iluvathar:
Sure 2A ... the engine manufacturer output.
I don't think that's what it means. It suspect it means maximum 2A. It makes no sense to say the output is always 2A, if you are also saying the voltage is fixed (12V). The current depends on the resistance. If you only want 1mA, you can have that by choosing the resistance you use.
Also not convinced that an opto-isolator is going to protect your circuit by itself. Sure, it will help prevent high voltage spikes reaching the Arduino. But won't the opto be damaged by these spikes? Whichever component fails, the circuit will still need repair. If this were a commercial product, the labour cost of the repair would dwarf the cost of any of the components. Surely its better to include some components to protect against those spikes? Like a zenner diode perhaps? I know ultimately nothing can protect 100%. But how to get from 99% to 99.99% protection...