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 the diagnostic scan tool using the Programmable Communications Interface (PCI) data bus. 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 passenger airbag protection is appropriate based upon the seat cushion load as signaled by the bladder pressure sensor and the seat belt cinch load as signaled by the belt tension sensor. 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) (also known as the Occupant Restraint Controller/ORC), 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 ON or START positions.

The hard wired inputs and outputs for the OCM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the OCM or the electronic controls and communication between other modules and devices that provide features of the OCS. The most reliable, efficient, and accurate means to diagnose the OCM or the electronic controls and communication related to OCM operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.