Occupant Classification Module (OCM)

Occupant Classification Module:

The Occupant Classification Module (OCM) is secured with two screws to a stamped steel mounting bracket welded onto the underside of the passenger side front seat cushion frame near the inboard front corner. Concealed within a hollow in the center of the molded plastic OCM housing is a microprocessor and the other electronic circuitry of the module. The module housing is sealed to enclose and protect the internal electronic circuitry. The OCM software is flash programmable.

Two mounting tabs, a connector lock tower, and a connector receptacle are integral to the OCM housing. The connector receptacle contains terminal pins that connect the OCM to the vehicle electrical system through a dedicated take out and connector of the passenger side front seat wire harness. A molded plastic lock pin is integral to one end of a long tether, while the other end of the tether is secured to the seat wire harness. After the wire harness has been connected to the module, the lock pin is engaged through the lock tower on the module housing preventing the connector latches from becoming disengaged and ensuring that a secure electrical connection is maintained at all times.

A non-calibrated OCM is the only component of the Occupant Classification System (OCS) that is available for separate service replacement. The OCS components of the passenger side front seat cushion including the cushion frame, springs, insulator pad, seat weight bladder and pressure sensor, seat cushion foam, wiring harness and the OCM are a factory-calibrated and assembled unit. Once this unit is connected to a vehicle electrically, the calibration settings are uploaded from the calibrated OCM and stored in the memory of the ACM. If only the OCM is subsequently replaced, the new, non-calibrated OCM learns the proper calibration settings from the ACM after it is connected to the vehicle electrically.

The OCM cannot be adjusted or repaired and, if damaged or faulty, it must be replaced. The components of the passenger side front seat cushion 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 microprocessor in the Occupant Classification Module (OCM) contains the Occupant Classification System (OCS) logic circuits. The OCM uses On-Board Diagnostics (OBD) and can communicate with other electronic modules in the vehicle as well as with the DRBIII(R) scan tool using the Programmable Communications Interface (PCI) data bus network. This method of communication is also used for OCS diagnosis and testing through the 16-way data link connector located on the driver side lower edge of the instrument panel.

The OCM provides a nominal five volts to both the pressure sensor of the seat weight bladder beneath the passenger side front seat cushion and to the belt tension sensor on the passenger side front seat belt lower anchor through dedicated hard wired sensor voltage and sensor ground circuits. The OCM then monitors the return voltage from each of the sensors on dedicated hard wired data communication circuits. The bladder pressure sensor input allows the OCM to determine whether the passenger side front seat is occupied and the relative size of the occupant by providing a weight-sensing reference to the load on the seat cushion. The belt tension sensor provides an additional logic input to the OCM microprocessor that allows it to distinguish between the lower seat belt cinch loads of a belted occupant and the higher loads associated with a belted child seat.

Pre-programmed decision algorithms and OCS calibration allow the OCM microprocessor to determine when the seat cushion load as signaled by the bladder pressure sensor and the seat belt cinch load as signaled by the belt tension sensor indicate that passenger airbag protection is appropriate. When the programmed conditions are met, the OCM sends the proper electronic occupant classification messages over the PCI data bus to the Airbag Control Module (ACM), and the ACM enables or disables the deployment circuits for the passenger front supplemental restraints. The ACM also provides a control output for the passenger airbag on/off indicator in the instrument panel grab handle based upon the electronic occupant classification messages it receives from the OCM.

The OCM microprocessor continuously monitors all of the OCS electrical circuits and components to determine the system readiness. If the OCM detects a monitored system fault, it sets an active and stored Diagnostic Trouble Code (DTC) and sends the appropriate electronic messages to the ACM over the PCI data bus. Then the ACM sets a DTC and sends messages to control the airbag indicator operation accordingly. An active fault only remains for the duration of the fault, or in some cases for the duration of the current ignition switch cycle, while a stored fault causes a DTC to be stored in memory by the OCM and the ACM. For some DTCs, if a fault does not recur for a number of ignition cycles, the OCM will automatically erase the stored DTC. For other internal faults, the stored DTC is latched forever.

The OCM receives battery current on a fused ignition switch output (run-start) circuit through a fuse in the Junction Block (JB). The OCM receives ground through a ground circuit and take out of the body wire harness, which it shares with the ACM. This take out has a single eyelet terminal connector that is secured by a ground screw to the top of the right front seat riser on the floor panel beneath the right front seat. These connections allow the OCM to be operational whenever the ignition switch is in the Start or On positions.

The hard wired inputs and outputs for the OCM may be diagnosed and tested using conventional diagnostic tools and procedures. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the OCM, the PCI data bus network, or the electronic message inputs to and outputs from the OCM. The most reliable, efficient, and accurate means to diagnose the OCM, the PCI data bus network, and the electronic message inputs to and outputs from the OCM requires the use of a DRBIII(R) scan tool. Refer to the appropriate diagnostic information.