AIR BAG BASICS
By Carl O’Reilly
AIR BAG OVERVIEW
Passenger cars and light trucks are equipped with air bag systems in compliance with federal motor vehicle safety standards. These standards require installation of passive automotive safety systems, to lessen the chance of serious injury in the event of a collision. A passive safety system is one that operates automatically to provide restraint of the driver and passengers in the event of a collision.
Protection during a collision is provided by a number of safety devices. The vehicle itself is designed to absorb the energy of a collision by crumpling, to cushion the force of an impact. The front and rear sections of a vehicle are designed to collapse while leaving the passenger compartment intact, creating a safety zone for the occupants. The use of safety belts helps to enhance this feature by restraining the occupants inside the safety zone of the passenger compartment. Steering columns are designed to collapse so as not to crush the driver on impact. Energy absorbing beams are mounted in the doors and the sides of the vehicle to cushion side impact collisions. The air bag is designed to work in conjunction with these safety devices, to allow greater protection of the occupants during a collision. The use of safety belts is the single most important safety device in the vehicle and their use is required for the air bag system to function properly, to protect the occupants.
Air bags are designed to deploy only under certain conditions. Air bag systems are designed to deploy during front end collisions of sufficient force, to cause injury to the occupants of the vehicle. The collision area must be within a specific range of the centerline at the front of the vehicle and the force of the impact must reach a specific level to cause deployment of the air bag system. Sensors are used to determine if the crash meets the criteria for air bag deployment.
AIR BAG CONTROL MODULE AND ENERGY RESERVE MODULE
The air bag control module and the energy reserve module functions are often combined into one unit. The air bag control module is used to monitor the air bag system circuits and components for failures, that may affect air bag operation. The control module has the ability to generate and store diagnostic trouble codes, relating to wiring and component failure and illuminate the air bag warning lamp, as a trouble indicator for the driver. Most air bag control modules have the ability to record and store data from sensors during a deployment event in a collision. The data recorded contains information pertinent to the order in which the crash sensors commanded deployment, as well as other vehicle information that the manufacturer may choose to have recorded.
Diagnostic information is generally only available through the use of a scan tool. Reading and clearing trouble codes usually requires a scan tool as well. Some diagnostic codes are designed as latching codes and cannot be cleared. Latching codes are generated by deployment during a collision and require replacement of the control module to be removed. Diagnosis and repair of air bag systems should only be attempted by trained professionals. Energy reserve is used to provide back up power to the air bag system, in the event that battery power is disabled during the collision. Energy reserve functions are often combined into the air bag module however, some manufacturers may use stand alone energy reserve modules. Energy reserve is accomplished by the charging of capacitors located in the module. The energy reserve can be as high as 35 volts and last for a number of minutes after power is removed.
THE AIR BAG
The air bag is inflated by expanding gas, from the ignition of a propellant contained in the base of the air bag. The inflation speed of the air bag is designed to provide an impact cushion before the occupants are forced against the interior components of the vehicle, by the force of a collision. The air bag will deflate almost immediately due to large holes placed in the bag, to allow gas to escape.
Driver side air bags are mounted in the center of the steering wheel. Wiring to the air bag is routed inside the steering column and is designed to allow the steering wheel to rotate without affecting the continuity of the air bag circuits. Some vehicles use a coiled flat wire, placed in a housing that winds and unwinds with rotation of the steering wheel.
Crash sensors are used to supply voltage and a ground source to inflate the air bag. Crash sensors contain a switch that closes during the force of a collision, to complete a circuit to the air bag. A typical crash sensor switch consists of a gold plated ball, held in place by a strong magnet. When a collision occurs, the force of the impact causes the ball to jar loose from the magnet and travel down a small ramp and close the circuit to the air bag. The crash switch is located inside the sensor and the sensors are designed to be installed in a specific direction, relative to the forward motion of the vehicle. Most sensors have arrows indicating the direction they are installed in.
Most vehicles are equipped with three crash sensors. One sensor will supply voltage to the air bag, it is referred to as the arming sensor. It is generally placed in the passenger compartment of the vehicle. There are two sensors used to provide a ground circuit to the air bag. They are referred to as the discriminating sensors. One sensor is usually located at or near the front end of the vehicle and the other is placed in the passenger compartment. Either one of these sensors can be used to complete the ground circuit to the air bag.
AIR BAG WARNING LIGHT
The air bag warning light serves to inform the driver about the condition of the air bag system. The lamp is placed in or near the instrument cluster and is operated by the air bag control module. The air bag warning lamp will illuminate when the key is turned to on, as a bulb check. When the vehicle is started, the lamp will briefly remain on or flash for a few seconds to indicate the system is operational. A failure in the air bag system will result in the lamp remaining illuminated while the engine is running. Air bag operation is almost always disabled when the air bag light is illuminated to indicate a system failure.
TRACTION CONTROL SYSTEM
The traction control system is used to maintain tire grip on the road surface during acceleration. When a vehicle is accelerated quickly, excessive engine torque can be applied to the drive wheels causing them to slip on the road surface. When this loss of traction occurs, vehicle acceleration time is increased, while the stability and control of the vehicle is reduced. The typical traction control system components consists of a traction control module, a warning lamp, a disengagement switch (some models), and components of the anti-lock brake system and the engine control system. The traction control module can be a stand alone component or part of the anti-lock brake module.