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.
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 ultrasonic sensors 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 them.
The good news is that most decent alarms have the option to turn the sensors off when arming.
Both Vodafone Automotive and Autowatch alarms allow you to turn the ultrasonic input off by pressing the arm button on the remote a second time upon arming.
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.
I like to loop some extra cable up to make it easier for anyone who works on the vehicle at a later date. This way there is less change of damage being done in the future.
There is nothing more frustrating then having to work around someone mess from previous work. It’s best practice to think about the next guy.
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.
Other brands of ultrasonic sensors
If I’m not installing Cobra alarm then my go to sensors are either Autowatch or DEI (Viper).
Both brands make stand alone sensors which can be added to their alarms, plus most of their alarms give you the option to disable the sensors upon arming.
My main frustration is that the sensors just don’t look good. If I had them in my own vehicle the aesthetics would bug me every time I got in the car. Maybe that’s a me thing, but it’s a deal breaker!
As for the AVS sensors, well they look really shit. I could probably mould something nicer out of playdough.
Factory Located Ultrasonic Sensors
Some vehicles have factory ultrasonic slots available. I love using these as it makes for a super clean installation.
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.
