Wednesday, 31 August 2005 17:00

Interior rearview mirror provides place for modern electronics

Written by I-CAR Advantage Online
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What's in an interior rearview mirror? You name it because it may be there! Because of its optimal location, the rearview mirror has become a centerpiece for modern day electronic devices. 

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Figure 1. This moisture sensor, used for an automatic windshield wiper system, stems from the base of a rearview mirror.
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Figure 2. A light sensitive sensor, used for an auto-dimming high-beam system, is located at the base of this rearview mirror.
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Figure 3. Among the controls found on this rearview mirror is a button for OnStar.
Every vehicle on the road is equipped with a rearview mirror. This makes it easy for vehicle makers to include the newest features to a wide range of vehicles without major changes to design. Its position relative to the sky is perfect for the reception and transmission of analog and digital signals. Being close to the driver's normal line of sight makes it safe for displaying status information or placing accessory controls.

Some of the new electronic accessories that are integrated with the interior rearview mirror include: automatic windshield wipers; auto-dimming high-beam systems; wireless connection devices; computer control modules; and auto-dimming rearview mirrors.

Automatic windshield wipers

An interior rearview mirror is a common location for moisture sensors used by automatic windshield wiper systems. The moisture sensor and control module may consist of a single assembly and be attached to the base of the rearview mirror (Figure 1), or only the sensor is placed there and the module is located somewhere else in the vehicle. The sensors project infrared beams onto the outer layer of the windshield. The presence and amount of moisture accumulated on the windshield is determined by how the beams reflect back into the sensor.

When the windshield is dry, most of the beams reflect back into the sensor. If there is moisture, the beams are interrupted and deflect in different directions. The more rain that is on the windshield, the fewer beams that make it back to the sensor. A special type of windshield may also be required for some systems.

Auto-dimming high beams

The new auto-dimming high-beam systems equipped on the 2005 Jeep Grand Cherokee and some of the new Cadillac vehicles also use a sensor that is located on the rearview mirror (Figure 2). The sensor for these systems is used to detect light projected from other vehicles. When the sensor detects light, either from oncoming headlamps or preceding tail lamps, the high beams automatically transition to the low beams.

The new system is much more advanced than older versions. The sensor is one type commonly used in digital cameras. It can distinguish between light produced from vehicles and light produced from street lights (DC vs. AC light). Older versions of this feature would often react to non-vehicle lights and flick on and off erratically. The new systems are also designed to transition more smoothly between high and low beams, but will instantly turn off if a vehicle suddenly appears.

Wireless connections

Wireless technology systems like OnStar and Bluetooth also use the convenient location of the rearview mirror for system controls and microphones (Figure 3). OnStar is a wireless service that provides a wide range of assistance to vehicle passengers. The OnStar button may be located on the rearview mirror, and is pressed to contact a service operator that can provide directions to the nearest hotels and restaurants or contact emergency personnel and provide them with the GPS location of the vehicle. This system may also automatically activate upon air bag deployment.

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Figure 4. The electronic control module for the Volvo S60 is located inside the rearview mirror.

Bluetooth is a wireless communication system used to link mobile phones, computers, and other devices over short distances. These systems are being added to vehicle designs to provide hands-free mobile phone use using the driver's own mobile phone, provided that the mobile phone is Bluetooth compatible. Many of the newer factory installed systems are voice activated, with the microphone located on the rearview mirror, and broadcasts through the vehicle speakers.

Computer control modules

Late model Volvo vehicles use the interior rearview mirror for housing the upper electronic module (UEM) (Figure 4). This module controls the interior accessories of the vehicle including, but not limited to, receiving lock and unlock commands from the key fob. The UEM cannot be separated from the mirror and both will require replacement as a single unit if either is damaged. If the rearview mirror/UEM is replaced, special software is required during the installation process.

Auto-dimming rearview mirror

Not only can a rearview mirror house many electronic accessories, but it may actually be an electronic accessory itself. The function of an auto-dimming rearview mirror is to shade out headlamp glare from vehicles in the rear. Glare from tailing headlamps are sometimes so bright that they can temporarily blind a driver.

These mirrors use an electrochromic material sandwiched inside the glass of the mirror. Electrochromic is the combination of two words - "electro" referring to electric, and "chromic" referring to color. The material changes color, or darkens, when exposed to an electrical current and dims the reflection of the mirror.


