How Rain-Sensing Wipers Know It's Raining Before You Do

Yahoo

04-03-2025

Cars are packed to the gunwales these days with sensors, hand-holding driver-assist systems, and various arcane doodads that serve to isolate the driver from the actual physical act of driving. The more money you spend the less you have to be present, and you end up with systems that dim your headlights automatically, adjust your cruising speed, and even keep you in your lane as you drive. Feels kinda excessive to me, but then again, I consider anything made since the 1990s a "new" car, and mostly commute on either Ural sidecar rigs or 50-year-old Japanese bikes. Not a lot of driver-assist systems on a Yamaha XS850, unless you count the self-canceling turn signals.
Ah, but I digress. It's not all bad. One modern, high-tech doohickey that's actually cool and useful are rain-sensing wipers. It is kinda nice to just set your wiper stalk to the automatic position and let the sensors do the math of figuring out when to turn them on and at what speed to set them. As much as I love my 2022 Kia Soul, I do spend a lot of time when it's raining dicking around with the right-hand control stalk trying to find the right speed for the intermittent wipers. It'd be nice to just push a button and have a little computer do it for me. Not even I am immune to convenience.
How do they work, though? How do rain-sensing wipers actually, you know, sense the rain? Sorcery? Machine telepathy? A tiny faerie sitting behind your rearview mirror looking out for rain? Let's find out.
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Rain-sensing wipers have been around, in one form or another, since the 1960s. Based on technologies developed for the aviation industry — and primitive rain-sensing technology showcased in GM's 1951 LeSabre concept — early systems used a variety of mad science and trickery to sense when rain was in the air. In the late '60s, Ford engineers figured out how to use windshield-mounted conductive assemblies to sense rain. Japan's Denso was also tinkering with automatic wipers, patenting their own in 1968.
Citroën unveiled (probably) the first mass-produced rain-sensing wipers on its high-tech SM halo car in 1970. This system used an absurdly complicated system — very on-brand for the SM, honestly — that monitored the wiper motor's current draw. If the wipers were running and the motor was drawing a lot of power, say when the windshield was dry and there was more resistance as the wipers moved across the glass, the wiper system would park the wipers for a set amount of time. As the wipers were parked, rain would build up on the windshield which allowed for smoother operation once the wipers kicked back in.
Other systems followed over the next two decades, with a real standout patented by Nissan in the early '80s. It wasn't until the mid '90s, however, when we got the first truly modern rain sensing system.
GM unveiled its "Rainsense" automatic wipers in 1996 on that year's Cadillac STS, Eldorado, and DeVille. It used a simple system wherein a tiny module mounted behind the rearview mirror shone infrared LEDs through the windshield then measured the degree of refraction. If there was no water on the windshield, all the light reflected back to the sensor and all was good. No need for the wipers. If there was rain on the windshield, less light would be bounced back and the sensor would know something was up. Something moist. Using a bunch of fancy math, the system calculated how much water was on the windshield based on the refraction, then turned the wipers on and adjusted them accordingly as long as it was raining and the auto system was engaged. Pretty clever stuff.
Rainsense quickly spread from Cadillacs to Buicks and on down The General's food chain until even humble Chevys were equipped with it. Nowadays, nearly every marque has its own style of rain-sensing wiper, the majority of which are based on the same technology that underpins GM's Rainsense system. Just a couple of IR lights, a sensor, and a little computer that's good at physics and a driver never has to adjust or even turn on their wipers ever again. Pretty cool, if you ask me.
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