Liberty 2WD V6-3.7L (2008)
Ambient Light Sensor: Description and Operation
DESCRIPTION
The Light Rain Sensor Module (LRSM) (1) is the primary component of the automatic wiper system. In vehicles equipped with the automatic wiper
system and the automatic headlamp system, the LRSM also performs the ambient light sensor function; and, therefore, is alternately referred to as the
Light Sensor Module (LSM) or the Rain Sensor Module (RSM). The LRSM is located on the inside of the windshield just below and to the right of the
inside rear view mirror mounting button. The LRSM is concealed from view within the vehicle interior by a molded plastic trim cover that snaps over the
top of the LRSM housing onto the LRSM mounting bracket, but is visible through the windshield glass from the exterior of the vehicle.
The molded black plastic LRSM housing has an integral connector receptacle (2) at the top with provisions for up to four terminal pins. These terminal
pins connect the LRSM to the vehicle electrical system through a dedicated take out and connector of the left body wire harness that extends from above
the headliner. Openings on the windshield side of the housing are fitted with four convex clear plastic lenses. A spring steel retaining strap (3) extends
forward on each side of the housing to latch the LRSM to a molded black plastic mounting bracket, which is permanently bonded to the inside of the
windshield glass.
Concealed and protected within the LRSM housing is the electronic circuitry of the module, which includes two InfraRed (IR) diodes, two photocells,
and a microprocessor. The LRSM is a Local Interface Network (LIN) slave and communicates over a single LIN bus circuit with the Wireless Control
Module (WCM). The WCM is a LIN master and a gateway to the Controller Area Network (CAN) data bus. The WCM controls the exchange of
electronic messages back and forth between the LRSM and other electronic modules in the vehicle.
The LRSM cannot be adjusted or repaired. If ineffective or damaged, the entire module must be replaced. The LRSM mounting bracket is serviced only
as a unit with the windshield glass.
OPERATION
The microprocessor-based Light Rain Sensor Module (LRSM) (also known as the Light Sensor ModuleILSM or the Rain Sensor ModuleIRSM) senses
moisture in the wipe pattern on the outside of the windshield glass and sends electronic messages to the Wireless Control Module (WCM) on the Local
Interface Network (LIN) data bus. The WCM relays messages back and forth between the LRSM and other electronic modules in the vehicle.
Two InfraRed (IR) diodes within the LRSM generate infrared light beams that are aimed by two of the convex optical lenses of the sensor through the
windshield glass. Two additional convex optical lenses of the sensor are focused on the infrared light beams reflected back from the windshield glass and
allow the two photocells within the sensor to sense changes in the intensity of these infrared light beams. When sufficient moisture accumulates within
the wipe pattern on the windshield glass, less of the infrared light is reflected back and the sensor detects a change in the monitored infrared light beam
intensity.
