adding sensor to oem ford alarm

I want to add a glass break sensor to my oem alarm on a 2002 ford explorer.

I want to utilize the oem door pin circuit so that the glass break sensor will create an alarm condition just like if the door had been opened. I presume this would be the easiest or only way to connect an additional sensor.

I havent really read much about people adding to oem systems (although i have seached), but the oem alarm is all i need once i add the glass break sensor. 

Basically I want the glass sensor to feed the door pin and make it as if a door was opened (when it senses glass break), thereby triggering the alarm, and I know it can be done, and ,may require a relay or diodes, I just dont know what or how to make it work.

I have found a sensor I would like to use, but it is a "negative trigger input", and I need help connecting this to the existing door pin system.

I truly appreciate any input or help, this is my first post here, and i will be proud to redistribute this info to many who would also like this additional protection, without having to buy an aftermarket alarm.

The procedure is pretty basic - locate your OEM alarm trigger:

|     DOOR TRIGGER|BLACK/ lt. blue      |+ |driver kick panel           |

and connect the sensor output to this wire.  You should isolate the OEM door pin from the sensor output, and in your case you will have to, because you will also need to convert the polarity.

If you can, find the specs for the output of the glass break sensor - if it is at least 100mA you can drive a standard automotive relay with it.  If it is less then 100mA you will need to get a reed relay (or some other type of switching device that doesn't require a lot of current).

Assuming it has a decent output, connect the (-) output of the glass break sensor to Pin 86 of a Bosch style relay.  Connect Pin 85 and Pin 30 to constant power.  Connect Pin 87 to the wire listed above.

For your glass break sensor you will have a few options, I would most likely connect the 12vdc wire directly to 12vdc, connect the ground to an ignition wire, and connect the output as listed above.  When the ignition is off it will rest at ground and provide ground to the sensor. 

One thing I would also look in to is if you truck has an OEM hood pin that triggers the alarm.  If it does, it may be a better signal to tap in to, as it won't cause your dome light to come on when it triggers!

What a fantastic and prompt answer!  I may have a few q's about the details here, but you have given me a great start!

First i will get specs on the output of the sensor, and try to find one with sufficient amperage.

"You should isolate the OEM door pin from the sensor output, and in your case you will have to, because you will also need to convert the polarity."

could you explain this part a little for me? How do I isolate the door pin from the sensor output, and how do i convert the polarity?

I assume that the negative trigger needs to be converted to positive trigger? How do I do that?

Many many thanks!

OK, I understand everything thus far, but I have two more questions...

Ive decided to use the hood pin as you suggested, so my dome lights dont blink while I drive.

However, i need to know which wire on the hood switch to use when wiring the relay as you suggested, and Is there a better spot to access that wire from the interior? (I have no wiring diagram)

Also, the sensor in question is a directed 506t, and I cant find any specs on it. Is the amperage output sufficient to use a bosch relay?

KPierson wrote:

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One thing I would also look in to is if you truck has an OEM hood pin that triggers the alarm.  If it does, it may be a better signal to tap in to, as it won't cause your dome light to come on when it triggers!

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You did that too, didn't you?  Mine was a first gen 300ZX.  I just thought I was through for the day, then I got the phone call, if it would have been the dome light, he could have turned it off, it was the passenger door light. 

Believe it or not I've never hooked up a sensor this way, but I've read a lot about it in the G35 / 350Z world - it's a very popular mod (adding a shock sensor to OEM alarm).  The issue the G35 people were having is everytime the shock sensor was tripped the window would dip down 30mm then go back up - not the best integration!  I, myself, have been waiting for someone to come up with a module to hook a dual stage shock sensor up to an OEM alarm - that would be cool!

Finding the OEM hood pin wire may be difficult to do - you will just have to trace it.

The light won't actually come on while you are driving because it will be grounded to the ignition wire - therefore when the car is on the glass break sensor won't actually have power (it will have 12vdc on the power wire and 12vdc on the ground wire).

Well,, I found the door trigger wire and the hood pin wires. One problem though...  They rest at 0v when door closed, but only rise to 1.2-1.4v when door open, instead of 12v as other have specified. Apparently that circuit runs on lower voltage. I tested all four doors and the hood pin and got the same voltage.

So now I am left with 12v that I need to step down to the 1.2-1.4v of that circuit.  I tried to hook it up with the 12v wire out from the relay, as specified in post #2, and the wire started smoking.

