The new, transformed PWM output from the Arduino, must be 12V so that the motor control unit in the vehicle can read the new signal.

Are you absolutely sure of that? It's probable that the ECU has a 5V supply and is expecting a 5V input.
Depending where you live, any vehicle modification such as this may invalidate your vehicle insurance and make the vehicle no longer street-legal.

 

Are you absolutely sure of that? It's probable that the ECU has a 5V supply and is expecting a 5V input.

This is a very common idea that people have. A car uses a 12V battery so all electrical things involved with the car must be 12V. It was that way in the past, before computers in a vehicle, so people that don't understand how things work think it is that way now, but it isn't.

 

Every automotive, heavy truck, heavy equipment system that runs on 12 volts that I know of, will provide 12 vdc to the ECU, which will have a voltage reference system providing 5 vdc to various sensors for feedback. It will also have various output voltages to run injectors, actuators etc but always 12 vdc input + 12v switched input + ground.

 

Thanks for the clarification.

 

Which Hall sensor are you connecting to and what outcome are you looking for? You have my curiosity.

I don't know what type of hall sensor this is. It has no labels on it, its for an ATV. As mentioned earlier, this is a 3-pin hall sensor, where the supply voltage for the sensor is 12V. I've measured that!
Here's how it looks : https://i.ebayimg.com/images/g/JgMAAOSwFAZTtZce/s-l300.jpg

I'm aware of the vehicle insurance issues and all that, but thats not an problem I have to deal with.

How can I create a 12V square signal from a 12V supply (vehicle battery for instance) and send this to the ECU? I use an Arduino UNO and control a BC547A NPN transistor from it. What kind of circuit must I have, in order to "limit" the current (10mA maybe), but at the same time maintain 12V signal for the ECU?
All answers related to the actual topic will be greatly appreciated.
Thank you.