder belt motor, a cable, a track, a front position switch, two rear lock position switches (one indicates that the belt is anchored in the rear position, the other indicates that the belt is buckled), and a shoulder belt receptacle. The control unit signals the motor to drive the belt (via the cable) from front to rear (along the track), and monitors the position switches to determine where the belt is and if it's buckled.
Operation
The control unit positions the shoulder belts according to inputs from the ignition switch, the door latch switches (located in the latch assemblies), and the front and rear position switches.
When the ignition switch is turned ON, the control unit monitors the door latch and position switches to determine the shoulder belt position. If the belt is not in the "proper" position, the control unit moves it to that position.
When the ignition switch is turned OFF and the key is removed, the driver's shoulder belt travels forward and remains there, regardless of the driver's door position. The passenger's belt remains in the rear position until the passenger's door is opened, at which time the belt travels forward and remains there.
Here's how the control unit "reads" each switch:
^ Each door latch switch has two positions and two wires running to the control unit. When the door is closed, one wire is switched to body ground and the other is open. When the door is open, the conditions reverse.
^ Each front position switch is closed and provides a path to ground when the shoulder belt is not in the front position. The switch and circuit are open when the shoulder belt is in the front position.
^ Each rear lock position switch (anchor) is closed and provides a path to ground when the shoulder belt is not in the rear position. The switch and circuit are open when the shoulder belt is in the rear position.
^ Each rear lock position switch (belt) is closed and provides a path to ground when the shoulder belt is not in the rear position. The switch and circuit are open when the belt is in the rear position and the shoulder belt is buckled.
^ The driver's lap belt switch (located in the driver's shoulder belt retractor) is closed and provides a path to ground when the driver's lap belt is not buckled. The switch and circuit are open when the belt is buckled.
^ Each solenoid switch is closed and provides a path to ground when the solenoid is not energized. The switch and circuit are open when the solenoid is energized.
Troubleshooting Tips
^ The shoulder belt retractor may lock under normal driving conditions, which may not be apparent until the motor tries to drive the belt forward. If the retractor is locked, relieve the tension on the belt and allow it to retract slightly to unlock the retractor.
^ If the indicator light and beeper are on, you will find the problem if you follow the control unit input test faithfully.
^ When the input test indicates the voltage should be 1V or less, or 0.03V or less, that's exactly what it means. If the voltage exceeds the spec at all, there's too much circuit resistance.
^ If the system fails one of the input tests, you must repair that problem before testing further. Otherwise, you may get incorrect results on a later test.
^ The "Test: Desired result" section of the input test may specify the shoulder buckle position. If the buckle is not in that position, the test results will be incorrect.
^ You must check the entire circuit if it fails a voltage check. That means the control unit connectors, the wire to the switch, the switch, the wire from the switch, and the ground connection.
^ For poor connections at the control unit, remove the female terminals from the back of the control unit connector and adjust them so they fit the male terminals snugly.