Operation

OPERATION

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 a 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 (DLC), 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 front seat cushion and to the belt tension sensor on the passenger front seat belt lower anchor. The OCM then monitors the return voltage from each of the sensors. The bladder pressure sensor input allows the OCM to determine whether the passenger 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 Occupant Restraint Controller (ORC), and the ORC enables or disables the deployment circuits for the passenger front supplemental restraints. The ORC also provides a control output for the Passenger Airbag Disabled (PAD) indicator in the instrument panel center stack, 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 ORC over the PCI data bus. Then the ORC 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 ORC. For some DTC's, 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 from an IPM high side driver (Run/Start). The OCM receives ground through a ground circuit of the body wire harness, which it shares with the ORC. These connections allow the OCM to be operational whenever the ignition switch is in the Start or ON positions.


To diagnose and test the OCS, use a scan tool and the appropriate diagnostic information.


WARNING: On vehicles equipped with the Occupant Classification System (OCS), never replace both the Occupant Restraint Controller (ORC) and the Occupant Classification Module (OCM) at the same time. If both require replacement, replace the OCM first. Then perform the Airbag System test including an ignition-On time of at least one minute before replacing the ORC. Both the ORC and the OCM store OCS calibration data, which they transfer to one another during the first minute of ignition-On time after one of them is replaced. If both modules are replaced at the same time, an irreversible fault will be set in both modules.