Exterior Lamps

Figure 8D3-2 Typical OSM Pinout:

Figure 8D3-3 OSM Usage Table:

The distributed lighting system consists of nine output switching modules (OSM's), the BCM and the lighting control panel or Left Switch Pod (LSP). Each of these components communicate via a serial data circuit that is controlled by the Body Control Module (BCM). As a backup system, the BCM also controls two redundant relays that will allow for essential lighting in the event of major system fault.


OUTPUT SWITCHING MODULES (OSM'S)
Each OSM is a solid state switching device that consists of: 1) four outputs to power lamps, 2) Address pins that will uniquely identify the OSM when connected to the vehicle harness, 3) two data line inputs that are used to communicate with the BCM, 4) A power and ground connection. Based on the commands the OSM receives from the BCM, the OSM will turn on and off its outputs, which results in the headlamp, taillamp, turn signal lamp, or courtesy lamp operation required under the particular operating condition. Figures 8D3-2 and 3 details the general and specific bulbs, addresses, power and grounds for each OSM.

Data Line Detail - Circuit 928:

COMMUNICATION
The communication between the nine OSM's the BCM, and the Left Switch Pod is achieved via the lighting data line. The BCM will continuously scan from the Left Switch Pod to each of the nine OSM's one at a time, making commands to each, and requesting information from each. The data line is designed as a complete loop so that each element on the data circuit has two data circuit inputs. This loop design will allow the circuit to operate with one open in the circuit. Figure 8D3-5 details the data circuit.

ADDRESSING
In order for an OSM to know which commands from the BCM it is to respond to it must have an "address". The OSM's are addressed by the vehicle wiring harness through the presence or absence of a wire (voltage) on OSM pins C, D, E, or F. Any pin without a wire will have a plastic "plug" in that cavity or pin location. Each of these four pins has a different "value", the total of all the pin values with a wire is the OSM number, or address. The OSM "value" for pin C is 8, D is 4, E is 2, and F is 1. For example, an OSM with a "plug" at pin C and F, with wires at D and E would be OSM # 6 (D=4, E=2, 4+2=6). Specific OSM addresses are detailed for each OSM in Figure 8D3-3.

FAULT DETECTION
As the BCM scans each OSM and the switch pod. It makes both commands and requests of each element. The information requested from each OSM informs the BCM about the integrity of each of the four outputs of the OSM. Therefore, if an OSM detects an open or short on one of it's outputs it informs the BCM of this problem. The BCM in turn will inform the driver of the fault via the CCDIC display. If the BCM becomes unable to communicate with an OSM or the LSP, it will log a lighting malfunction code and inform the driver of a "Lighting Fault" via CCDIC display. In some cases the BCM will substitute one lamp for another to maintain near normal operation. The following is a list of substitutions the BCM will make:

LAMP SUBSTITUTION:
Fault/Substitution (Same Side)
Low Beam - Fog Lamp
High Beam - Low Beam
Front Turn - Park Lamp

If a system fault is detected that would disable the communication among all OSM's and the BCM, the BCM will activate the redundant relays which will power the left low beam lamp and the left outer-upper taillamp. This action eliminates the possibility of total loss of lighting.