A voltage resistor divider would be needed! I would use 33K/12K or similar changing your 0-15VDC to 0-5VDC. I would first verify that your alternator produces voltage as soon as the engine starts. Newer vehicles with computer controlled alternators sometimes don't turn on immediatly. I have an automatic engine starter I built on a diesel engine and I am using a 5PSI oil pressure switch to check if the engine is running or not. Perhaps your motor has one of these already for an idiot light on the dash to show that there is no oil pressure.