A technician is diagnosing an intermittent issue where a vehicle's power windows occasionally operate slowly or stop midway. After performing a quick voltage drop test across the window motor's power and ground circuits, the technician notes a 1.2V drop on the power side and a 0.5V drop on the ground side when the window is operating. The battery voltage is 12.6V. What do these voltage drop readings indicate?
- AThere is excessive resistance in both the power and ground circuits.
- BThe window motor is drawing excessive current.
- CThe battery is undercharged and needs trickle charging.
- DThe window switch is faulty and causing intermittent operation.
Show answer & explanationAnswer & explanation
Correct answer: A. There is excessive resistance in both the power and ground circuits.
A voltage drop of 1.2V on the power side and 0.5V on the ground side, especially for an accessory circuit like a power window, is excessive. Typical acceptable voltage drops are usually less than 0.2V-0.3V for power and even less for ground (e.g., <0.1V). These high readings clearly indicate excessive resistance in both the power supply wiring/connections and the ground return path, which would restrict current flow and cause the motor to operate slowly or stop. (1.2V + 0.5V = 1.7V lost before the motor).
Why the other options are wrong
- B. Excessive current draw would cause voltage drops, but the primary indicator from a voltage drop test is resistance. High resistance causes voltage drop and *limits* current, not necessarily indicates excessive draw unless the motor is binding.
- C. The battery voltage of 12.6V indicates a fully charged battery, so an undercharged battery is not the issue.
- D. While a faulty switch can cause intermittent operation, the voltage drop test specifically points to resistance in the wiring/connections, not necessarily the switch's internal contacts being the sole cause of the resistance.
Excessive Voltage Drop (Accessory)
Excessive voltage drop in an accessory circuit (e.g., >0.2V on power, >0.1V on ground) indicates high resistance in the wiring or connections, restricting current flow and causing components to operate slowly or fail.
- Voltage drop measures resistance under load.
- Common causes: corroded terminals, loose connectors, damaged wires.
- High drops on both power and ground indicate issues in both paths.
Memory trick: Slow window, high drop, resistance makes the current stop.