I posted this on the mp3car.com forums already, but I figure I'll throw it in this thread too.
How To : Remote start CarPC using your car alarm
I just thought I'd post this since a lot of people might be interested in doing something similar. My problem was that I didn't like waiting for my CarPC to boot up when I got in the car. Further complicating things was the fact that the weather in Nebraska where I live gets anywhere from extreme cold to extreme hot. For this reason I did not want to simply wire my computer to the remote start. Having it start separately means that in the winter when it's cold I can remote start the car, then remote start the computer after it warms up while I'm walking out, or during the summer just remote start the computer without wasting gas.
Now this solution isn't for everybody, you need to have some sort of unused auxillary function on your car alarm (or any method of remotely triggering a constant +12v line). I had door lock and trunk pop and two unused ones. I also have an OPUS power supply for my PC, meaning that it can turn on and off with only a simple trigger wire, most people use the ignition, exactly how an amp turns on and off. You can do this even if you don't have an OPUS power supply with an ignition trigger, the wiring will just be a bit more complicated. You must have a latched output on your alarm, not a pulse or a timed output, the trigger must go live and stay on until the ignition is triggered. (If you don't have a latched output on your alarm, you can make your own relay system that converts a single pulse to a latched output, see
THIS article on the12volt.com)
So now without further hesitation, the simple solution :
All you need is a standard relay, some wire, and a few different styles of crimp connectors. You need access to a constant +12V wire (low amperage is no problem, I think mine was 22Gauge), your computer-turn-on wire (most people use ignition), and your alarm's trigger wire.
Step one : Program your alarm function. The extra function I used on my alarm only turned on for a brief time by default, however it could be programmed to turn on and stay on until the ignition is triggered. This was perfect for my purposes so I switched it over to that mode, now when I trigger that function the wire goes live and stays on until the key is turned on.
Step two : Locate a constant +12V power wire.
http://www.installdr.com has a nice wire harness diagram that I used to locate one. A multimeter is your friend.
Step three : Cut your computer turn on wire. Usually this is the ignition wire.
Step four and up : Attach your alarm's trigger wire (the one that goes live when you hit the function key) to terminal 86 on the relay. Attach your constant +12v source to both terminals 85 and 87. The ignition wire should run through the relay entering on terminal 87a and exiting on terminal 30.
Here is the diagram I made to explain this better
That's it, you're done. Just tidy up your wiring,mount your relay down somewhere, and put everything back together that you might have torn apart to access the wires used for this simple mod. If everything was done correctly when a charge is activated to terminal 86, current from 87 should flow through 30 triggering your computer to start. When there is no current on 86, the relay connects 87a to 30, which means your computer will turn on and off with your key just like normal. In simpler terms, triggering your alarm function should now start your computer and it should still work with the key as well.
Note : It is important that you program your alarm correctly. The trigger has to go live and stay on, but also turn off when the ignition is turned on. Most alarm functions have this option as well as a timed mode. If you do not set it properly it might only start your computer for a short time, or it might start it and never turn it off unless you trigger the alarm functions again, this would effectively make the key not do anything.
