Rain Sensor Module: Operation

WARNING: This page is about a different car, the 2004 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) senses moisture in the wipe pattern on the outside of the windshield glass and sends wipe commands to the Body Control Module (BCM). Four InfraRed (IR) diodes within the RSM generate IR light beams that are aimed by four of the convex optical lenses near the base of the module through the windshield glass. Four additional convex optical lenses near the top of the RSM are focused on the IR light beams on the outside of the windshield glass and allow the two photocells within the module to sense changes in the intensity of these IR light beams. When sufficient moisture accumulates within the wipe pattern of the windshield glass, the RSM detects a change in the monitored IR light beam intensity.

The internal programming of the RSM then sends the appropriate electronic wipe command messages to the BCM over the Programmable Communications Interface (PCI) data bus. The BCM responds by activating or deactivating the front wiper system. The BCM also sends electronic sensitivity level messages to the RSM over the PCI data bus based upon the driver-selected sensitivity setting of the control knob on the control stalk of the right (wiper) multi-function switch. The higher the selected sensitivity setting the more sensitive the RSM is to the accumulated moisture on the windshield glass, and the more frequently the RSM will send wipe commands to the BCM to operate the front wiper system.

The RSM operates on battery current received through a fuse in the Junction Block (JB) on a fused B(+) circuit. This circuit is switched by the power accessory (sunroof) delay relay in the JB so that the RSM will operate whenever the relay is energized by the BCM. The RSM receives ground at all times through a take out of the left body wire harness with an eyelet terminal that is secured by a ground screw to the front seat crossmember on the floor panel under the left front seat. It is important to note that the default condition for the wiper system is automatic wipers Off; therefore, if no message is received from the RSM by the BCM for more than about five seconds, the automatic wipers will be disabled and the BCM will default the front wiper system operation to the low speed continuous wipe mode.

The RSM ground and battery current inputs may be diagnosed using conventional diagnostic tools and methods. However, conventional diagnostic methods may not prove conclusive in the diagnosis of the RSM internal circuitry, the BCM, the PCI data bus network, or the electronic messages received and transmitted by the RSM over the PCI data bus. The most reliable, efficient, and accurate means to diagnose the RSM requires the use of a DRBIII(R) scan tool. Refer to the appropriate diagnostic information.