Seat Sensor/Switch: Description and Operation

Seat Weight Bladder & Pressure Sensor:

Vehicles equipped with the Occupant Classification System (OCS) have a seat weight bladder and pressure sensor unit that is integral to the passenger side front seat cushion. The bladder is sandwiched between the seat cushion springs and seat cushion foam. A heavy jute-like insulator pad is installed between the lower surface of the bladder and the seat cushion springs. The pad is secured to the underside of the bladder by diagonal straps that form pockets at each of the two rear corners of the bladder, while two plastic push-in fasteners locate and secure the forward edge of the bladder and the pad to the front edge of the seat cushion frame. The seat cushion foam is installed loosely over the top of the bladder, then secured by the installation of the trim cover to the seat cushion.

The bladder consists of two rectangular sheets of an elastomeric material and a molded plastic elbow fitting. The two sheets of material are sealed together around their perimeter and heat staked to each other at numerous regular points within their field. The elbow fitting is sealed to a small round hole in the lower surface of the bladder and is pointed downward where it passes through a clearance hole in the insulator pad and extends to just below the seat cushion springs. Then the bladder is filled with a silicone fluid to become a pliable, quilted membrane.

Under the seat cushion a short tube is securely clamped at one end to the bladder nipple, and at the other end to a nipple on the electronic pressure sensor. The pressure sensor is contained within a molded plastic housing with an integral nipple formation near one end and an integral electrical connector receptacle at the opposite end. The sensor housing also features an integral mount that snaps over a tab integral to the stamped steel Occupant Classification Module (OCM) mounting bracket welded to the underside of the passenger side front seat cushion frame. The pressure sensor connector receptacle contains terminal pins that connect the sensor to the vehicle electrical system and the OCM through a dedicated take out and connector of the passenger side front seat wire harness.

The seat weight bladder and pressure sensor cannot be adjusted or repaired. The components of the passenger side front seat cushion of a vehicle equipped with the OCS including the cushion frame, springs, insulator pad, seat weight bladder and pressure sensor, seat cushion foam, wire harness and the OCM are serviced only as a factory-calibrated, assembled and tamper-evident unit. Only the OCM and the seat cushion trim are available for separate service replacement. Once a service replacement package has been installed in a vehicle, the OCM can thereafter be serviced only by replacing the entire passenger side front seat cushion unit with another complete service replacement package.

The seat weight bladder and pressure sensor unit is designed to sense the relative weight of a load applied to the passenger side front seat cushion, which provides a logic input to the microprocessor of the Occupant Classification Module (OCM). When a load is applied to the seat cushion, fluid within the bladder becomes pressurized. These changes in bladder fluid pressure are measured by the pressure sensor under the seat cushion through the bladder tube. As the pressure within the bladder changes, the circuitry of the pressure sensor changes the output voltage of the sensor.

The pressure sensor receives a nominal five volts and a ground through dedicated hard wired circuits from the OCM. The OCM then monitors the pressure sensor output voltage on a dedicated hard wired data communication circuit. The hard wired circuits between the pressure sensor and the OCM may be diagnosed and tested using conventional diagnostic tools and procedures. However, the most reliable, efficient, and accurate means to diagnose the bladder and pressure sensor input to the OCM, and the electronic message communication between the OCM and the Airbag Control Module (ACM) requires the use of a diagnostic scan tool.