[valiant67]

I had this issue on my 2005 300C with the automatic wipers. If the setup is the same, there was a senors (maybe an infrared sensor) mounted to the windshield up by the rear view mirror. I'd occasionally get the random wipe on a dry windshield as well. The cleaner the windshield the better this worked.

 

[ImperialCrown]

FALSE WIPES (AUTOMATIC WIPER ONLY)

False wipes are unnecessary wipes that occur when the control knob of the multi-function switch is in one of the five automatic wiper sensitivity positions and no rain or moisture is apparent within the wipe pattern on the windshield glass. Because this system is designed to operate whenever it detects moisture, if the switch is in one of the automatic wiper sensitivity positions any road spray, bug splatters or mist from passing cars or trucks may occasionally cause the wipers to cycle. These are normal characteristics of this system and are not false wipes.
False wipes generally are the result of foreign material or flaws that interfere with the system optics. The optics for this system allow the Light Rain Sensor Module (LRSM) (also known as the Light Sensor Module/LSM or Rain Sensor Module/RSM) to precisely transmit several InfraRed (IR) light beams and precisely monitor the intensity of these beams reflected from the outer surface of the windshield glass to determine the presence of moisture. The sensitivity of this system is such that anything that distorts the focus or intensity of the IR light beams or affects the ability of the photo diodes within the LRSM to see and accurately measure these light beams can result in the LRSM logic misinterpreting the resulting input data as moisture on the windshield. The optics for this system include the lenses of the LRSM, the lens of the LRSM bracket, the adhesive pad layer that bonds the LRSM bracket to the inside of the windshield, and the windshield glass.
The following procedure should only be used if false wipe occurrences are frequent and persistent. Perform diagnosis of the front wiper system, the multi-function switch, the Steering Control Module (SCM) and the LRSM using a diagnostic scan tool and the appropriate diagnostic procedures before performing the following:


NOTE:
The Light Rain Sensor Module (LRSM) mounting bracket is serviced as a unit with the windshield glass. If either the bracket or the glass is ineffective or damaged, the entire LRSM bracket and windshield glass unit must be replaced.

1. Carefully inspect the outer surface of the windshield glass for physical damage, including scratches, cracks or chips in the vicinity of the LRSM mounting bracket lens. Scratches, cracks or chips in the windshield glass outside the area of the LRSM mounting bracket lenses will not cause false wipes. If OK, go to Step #2. If not OK, replace the damaged windshield.
2. From the outside of the windshield glass, carefully inspect the adhesive layer between the windshield glass and the LRSM bracket for any large voids. If an adhesive void greater than about 1 millimeter (0.04 inch) is observed, replace the flawed LRSM mounting bracket and windshield unit. If OK, go to Step #3.
3. Lightly pull the LRSM away from the windshield bracket to confirm that both sides of the spring steel LRSM mounting band are fully engaged with the mounting bracket on the inside of the windshield glass. If OK, go to Step #4. If not OK, reinstall the LRSM onto the bracket as required.
4. Remove the LRSM from the mounting bracket. Inspect the LRSM lenses and the mounting bracket lens for contamination. If OK, go to Step #5. If not OK, clean any foreign material from each of these lenses using rubbing alcohol and a lint-free cloth.
5. Carefully inspect the LRSM for any physical damage, including scratches on the LRSM lenses. If not OK, replace the damaged LRSM.
6. Carefully inspect the LRSM mounting bracket for any physical damage, including scratches on the LRSM bracket lens. If not OK, replace the damaged LRSM bracket and windshield as a unit.

 

[saltydog]

When my wife drives the van she parks near a tree that leaves sap drops all over the car, though hard to see they will cause the wipers to cycle.

Also if the windshield is dirty they seem to cycle at random.

I do my best to keep the window clean and set the sensitivity to level 3 and I don't have many false wips.