To my understanding, a simple first-step for testing the alternator and related components on a vehicle is to monitor the voltage (using a multi-meter) at the battery while the vehicle is running. According to my research, one should get around 13 to 14+ volts; however, what is "proper" seems to vary by vehicle.
With that in mind, how does on accurately determine what the voltage should actually be for a particular vehicle. For example, is there a relationship between engine displacement, number of cylinders, etc. and the expected reading?
To provide some context, my sister has a 1991 Cadillac Deville with a 4.5L engine. She had the electrical system tested (at a parts/battery store) and was told that the voltage was "low" for that particular vehicle. I tested it myself, and received readings in lower to mid 13.x range. While I believe such would be acceptable for some vehicles, I am not certain if it is acceptable for that particular model and year of vehicle. I could not find the satisfactory range within the Chilton's manual for that car.
How does one determine what the appropriate voltage (at idle) is for a specific vehicle?