DIY: Adding a shock sensor to the OEM alarm

[B-R-Z]

This DIY will show you how to add a shock sensor, or any type of aftermarket sensor to your OEM alarm system (if equipped).

Parts needed:
DEI 504D Shock Sensor - Bought off of eBay for $12 shipped.

SPDT 12VDC Relay (Referred to as relay #1) - I bought the below from eBay for $4.50 shipped: Link

SPST or SPDT 12VDC Relay WITH 30mA Nominal coil current (Referred to as relay #2) - I used SPDT Radio Shack p/n 275-0031, it's $4.99. The coil current is important here because if it's too high, the sensor might not be strong enough to trigger it. 30mA definitely works with this sensor. You can use SPST or SPDT for this one, if you use SPDT, you do not use one of the pins where if you use SPST, all pins are used.

Option Connector Harness from svxDC (Optional) $4 - This is a harness that connects to an unused plug behind your radio. It is useful because it provides Constant +12VDC, GND, and Accessory +12VDC. If you have some other means of getting these 3 lines of power then you do not need this harness. I already had it from a backup cam that I installed so I added wires to the existing wiring I had.
Radio Shack small project box, p/n270-1801: Not required, but I used it to house relay #2.
Radio Shack diodes, p/n IN4003: This probably isn't required but when I was doing research many people recommend this to isolate your sensor from anything else in the path of the alarm trigger wire. I'm pretty sure you do not need this, but I had the diodes already from a shock sensor install in my S2000 so I decided to throw it in. If you decide to use this, put it on the end of the blue sensor wire, gray strip towards the sensor.

Wire - I believe I used 18-20awg, nothing thick is needed.
Soldering iron, solder, shrink tubing: Not really required, but I prefer to solder all connections. Crimp connectors can be used instead.
Zip ties

Explanation of how it works:
First part is the shock sensor itself. The sensor isn't needed while the car is running so relay #1 is used as an on/off switch for the sensor to make it only have power when the car is off. When accessory power exists, the coil in the relay engages which disengages the sensors power supply. Likewise, when accessory power is lost, the sensor gets a power supply and is enabled. A 'side effect' you will notice is if you have your back seats down and your car (accessory power) is off, you may see the trunk light flicker when the shock sensor senses impact. Your alarm will not be triggered unless it is armed, key fob is 10+ feet away (BRZ Limited only), and the alarm has been armed for a few minutes; these may not be exact numbers, but it's approximates of what I noticed in testing.

The next part is triggering the alarm. The sensor outputs a ground to the blue wire when it senses impact. You should technically be able to connect the blue wire directly to the alarm trigger wire, since the alarm will go off when this trigger wire is grounded--and this is how I wired it on my first attempt. I found that the sensor was making the trunk light turn on when it was impacted, but the alarm would not trigger. This is where relay #2 comes in. It turns out that the ground outputted by the sensor is not strong enough to trigger the alarm. So to fix this issue, the sensor trigger output wire is used to fire relay #2 which then gives the alarm a higher power ground signal to trigger the alarm.

1. The wire that is used to trigger the alarm is the wire that makes your car and alarm think that the trunk was opened. There is a harness under the rear deck on the drivers side that has a black/white wire in it. You need to connect a wire to that wire and then run it up to the front of the car. I removed the pin from the plug, wrapped the wire around the metal pin, and then re-inserted the pin in the plug. I ran the wire behind the drivers side rear arm rest and under the door sill and then under the drivers side dash, zip tying along the way to keep it secure. You could find where this particular wire ends up under the dash instead of running a wire from the trunk, but since there are so many wires I didn't want to take the risk of choosing the incorrect wire and breaking something.


2. Wire up your relays and sensor as outlined below:



Once all of your wiring is done, zip tie all wires and relays to secure them.

3. The sensor itself should be zip-tied to a thick bundle of wires, or if you prefer less sensitivity, it can be secured to a solid surface. The sensor does have an adjustment screw on it to allow adjustability. In my BRZ and S2000, I zip-tied it to a bundle of wires and it seemed to work perfect.

The sensor zip tied onto wires. Notice the orange sensitivity adjustment screw:


The wiring:


The shock sensor has 2 output types. With minor shock it sends out a short pulse of ground which would trigger an aftermarket alarms 'warn-away' beeps. With major shocks, it sends out a longer pulse of ground which fully triggers the alarm. In my testing, I've only seen the alarm trigger when it gets the full shock--there is no warn away functionality.

To test, make sure the accessory power is off, put the back seats down, and tap the sensor with your hand. You should hear relay #2 click and see your trunk light turn on for .5-1 second, depending on how hard you tapped the sensor; short flash is warn away, long flash is full alarm. If that works, arm the alarm, throw your keys a few feet away, wait 3-5 minutes, and give the car a decent bump.

I think I covered everything, if you have any questions let me know.

Mods, please move to DIY section.

[phanguy]

i don't have a brz, or the need for this since i have an aftermarket one, but well written, well organized instructions always give me a raging nerd boner. good work on this!

[k.houan]

SPST or SPDT 12VDC Relay WITH 30mA Nominal coil current (Referred to as relay #2) - I used SPDT Radio Shack p/n 275-0031

Is there another relay I can use in relay #2's place? I cant seem to find the exact one that you used. Im really trying to get all of the same parts you used so I can follow your guide correctly.

[B-R-Z]

This should work: http://pages.ebay.com/link/?nav=item...281560&alt=web

If you don't want to wait for it to be shipped, just go to radio shack and look in the relay drawer and find one with a 12v 30ma coil rating.

[k.houan]

Originally Posted by B-R-Z

This should work: http://pages.ebay.com/link/?nav=item...281560&alt=web

If you don't want to wait for it to be shipped, just go to radio shack and look in the relay drawer and find one with a 12v 30ma coil rating.

I was about to order that exact relay, so the blue one is layed out the same as the one you used? So I wouldnt have to wire it any differently?

[B-R-Z]

I haven't compared it to the one I used, but they do the exact same thing. There is a diagram in the description so I can tell you how to wire it.