Rain sensing wipers have been a fairly recent development on vehicles. Rain sensors automatically detect rain, snow or moisture on the windshield. They adjust the wiper speed depending upon the accumulation of water drops on the window's surface. This allows the vehicle owner a hands-off posture while driving through rain, mist and snow. A sensor attached to the front windshield behind the rear view mirror operates by using an infrared light. The infrared light shoots a beam through the windshield at a 45-degree angle. Anything on the window that disrupts that beam triggers the sensor to activate the wiper.
Place the vehicle in park or neutral with the emergency brake set. Use a spray bottle of window cleaner to soak the inside of the front window. Wipe the window's surface clean with towels, removing all stains, such as coffee, tobacco, any haze and hand prints.
Clean the outside of the front window with window cleaner and towels. Remove all bug stains, bird droppings, oil film, mud or other debris. Clean the window until spotless. Use extra effort around the location of the rain sensor mounted on the inside window in front of the rear view mirror.
Use a magnifying glass to inspect the entire outside surface of the windshield. Look for recent cracks, chips, nicks or "bull's eye" deformations. Study the area where the rain sensor sits. There can be no cracks or chips at that location. If you find chips or cracks, particularly in the area above the sensor, you will have to repair the crack or replace the windshield. The infrared sensor beam will deflect off any crack or chip and fail to function properly.
Look at the small boxlike sensor on the inside of the windshield in front of the rear view mirror. Make sure it has not become unglued and it sits firmly adhered to the window's surface. If it sags, it will cause misalignment with the infrared beam and give false information. If loose and hanging, glue it back into position with some window adhesive.
Raise the hood. Disconnect the negative battery cable with an end wrench. Go to the wiper control switch on your vehicle. Spray electrical contact cleaner into the switch seams and rapidly work the control on and off, or turn to the knob dial. Spray contact cleaner on the "On/Off" switch to the rain sensor and activate the switch several times. Let the switches air dry.
Reconnect the negative battery cable with an end wrench. Use a spray bottle to wet the outside surface of the front windshield until you see water heavy beads. Turn the ignition key to the "On" position. Turn the rain sensor switch to the "On" position. If the wipers do not move, refer to your owner's repair manual for the location of the rain sensor module. It will be a small plastic box that receives the wires from the sensor. Most modules have location mounts on the firewall inside the engine compartment.
Take a test light and stick the sharp probe of the test light into the red power wire that comes from the sensor down to the module. The key must be on, and the window must be wet. Look for the test bulb light to illuminate. If it does not illuminate, check the wire connection at the module. The other two wires on the module will be a ground wire and a signal wire. Check the signal wire for voltage with the test light. If it does not light, the wiring has a fault from the cabin windshield wiper switch to the module.
Check the ground wire to make sure it has a tight fitting on a metal ground source. Remove the ground wire with an end wrench or screwdriver and clean the connection with contact spray. Refasten it with an end wrench or screwdriver.