OBD? CAN H CAN L? Protocol? PID? FMS? J1379?
Welcome in the Jungle of digital vehicle communication.
1.) There are NO free professional ressources because all car/truck maufactors hide this informations totaly secret.
2.) On OBD Connector which ALL vehicles since 2002 must have you can get some standard informations like RPM, Motor Temerature and some other not importand informations, (Look in google for OBD2 PID)
3.) Interesting informations like Fuel Level, Real Odometer value, axis weight, drivercard ID etc. you get only, if your vehicle has a FMS connection (look in google for FMS fleet management protocol), or if your gps tracking manufactor "hacks" the canbus information. Good gps tracking manufactors provide YOU with all informations you need.
4.) We (my company) are experimenting since 2013 with many different hardware providers and units and CANBUS and we hate it. No standards, a huge time for EACH vehicle to get it running and so on.
5.) Better spent your time in more useful things :-))) (for example the new Wialon Logistics)
Politeness dictates it to write his name on a post