Don't worry about unplugging any of the data cables because they are not connected to the power supply. Check the sticker on the power supply and locate the voltage information.

Power on the computer. The multimeter should read between 11 and 12 volts. If it's too high or too low, the power supply is failing and needs to be replaced. Test pin 12 as well.

You can use a multimeter with capacitance-measurement capability to measure the length of wire or cable to an open circuit. The capacitance of a pair of ...