“The main focus of this mandate is to reduce injury to children and small adults (weighing less than 100 pounds) who may be seated close to the airbag.”

Recent changes to the Federal Motor Vehicle Safety Standard mandate the design and installation of advanced airbag systems that reduce the risk of airbag-related injuries. The main focus of this mandate is to reduce injury to children and small adults (weighing less than 100 pounds) who may be seated close to the airbag.

The mandate states the passenger front airbag must be suppressed when an infant in a rear-facing infant seat occupies the passenger seat. Suppressed means that based on the classification of the occupant in the front seat, the passenger airbag will be turned off. In addition, the passenger airbag disable lamp (PADL) must be illuminated whenever the seat is occupied and the front passenger airbag has been suppressed by the restraint system. If the seat is not occupied, the lamp will not be illuminated.

To meet these new mandates, DaimlerChrysler has implemented an occupant classification system (OCS), using sensors in the passenger front seat and an occupant classification module (OCM) to detect the classification (or weight) of the occupant. The OCM provides the airbag control module (ACM) information on the passenger's classification. The ACM will then suppress or modify the deployment of the passenger airbag, as needed. DaimlerChrysler currently uses two types of occupant classification systems: bladder type and strain gauge type. On the bladder type system, a silicone-filled bladder is mounted between the seat foam and the seat support or pan. A pressure sensor that looks similar to a manifold absolute pressure (MAP) sensor is mounted under the seat and is connected by a hose to the bladder. The pressure sensor, hardwired to the OCM, can sense a change of pressure applied to the bladder. The pressure sensor has three circuits: 5-volt supply, signal wire and sensor ground. When the seat is not occupied, the OCM compares the sensor voltage with the value stored in memory. This value is stored when the seat is initially installed and must be relearned any time service is performed to the seat cushion or if components are replaced.

The voltage values from the sensor change as weight is added to the seat. Based on the change of voltage from the sensor, the OCM can determine the weight on the seat. Due to the natural aging of the components, such as seat foam, the sensor value for an empty seat will change over time. The OCM compensates for the natural aging of the seat components by monitoring the gradual changes. The OCM stores seat aging and calibration information in the ACM. If a new OCM is installed, it can "learn" the aging characteristics of the same seat from the ACM.

The following is the logic for the bladder type system:

• When the OCM detects that the seat is empty, the PADL is off and the airbag is suppressed.
• When the OCM detects a weight equivalent to a 6-year-old child (approximately 12-60 pounds), the PADL is on and the airbag is suppressed.
• When the OCM detects a weight equivalent to or greater than approximately 99 pounds, the PADL light is off and the front passenger airbag is enabled and if necessary, deployed depending on the severity of the impact.

On the strain gauge type system, four strain gauges are located at each corner of the seat frame where the frame attaches to the seat riser. Data from each sensor is sent to the OCM. The strain gauge has three circuits: 5-volt supply, signal wire and sensor ground. Each strain gauge is attached to the seat riser with two bolts. A center bolt attaches the seat to the strain gauge. The strain gauge supports the weight of the seat. It is critical that the correct fasteners and torque on the strain gauge are used.

The OCM compares each of the sensor voltages with the values stored in memory. These values are determined when the seat is initially installed and must be relearned each time service to the seat is performed. As weight is added to the seat, the voltage values of the sensors change up or down. Based on the change of voltage from each sensor, the OCM can determine the amount of weight on the seat. When the seat is not occupied and the ignition is off, the bus system on the vehicle is "asleep." Then the OCM attempts to "re-zero" the sensor values.

The following is the logic for the strain gauge type system:

• When the OCM detects that the seat is empty, the PADL is off and the airbag is suppressed.
• When the OCM detects a weight equivalent to a rear-facing infant seat, the PADL is on and the airbag is suppressed.
• When the OCM detects a weight equivalent to a child, the PADL is off and the airbag is enabled and if necessary, deploys but is limited to a low risk deployment level.
• When the OCM detects a weight equivalent to or greater than approximately 99 pounds, the PADL is off. The front passenger airbag is enabled and may deploy, depending on the severity of the impact.

Service concerns include the following:

• Occupant Classification Seat Weights tool No. 9077 is needed for calibrating the system after airbag deployment or service to the seat or its sensors along with a full function scan tool.
• PADL lamp is on or flickering. This concern will be present when items heavy enough to trigger the light are on the passenger seat. This is a normal condition.
• Do not modify the passenger seat, seat covering or mountings in any way, or the system may not operate as designed.

Greg Montero is an Identifix DaimlerChrysler, Hyundai and Mitsubishi specialist. He is an ASE master with L1 certifications and has recently received his Accredited Automotive Manager certificate from the Automotive Management Institute.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2006 IDENTIFIX. All Rights Reserved.

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