Rain Sensor Module: Operation

WARNING: This page is about a different car, the 2006 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

The microprocessor-based Rain Sensor Module (RSM) or Rain/Light Sensor Module (RLSM) senses moisture in the wipe pattern on the outside of the windshield glass and sends electronic messages to the Front Control Module (FCM). Four InfraRed (IR) diodes within the sensor generate infrared light beams that are aimed by four of the convex optical lenses near the base of the sensor through the windshield glass. Four additional convex optical lenses near the top of the sensor are focused on the infrared light beams on the outside of the windshield glass and allow the four 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, the sensor detects a change in the monitored infrared light beam intensity.

The internal programming of the sensor then sends the appropriate electronic wipe command messages to the FCM over the Controller Area Network (CAN) data bus. The FCM responds by activating or deactivating the front wiper system. The Steering Control Module (SCM) sends electronic sensitivity level messages to the RSM over the CAN data bus based upon the driver-selected SENSITIVITY setting of the control knob on the control stalk of the right multi-function switch. The higher the selected SENSITIVITY setting the more sensitive the rain sensor is to the accumulated moisture on the windshield glass, and the more frequently the sensor will send wipe commands to the FCM to operate the front wiper system. The third (middle) SENSITIVITY setting is set up for optimal operation of the automatic wipers under most typical driving conditions.

Similarly, the photocells within the RSM are able to sense ambient light levels through the windshield glass and provide electronic light level messages to other modules in the vehicle over the CAN data bus. These messages can be used by the ElectroMechanical Instrument Cluster (EMIC)/Cab Control Node (CCN) instead of the sun load sensor input to provide the automatic headlamps feature.

The RSM operates on battery current received through a switched fused B(+) fuse on a switched fused B(+) circuit. The RSM 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 lower left inner C-pillar. 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 FCM from the RSM for more than about five seconds when the AUTOMATIC wipe mode is selected, the wipers will default to the low speed continuous wipe operation.

The hard wired circuits of the RSM may be diagnosed using conventional diagnostic tools and procedures. Refer to the SYSTEM WIRING DIAGRAMS article. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the RSM or the electronic communication between other modules and components that provide features of the automatic wiper system. The most reliable, efficient, and accurate means to diagnose the RSM or the electronic communication related to RSM operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.