Automatic wipers and automatic headlamps both come from a light-and-rain sensor at the glass. The driver never thinks about it, which is the point. It reads how much light is reaching the cab and whether there is water on the glass, hands those two readings to the vehicle, and lets the wiper and lighting logic do the rest. It is a small sensor with an outsized effect on how finished a cab feels to drive.
This guide is the version of the rain-and-light conversation we have with commercial-vehicle OEM buyers during project scoping. It covers what the sensor is responsible for, how the light and rain readings travel to the wiper and lighting controller, how the published ambient-light assembly differs from the windscreen part, why the glass at the installation area is part of the specification, and what to put in a request for quotation so a supplier can price it without a round of clarification emails.
1. What a rain and light sensor actually does
On a basic cab the driver flicks the wipers and the lights by hand and no sensor is needed. The moment a programme wants those functions to happen on their own, something has to measure the weather and the daylight, and that is the sensor's job. It does a few distinct things, and the split matters when you come to specify one.
- Measuring ambient light. How much daylight is reaching the cab, for automatic headlamps.
- Measuring rain on the glass. An optical reading of water on the glass, for automatic wipers.
- Sun load for the climate system. Some light sensors add a sunlight and temperature channel for air-conditioning compensation. On the CGQ-012A those pins exist but are reserved and undefined.
- Reporting, not deciding. The sensor is an input. It reports its readings to a wiper and lighting controller, which owns the wiper-speed and headlamp decisions.
- Working temperature. The glass area cold-soaks and bakes in sun, so the working temperature is part of the spec.
The common mistake is to treat “rain-light sensor” as one fixed thing. The first question is which of those jobs the cab automates, because automatic wipers need the windscreen light-and-rain part. For where this sensor sits among the cab inputs and the rest of the switching layer, the Switches and Sensors technical guide covers the full picture.
2. The two channels: light and rain
A windscreen sensor carries two readings in one housing, and they feed two different vehicle functions. Following that path from the glass to the wiper motor is what turns “we want automatic wipers” into a specification a supplier can build.
The pieces work like this:
- The sensor reads light and rain. The CGQ-024A light-and-rain sensor reads both channels through the glass: daylight level for the lighting logic, and water on the glass for the wiper logic.
- The readings travel on CAN. On CGQ-024A-SYN the harness connector is a Deutsch DT04-4P: power, ground, CAN− and CAN+.
- The controller owns the decisions. A wiper and lighting controller takes the rain reading and sets the wiper state, and takes the light reading and sets the headlamp state. Where a platform splits these across modules, the sensor still reports to the controller that carries the CAN interface, while a dedicated wiper module such as the EBX-2162 handles the wiper drive. For how the front exterior lighting group itself is controlled, see the vehicle lighting control module guide.
3. Windscreen light-and-rain versus the ambient-light assembly
This is the choice that actually decides the part number, and it comes down to one question: does the cab automate the wipers? Youlai builds two related sensors for the two answers, and picking the wrong one is the most common way a rain-light line item goes wrong.
Which light sensor do you need?
Start from the function the cab automates, not the sensor. One question sets the part.
Start here · the one question
Does the cab automate the wipers? If it does, only the part with a rain channel fits.
You need automatic wipers
Windscreen light-and-rain
Reads rain and daylight through the glass, and reports both on CAN.
CGQ-024AYou need an ambient-light input
Ambient-light assembly
A low-level ambient-light signal on pin 2, from a 5 V supply.
CGQ-012A| Windscreen light-and-rain (CGQ-024A) | Ambient-light assembly (CGQ-012A) | |
|---|---|---|
| Channels | Light + rain in one housing | Ambient light on pin 2; temperature and sunlight pins reserved |
| Drives | Automatic wipers and automatic headlamps | A low-level (low-side switch) input; the controller and the function it feeds are set by the programme |
| Supply | Rated 12 VDC and 24 VDC (CGQ-024A-SYN) | 4.5–5.5 V (+5 V on pin 6); reverse voltage 30 V max |
| Mounting | Glass 3–7 mm at the installation area | Position set by the programme; mates with Yazaki 7283-8660 |
| Sealing / temp | −40 to +85 °C work; −40 to +95 °C storage | IP53; −30 to +85 °C |
| Best fit | A cab that wants rain-sensing wipers as well as automatic headlamps | A controller that needs a 5 V ambient-light input on new-energy, commercial or construction vehicles |
Specify the CGQ-024A light-and-rain sensor when the programme wants automatic wipers. Where the controller only needs an ambient-light input, the CGQ-012A pin definition is on its own page. CGQ-024B-SYN is a separate catalogue part, with its own connector.
4. Mounting, and why the glass is part of the spec
Rain sensing reads through the glass, so the glass at the installation area is part of the specification.
- Glass at the installation area. On CGQ-024A-SYN that glass is 3 to 7 mm, with no interlayer, and with no static film, solar film, or other medium that affects light transmission over that area. Those media scatter or absorb the reading.
If a film covers that area, or the thickness sits outside that band, agree it with the glass supplier before the sensor is quoted.
5. How to write a rain-light sensor specification
A requirement a supplier can quote against, rather than guess at, covers five things. Leaving any of them implicit is what turns a quotation into a round of clarification emails.
- Which functions you automate. Automatic wipers, automatic headlamps, or both. Rain-sensing wipers need a windscreen light-and-rain part.
- Supply. CGQ-024A-SYN is rated 12 VDC and 24 VDC. For the ambient-light assembly, name the 5 V rail.
- Glass at the installation area. Thickness, interlayer and films. The mounting section states them for CGQ-024A-SYN. Agree that area with the glass supplier.
- Controller interface. CGQ-024A-SYN reports on CAN, on a Deutsch DT04-4P. Name the module that owns the wiper and lighting logic.
- Working temperature. −40 to +85 °C on CGQ-024A-SYN, storage −40 to +95 °C.
From a sourcing perspective, the function list is the line buyers leave implicit and regret. “A rain-light sensor” is ambiguous until you say whether the wipers are automatic; the moment that is clear, the windscreen part and the rest of the spec fall out of it. Provide the function list early and the sensor matches your vehicle plan instead of stalling in clarification.
6. What to look for in a supplier
The driver notices this sensor every time it rains. The supplier questions that matter are about capability and support.
- Both parts on one family. A supplier that offers the windscreen light-and-rain part and an ambient-light sensor covers a range of cabs without a second supplier. Confirm the two share the same quoting and validation workflow before committing.
- Glass at the installation area. Confirm the supplier states that glass requirement up front.
- Optical stability across temperature. Ask whether the optical performance is validated across the working temperature, not only at room temperature.
- Documentation on a project basis. EMC, IMDS and market-specific documentation are available upon project requirement rather than blanket-claimed across the catalogue.
Quality system and PPAP level are on the IATF 16949 and OEM sourcing pages. Youlai validates in an in-house environmental laboratory with EMC pre-compliance equipment, and the contact page reaches the project team directly for a sourcing conversation.
7. Suggested next step
If you are scoping automatic wipers or automatic lighting, the most useful things to bring to a first conversation are which functions you automate, and the glass at the installation area if you want rain sensing. That maps onto the CGQ-024A light-and-rain sensor or the CGQ-012A ambient-light sensor.
For the glass at the installation area, or a sensor-to-controller interface schedule against your vehicle programme, please use the contact page or message +86 134 6767 4786 on WhatsApp. Typical reply within one business day.