How Does a Windshield Rain Sensor Work?

There are many different techniques that automakers have experimented with to eliminate or substitute the wiper system. A rain sensor is a fairly recent addition to the windshield of a car. The technology has been around since the 50’s, however, rain sensors didn’t become prevalent in cars until the late 1990’s.  Today, they are fairly common in domestic and foreign passenger vehicles and trucks.  A rain sensor detects the presence of water on the glass of the windshield. The sensor then prompts the car to turn on the wipers automatically whenever rain is detected.

What is the mechanism behind the rain sensor?

The rain sensor is mounted beside the rearview mirror in such a way that it is in contact with the windshield. A small portion of the windshield is blackened to hide the rain sensor. However, there is a tiny view port which is not blackened and through which the sensor is able to detect rain.

The rain sensor works on the principle of total internal reflection. This technique makes use of infrared radiation. An infrared light beams at a 45-degree angle on a clear area of the windshield from the sensor inside the car. When it rains, the wet glass causes the light to scatter and lesser amount of light gets reflected back to the sensor. When the amount of light reflected back reduces to a level preset by the software inside the sensor, it automatically switches on the wiping mechanism of the car to start the drying process.

The software also determines the speed of the wipers depending on the amount of moisture accumulation. This speed changes according to the detection of moisture by the sensor.

What is the benefit of having a rain sensor?

It is very important for the windshield to be clear while you are driving not just for your own safety but for the safety of other people or vehicles around you. The visibility level decreases to a large extent when it rains. The wipers need to be turned on immediately to clear the windshield. The rain sensor detects rain and switches the wipers on so that you are not distracted with the effort of manually engaging the wipers. This helps you to have better control of the car and ultimately – stay safe.

Installing and Replacing a Rain Sensor

In order to detect whether your car has a rain sensor or not, you can pay attention the next time it rains and see if the wipers are getting switched on automatically or not. Rain sensor gel pads need to be replaced each time we perform a windshield replacement. If you’re not sure if your vehicle is equipped with a rain sensor, look at the photos below and if you still aren’t sure, just give us a call and we’ll help you. If all you need is a windshield repair from a stone chip or some other object making contact with  your windshield, you will not need a new rain sensor gel pad.

As always, feel free to reach out to us by phone or email.  Telephone 610-935-5588 or

Thanks for reading!


Examples of rain sensor buttons on Chevrolet, Ford, Lexus, Volvo and BMW vehicles.