Bonnet Switches

Car Alarm Bonnet Switch

Failed Bonnet Switch

Good effort with custom made bracket but the switch still failed due to corrosion!

One of the most basic, yet overlooked items on any security system is the car alarm bonnet switch. It’s job is to protect the engine bay which once breached often means the battery can be disconnected or the siren can be attacked and silenced. Needless to say that if it fails to work it then you may as well not have an alarm installed!

Some of the bonnet switches that come as standard with many alarms are of such poor quality that I refuse to use them. Below are a couple of photos showing ones that have rusted out becoming useless:

Here is another rusty one that I replaced recently. Often there is no visible sign that the base has corroded and no longer works until it is removed.

Rusty Bonnet Switch

Rusty AVS Bonnet Switch

The location of the switch is also a big factor. It’s good practice to make them difficult to access so they can’t simply be held down with a metal rule or unplugged by popping out one of the hazard lamps!

Bonnet Latch Switches

Many cars have a factory bonnet switch already, often built into the latch. These are normally reliable and I’ll always use them when I can. Again it irks me when I see an additional switch (normally a poor quality one at that) has been fitted as the installer has failed to notice that the car already has one. This year alone I’ve seen this done to a Ford Transit and a Hyundai iLoad, both by local car audio shops who really should know better!

Bonnet Latch

Many new cars have a factory switch built into the latch.

Stainless Switches do not rust!

Car Alarm Bonnet Switch

Stainless steel bonnet switch from Cobra

Given that most vehicles get wet and exposed to the elements you would have thought that stainless switches should come as standard with alarms.

Sadly Cobra would seem to be the only supplier that I deal with would provide them as standard.

I’m assuming it boils down to price. You pay a little more for Cobra which is justified by the quality of the components.

Cutting corners a using cheap, poor quality parts is not the way to go if you want reliability.

Rust proofing drilled holes

If I’m going to drill a hole I use cavity wax to treat the metal so it does not rust. Again it’s something many installers don’t bother with but makes a big difference in the long term.

Factory mounting locations

Where possible I like to install a switch by picking up on a factory mounting location.
Many cars have empty bolt locations on the inner wing where a bracket mounted switch can be fitted.

Waterproof Switch

Here is another waterproof car alarm bonnet switch which I used braided tubing around the cable to keep the clean look of the engine bay

Car Alarm Bonnet Switch Porn

Waterproof bonnet switch on bracket with braided cable

Other options

Tilt Bonnet Switch

Tilt/Ball Switch: Picks up tilting of the bonnet

There are occasionally vehicles where finding a good mounting location is simply not possible.

When this is the case I often resort to a tilt switch on the bonnet lid that simply rolls a ball-bearing to connect the circuit when the angle changes. Again these are waterproof and reliable.

Finally, if you have an alarm I highly recommend that you test to see if your switch still works, if it doesn’t or is showing signs of wear then I recommend getting it looked at asap.


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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6 Responses to Bonnet Switches

  1. Avatar Mike Huang says:

    Lol I remember the good old discussion between you and an obviously offended alarm installer regarding the bonnet pin switch and installation. When he showed one of his installation that he drilled through the weather rubber strip to install the bonnet pin the case is closed immediately.

  2. Julian Julian says:

    Haha, yep I recall that too. So many installers don’t like to change the way they do things, let alone develop and improve the standards!

  3. Avatar Jae Ekman says:

    Hi Julian,
    I have a Porsche 944 (we’ve discussed it a long time ago previously). It has a mercury bonnet switch for an under bonnet light. Can a bonnet switch be adapted to this in the same manner as the ball bearing one you describe above?

  4. Julian Julian says:

    Hi Jae,

    I don’t see why not. I’m assuming you want to hook it up to an alarm rather than adding an extra one?

  5. Avatar Jae Ekman says:

    Yes, would love to hook up the alarm bonnet switch wiring to this without interfering with the lights operation. If you remember, my original Dynatron alarms bonnet switch rusted out like the DVS one above.

  6. Julian Julian says:

    If your mercury switch is negative switching then it is easy, if not you’ll need a relay because Dynatron only supports negative inputs.

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