Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle where the power windows operate slowly. After performing a voltage drop test on the power window motor's power and ground circuits, the technician finds a 0.8-volt drop on the ground side and a 0.2-volt drop on the power side. What does this indicate?
- AExcessive resistance in the power circuit.
- BA good power circuit and excessive resistance in the ground circuit.
- CBoth power and ground circuits have excessive resistance.
- DA faulty power window motor.
Show answer & explanationAnswer & explanation
Correct answer: B. A good power circuit and excessive resistance in the ground circuit.
A voltage drop of 0.2 volts on the power side is generally acceptable for an accessory circuit. However, a 0.8-volt drop on the ground side indicates excessive resistance in the ground circuit, which would impede current flow and cause slow operation. This is because the industry standard for voltage drop on the ground side of an accessory circuit under load is typically 0.2V or less.
Why the other options are wrong
- A. The 0.2V drop on the power side is acceptable, not excessive.
- C. Only the ground circuit shows excessive resistance.
- D. While a faulty motor could cause slow operation, the voltage drop test points to a circuit issue, not directly the motor itself.
Voltage Drop Limits (Accessory)
Acceptable voltage drop for accessory circuits under load is typically around 0.2V on the power side and 0.2V on the ground side. Higher drops indicate excessive resistance.
- Measured under load.
- Separately for power and ground.
- Ground side drop indicates poor connection to chassis.
Memory trick: For accessories to run just right, keep power and ground voltage drops tight.