This type of mirror has a forward-facing sensor to detect the level of ambient light and a rearward-facing sensor to detect the intensity of headlamp glare coming from behind. When the front sensor senses low ambient light levels, the back sensor is activated. The darkness of the mirror is determined by the intensity of incoming light detected by the back sensor. This is done by applying the appropriate amount of voltage to the electrochromic material in comparison to the intensity of the glare.

The more voltage that is applied to the electrochromic material, the darker it gets. When the sensor does not detect any light, the voltage is removed and the mirror returns to its normal state.

Auto-dimming rearview mirrors have been around for a few years and are included in many vehicles. Aftermarket kits are also available. This feature is also available for the outside rearview mirrors on some vehicles. We may even see electrochromic or other dimming technologies for vehicle windows in the future.

Other rearview mirror features

Rearview mirrors may also be used for displaying various types of information such as temperature, direction, or air bag status. And, as mentioned earlier, the rearview mirror is a prime location for the antenna and transmission devices used for radio, communication, and global positioning systems. Some of these will require recalibration if the battery is disconnected or allowed to drain.

Identification and inspection

After repairs to a vehicle have been made, it is important to verify that all of the electronic accessories are in proper working order. The first step in this process is to identify what features the vehicle is equipped with. In the past this did not usually include an inspection of the interior rearview mirror. Today, this is an important step.

Although many of the features integrated into the rearview mirror are not required for the function of the mirror itself, it is important for the technician to be familiar with all of the parts. A visual inspection of the rearview mirror may help determine whether or not it has any electronic capabilities. Look for extra parts on or around the mirror. A wire harness stemming from the headliner to the base of the mirror is one indication.

Other signs may include sensor lenses, base modules, displays, or accessory buttons and controls. Many features are not standard equipment and are only offered as options. Electronic aftermarket rearview mirror kits are also available. Referencing the vehicle owner's manual or contacting the owner may be necessary to determine the functions of the rearview mirror. Always refer to the vehicle service manual or online service information before removing advanced rearview mirror systems.

A further inspection of the rearview mirror may be necessary when troubleshooting vehicle accessories. An obstructed optical sensor may be the cause of an improperly working accessory. Make sure that all sensors are clean and not blocked from view. When cleaning sensors, spray the cleaner onto a towel and gently clean the surface. Do not spray cleaner directly on any electronically equipped mirror. This is to prevent liquid from entering the housing and possibly damaging the electronic parts inside.

Check to see if other accessories that use the mirror are in proper working order. Inspect the wires and connection terminals that are routed to the mirror for damage. Look for pinched or cut wires. Damaged wires may be repairable using general wire repair guidelines. This should be verified with the vehicle service information.

Also inspect the connection terminals to make sure they are not damaged and fit firmly and secure. Refer to the vehicle service information for symptom flowcharts and electrical schematics. A DVOM or scan tool may be necessary for diagnosing the electrical system. Electronic parts are normally not repairable, and will most likely require replacement.

Handle the rearview mirror carefully when replacing a windshield. Refer to vehicle service information for removing any advanced rearview mirror system. To prevent the mirror from getting knocked to the ground or other accidental damage, avoid placing the mirror on the seats of the vehicle. Place the mirror in a safe location until the windshield is replaced and ready for the mirror to be reinstalled. Again, refer to the vehicle service information when reinstalling the mirror. Some features may require recalibration or installation of special software.


An interior rearview mirror might be made of more than just plastic and glass. It may include parts or contain the entire units of various electronic accessories. A computer module, essential for the function of the vehicle itself, may be located inside. If damaged, proper identification and verification of its functions will help ensure that it is properly replaced with the correct part. Unnecessary replacements can be avoided if the mirror is handled carefully during a windshield replacement process.

This Advantage Online article first appeared in the I-CAR e-newsletter, which is published and distributed free of charge. I-CAR, the Inter-Industry Conference on Auto Collision Repair, is a not-for-profit international training organization that researches and develops quality technical education programs related to collision repair. To learn more about I-CAR, and to subscribe to the free e-newsletter, visit or contact I-CAR Marketing Communications Specialist Brandon Eckenrode at


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