Light Rain Sensor Module (LRSM) - Operation

OPERATION

The microprocessor-based Light Rain Sensor Module (LRSM) (also known as the Light Sensor Module/LSM or the Rain Sensor Module/RSM) senses moisture in the wipe pattern on the outside of the windshield glass and sends electronic messages to the Body Control Module (BCM) (also known as the Common Body Controller/CBC) over the Local Interface Network (LIN) data bus. The BCM 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.

The internal programming of the LRSM then sends the appropriate electronic wipe command messages to the BCM over the LIN data bus. The BCM then responds by activating or deactivating the wiper system. Similarly, in some vehicle applications the photocells within the LRSM are capable of sensing ambient light levels through the windshield glass and providing electronic light level messages to other modules in the vehicle.

The Steering Control Module (SCM) sends electronic wiper switch status and sensitivity level messages over the CAN data bus to the BCM based upon the driver-selected settings of the control knob on the control stalk of the multi-function switch. The BCM receives electronic messages from other electronic modules in the vehicle over the CAN data bus, and the BCM relays the messages to the LRSM over the LIN bus.

The higher the selected sensitivity setting the more sensitive the LRSM is to the accumulated moisture on the windshield glass, and the more frequently the LRSM will send wipe commands to the BCM to operate the wiper system. The third (middle) sensitivity setting is set up for optimal operation of the automatic wipers under most typical driving conditions.

The LRSM operates on battery current received through a fused ignition switch output (run/accessory) circuit controlled by the run-accessory relay. The LRSM receives ground at all times through a take out of the body wire harness with an eyelet terminal that is secured by a ground screw to the body sheet metal. Therefore, the LRSM is operational only when the ignition switch is in the ON or ACCESSORY positions.

It is important to note that the default operation of the automatic wiper system is low speed continuous wipe. Therefore, if no message is received by the BCM from the LRSM for more than about five seconds when the AUTOMATIC wipe mode is selected, the wipers will default to the low speed continuous wipe operation. It is also noteworthy that the BCM must be properly configured for the automatic wipers option for the automatic wipers to function.

The hard wired circuits of the LRSM 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 LRSM or the electronic controls or communication between other modules and devices that provide some features of the automatic wiper system. The most reliable, efficient, and accurate means to diagnose the LRSM or the electronic controls and communication related to LRSM operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.