Ultrasonic Sensors

Ultrasonics Sensors are beyond doubt, one of the most effective sensors for protecting the internal space in a vehicle and are much more reliable than a glass break sensor.

Cobra ultrasonic sensora installed in a Toyota Hilux

Cobra ultrasonic sensor installed in a Toyota Hilux

There are normally two sensors which are typically installed at the top of the “A” pillars. One sensor transmits a high pitch sound wave (ultrasound) which is inaudible to the human ear, and the other sensor receives the signal.

The sound wave bounces off all the surfaces in the vehicle. If the wave pattern changes then the alarm system will trigger as shown in the video below.

Leaving a window open

If you do wish to leave a window open, or a dog or person in the vehicle when the alarm is armed the ultrasonics will need to be turned off to prevent a false alarm. Even a gust of wind through a slightly open window has the potential to trigger ultrasonics.

The good news is that most decent alarms have the option to turn the ultrasonics off when arming. Both Cobra and Autowatch alarms allow you to turn the ultrasonics off by pressing the arm button on the remote a second time upon arming.

You could add a COMFORT closure module to your vehicle, this way your windows will roll up every time you arm the alarm.

Intelligent CAN-BUS Systems

Some CAN-BUS alarms are able to detect if a window is left open, they will chirp the siren upon the alarm being armed to warn you and disable the ultrasonic sensors to avoid a false alarm. This feature is usually found on German vehicles and is only available if the vehicle has the window data and the alarm firmware reads it.

Installation Care

It’s vital that the ultrasonic sensors are installed with care and that they do not interfere with curtain airbags, or have the cables pinched.

Cobra Ultrasonics

Ultrasonics come as standard on all Cobra alarms sold in New Zealand. The latest generation of sensors come in a very tidy grey capsule that suits the interior or most cars. That said, not all cars have a grey interior and they can look silly if the car has black trim. The good news is that they can be painted to match. I try to carry some pre-sprayed black capsules with me to cover this at installation time.

Note: Cobra ultrasonics plug directly into the Cobra alarm ECU so are not compatible with other brands of alarms.

Painting ultrasonic capsules

Autowatch Ultrasonics

Autowatch Fixed Eye Ultrasonic Sensors

Autowatch Fixed Eye Ultrasonics

Both the fixed eye and the more common split eye version of the Ultrasonic sensor are available.

The Fixed eye sensor comes as standard with the 458RL sold in NZ and has an LED built into the center.

It can be added on to any Autowatch alarm system. Personally, I would recommend having an alarm that gives the option to disable the sensors upon arming.

Autowatch-split eye-ultrasonics

Autowatch Split Eye Ultrasonics

The Split eye Autowatch sensors lack the original equipment appearance that the Cobra capsule-style ultrasonics have.  They are still tidy and come in black which looks acceptable regardless of the vehicle interior colour.

They can be added to most alarm systems, even if the alarm is not an Autowatch.

Factory Located Ultrasonics

C Class (W204) alarm sensors

Some vehicles have factory ultrasonic slots available. I love using these as it makes for a super clean installation.

Here are a couple of examples:

Mitsuibishi Pajero, Click to see more…

Not Suitable for all vehicles

Ultrasonics are not suitable for vehicles that are not sealed such as rag-top convertibles.

If you have a cloth top car then a microwave sensor would be a better solution.

A full list of alarm sensors can be found here…

Car Alarm Sensors


About Julian

Owner and Installer at Obsessive Vehicle Security Limited. More details here... Please keep Comments relevant to the post and use the Contact form for enquiry's
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3 Responses to Ultrasonic Sensors

  1. Pingback: Cobra AK4218 | Obsessive Vehicle Security Blog

  2. vince says:

    Have you ever experimented with hiding ultrasonics?
    The first thing a thief would do is look at the A-pillars for their presence.
    Smash the window and rip one out … Then come back 10 mins later and empty the car undisturbed. I was thinking myself to put them behind the airvents or so. Would that work?
    PS : I also always heard to put them on the bottom of A-pillars, instead of at the top. At the front of the dash/windshield. Can’t remember why.

  3. Julian says:

    Hi Vince,

    No I have not experimented with hiding the ultrasonic sensors.
    The sensors need be placed so they have a good view to cover the back of the vehicle, if they are mounted low down then they will not get good coverage of the cabin as the sound waves would bounce off the front seats.

    Placing the sensors behind the air vents is a terrible idea. It’s the one spot inside the cabin where air movement is most likely, therefor becoming the prime location for a false alarm!

    Technically the area in front of the sensors is not covered, but the act of smashing a window would still trigger them due to the air movement caused by this.

    There have been cases of the front glass being scored and cut so that the OBD-II plug can be accessed without triggering the alarm (see OBD-II remote cloning). That’s the best argument for placing them low down, but it would limit the coverage at the back of most vehicles.

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