Could anyone tell me how to reduce the voltage I hgave coming out of the relay so I dont ruin the door pin circuit?

KPierson wrote:

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The procedure is pretty basic - locate your OEM alarm trigger:

|     DOOR TRIGGER|BLACK/ lt. blue      |+ |driver kick panel           |

and connect the sensor output to this wire.  You should isolate the OEM door pin from the sensor output, and in your case you will have to, because you will also need to convert the polarity.

If you can, find the specs for the output of the glass break sensor - if it is at least 100mA you can drive a standard automotive relay with it.  If it is less then 100mA you will need to get a reed relay (or some other type of switching device that doesn't require a lot of current).

Assuming it has a decent output, connect the (-) output of the glass break sensor to Pin 86 of a Bosch style relay.  Connect Pin 85 and Pin 30 to constant power.  Connect Pin 87 to the wire listed above.

For your glass break sensor you will have a few options, I would most likely connect the 12vdc wire directly to 12vdc, connect the ground to an ignition wire, and connect the output as listed above.  When the ignition is off it will rest at ground and provide ground to the sensor. 

One thing I would also look in to is if you truck has an OEM hood pin that triggers the alarm.  If it does, it may be a better signal to tap in to, as it won't cause your dome light to come on when it triggers!

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I hooked it up just like this ^^^ - except that BLACK/ ltblue wire is the dome light wire, and when I triggered the sensor (keys against glass) all that happened was the dome light lit for 2 seconds. Turning on the dome light even when the alarm is armed doesnt trigger the alarm.

So next i found the door pin wire, which rests at ground and shows +1.25v when door is opened.(tested)   Well, I connected to this wire instead of the BLACK/ ltblue, and what I got when I triggered the sensor was smoking wires, and no alarm. The sensor and relay are wired up as instructed, so that my output when triggered is +12v.

Any other suggestions on how to hook this up would be great, since so far I cant get it to work. I just want the glass break sensor to trigger the factory alarm.

The door pin is switch between "OPEN" and "GROUND" from door closed to door open. 

KPierson wrote:

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Are you sure the door pin doesn't rest "open" when the door is closed and ground when the door is open?

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I dont know, I checked the continuity, and its 1 when open, and 140 when closed.

Check it with a voltage meter - black lead on the suspect door pin wire and red lead on a 12vdc source.

One is close to zero meaning when the door is open the wire is grounded.  140 is still pretty low though - a voltage test should reveal the answer.

KPierson wrote:

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The door pin is switch between "OPEN" and "GROUND" from door closed to door open. 

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it sounds like this means it is negative trigger? Im not sure what this tells me. Well, I tried connecting it directly to the blue neg trigg output of the sensor with no success.  My car has been torn apart for two days and Im fraid I wont figure this out.

KPierson wrote:

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Check it with a voltage meter - black lead on the suspect door pin wire and red lead on a 12vdc source.

One is close to zero meaning when the door is open the wire is grounded.  140 is still pretty low though - a voltage test should reveal the answer.

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The whole 1 and 140 bit, my meter shows 1 with the leads not touching or probing anything, and it shows a number from 0000 to 1000 when probing opposite ends of a single wire, the closer the probes the lower the number.

And as for the voltage test, another clue? with black lead on suspect wire and red to 12vdc -  11.65v door open,  12.91 door closed.

If any of these things sound strange, know that the door ajar/int lights/keyless/oem alarm, everything works as it should and cooperatively as well.

86 to Constant, and 85 to the negative output wire from the sensor.  I know that is exactly what you said, it just didn't really look like that.

flobee4 wrote:

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All the doors/hood triggers on the Explorer rest at ground when closed, so your last measurements are correct. In order to hook up an external sensor you need to "open" or "break" the wire while your sensor is tripped. I usually would use the hood Pin wire and break that wire open with a relay. So the output of your sensor needs to be hooked up to a relay. not directly to the hood pin wire.
One end of the hood wire goes to 30 of the relay, and the other goes to 87a. Wire your relay for Constant 12V(86 on relay) and Ground(85 on relay) off your sensor output. When sensor is tripped the relay will open the hood wire, this will fool the factory alarm that the hood was opened and go off.

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HA!!! I love it!  I thought it out this morning and tried it this way ^^^ before reading this. It works perfectly!