Most car alarms sense voltage drop. If you open a door with the alarm armed it will register the voltage drop as the interior light comes on and trigger the alarm. Get a multimeter and check your battery. It should have a voltage of about 12.5 volts. Have someone turn the engine over while you check for voltage drop while its trying to start. If the voltage drops to low the alarm will sense this and activate and will cut power to the ignition and or the starter motor. If this happens check all conections on the battery for tightness and cleanliness. Check all earth points with the multimeter if an earth point shows no reading on the multimeter take it off whatever it is attached to and clean all the wires with a wire brush. Also clean the earth point the wires go to. You need bare clean metal to give a good earth point. If the battery voltage is low consider changing it. I have had two cars with the same problem and it only gets worse as the weather gets colder as its doing now.
Simon Mayes
October 2005
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