If a voltmeter between the power window motor and ground reads 13 volts when the switch is closed, what is a likely fault?

• A. Faulty motor
• B. Open in the power supply
• C. Short to ground
• D. Break in the ground connection

Answer: (D)

When a DC motor is supposed to run, current must have a complete path from the power source, through the switch and motor, back to the battery (ground). If you measure the full system voltage (about 13 volts) between the motor and ground with the switch closed, the supply side is energized but there’s no usable return path. The motor’s ground lead is open, so the circuit isn’t complete and the motor won’t operate. The voltmeter’s high input resistance can pull the floating motor terminal up toward the supply, making it read 13 volts even though current can’t flow to drive the motor. The most likely fault is a break in the ground connection.

If the motor itself were defective, or if the power supply were open, you wouldn’t reliably see the full voltage at the motor. A short to ground would usually cause a fuse to blow or the voltage to drop under load, not simply read as the full supply with no current.