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The code key system has evolved from its inception in 1995 into the system we have today but the system still works on similar principals. The first misconception is that the code key immobiliser system has anything to do with the alarm system they don't they are completely different systems and the alarm DOES NOT immobilise the car.
The code key system works like this, In the key there is a tiny bit of electronics called a transponder, this consists of a miniature inductive loop connected to a tiny transmitter that can broadcast a digital code (I am going to use 1234 as my code). Around the lock barrel is another inductive loop (called an Ariel by Alfa Romeo) and when you put a key in the ignition and turn the car on a high frequency AC signal is sent to the inductive loop around the barrel which in turn generates a current in the transponder which then transmits the transponders unique code in our case 1234 back to the inductive loop round the ignition barrel which now acts as an Ariel to receive the code and sends it to the code key box under the dash.
If the code key box recognises the code 1234 as one that is programmed into it it then sends another code, say ABCD to the injection system to allow the car to start or say WXYZ to say don't start. If the engine management system does not recognise the code it stops the fuel injectors and spark plugs from working but the engine will still turn over on the starter motor but will not fire up and the code key light will STAY on.
The most common problem we get with this system happens when people connect up jump batteries with the keys in the ignition and this spikes the code key box and it will not accept any key.
To cure the problem you have to remove the key from the ignition and disconnect the battery for over 1/2 an hour to get the system to reset then connect the battery and the jump leads again with the key out of the ignition then put the keys in the ignition and it will start.On GTV's, 145's, 146's, 155's and 164's Alfa Romeo used a master key system for programming keys. The master key is simply the first key that is put in the lock barrel and turned on it is stored in the same part of the memory as the other keys and it will start the car the same as any other key but in order to program a new key you have to have a master key. The master key does not have any affect on the starting and does not do anything magic except allow you to program other keys. If your car were to loose the part of the memory with the slave keys in it it would also loose the master key and if it did still work it would recognise the first key that was put in as its master key however the likely hood of the code system still being serviceable in this case are infinitesimally small.
When the 156 came out the system changed slightly and Alfa Romeo abandoned the master key system and changed to a key card system the difference being that you program the key using the Alfa Romeo diagnostic computer instead of a master key and Alfa Romeo also changed the key transponder to what is called a rolling code system, which means that every time you start the car it changes the code that starts the car, this is done so that theives cannot record the signal using an electronic scanner and then break into your car and use the scanned signal to start your car.
By the time the 147 came along the system changed again slightly in that the code key computer was integrated into the body computer and rather than having a seperate code to program the Alarm and the key as you have on the 156 the two systems although still seperate share the same programing code. It has to be remembered that the 156 and 147/GT whilst sharing the same platform did not share the same electronics package. On a 156 most of the computer systems stand alone while on the 147/GT all the computers are connected together through the body computer via CAN lines
http://www.alfaworkshop.co.uk/Keycodesystem.shtml
