A technician is diagnosing an intermittent issue where a vehicle's power windows occasionally operate slowly, or sometimes not at all. A voltage drop test on the positive side of the power window motor circuit (from power source to motor input) measures 0.8V while the window is attempting to operate. What does this reading indicate?
- AThe power window motor is drawing excessive current.
- BThe battery voltage is too low to properly operate the window.
- CThe ground circuit for the power window motor has excessive resistance.
- DThere is excessive resistance in the positive feed circuit to the motor.
Show answer & explanationAnswer & explanation
Correct answer: D. There is excessive resistance in the positive feed circuit to the motor.
A voltage drop of 0.8V on the positive side is excessive for most accessory circuits (typically less than 0.5V is acceptable). This indicates that the positive feed circuit (wiring, connections, switches, or relays) has too much resistance, causing a significant portion of the available voltage to be 'dropped' across this resistance, rather than reaching the motor. This reduced voltage at the motor will cause it to operate slowly or not at all.
Why the other options are wrong
- A. Excessive current draw would cause a voltage drop, but the 0.8V drop is across the circuit, not necessarily indicating the motor itself is the problem. It points to the circuit feeding the motor.
- B. While low battery voltage would affect operation, the voltage drop test specifically measures the loss *across* the circuit, indicating a resistance issue within the circuit.
- C. This test measures the positive side. Excessive resistance in the ground circuit would be identified by a voltage drop test on the ground side.
Voltage Drop (Accessory Circuit)
Measuring the voltage lost across a conductor or component in an accessory circuit while it is operating, indicating resistance in that part of the circuit.
- Acceptable voltage drop is typically 0.2V to 0.5V, depending on manufacturer and circuit type.
- Excessive voltage drop reduces the voltage available to the load, causing slow or inoperative function.
- Performed under load to accurately reflect circuit performance.
Memory trick: High drop on positive, means resistance is the culprit.