Hello. I'm going to go ahead and apologize in advance for being clueless on a lot of this. I am planning on trying to use an Arduino Mega as a Powertrain Control Module in a Nissan Pathfinder. The board will have to take input from the transmission and engine sensors in order to shift the transmission. Sensors include a 2 wire temperature sensor, RPM sensor (produces alternating current), 6 wires for gear selector, overdrive off switch, throttle switch, 2 wire A/T fluid temperature sensor, and 5 one wire solenoids. I have seen were similar systems have been made but I am unsure about how to go about doing it and if it can be done. The engine is from a 1985 Mercedes Benz 190D and is a 2.2L diesel. The transmission is a 4 speed automatic out of a 1994 Nissan Pathfinder. I have diagrams of wiring from both vehicles and how the transmission shifts if it will help. Anything I get will be greatly appreciated. Thank you.
It can probably be done. It is complex. Whether it can be done by a complete beginner is an entirely different matter.
There is also the question of whether a DIY transmission controller would be approved by your automobile insurer - I suspect not, but you should check with them before you start the project.
Start by writing short separate programs to learn how to get data reliably from each of the different inputs and display it on the Arduino Serial Monitor.