ASE G1 Auto Maintenance and Light RepairHeating and Air Conditioning (HVAC)Hard
A technician is diagnosing an automotive HVAC system where the blower motor operates only on the 'HIGH' setting. All other settings produce no airflow. What is the MOST likely cause?
- AA seized blower motor bearing.
- BA faulty blower motor.
- CAn open circuit in the 'HIGH' speed wiring.
- DA defective blower motor resistor or control module.
Show answer & explanationAnswer & explanation
Correct answer: D. A defective blower motor resistor or control module.
The blower motor resistor (or a solid-state control module) is responsible for regulating current to the blower motor to achieve different fan speeds. The 'HIGH' setting often bypasses the resistor entirely, sending full battery voltage directly to the motor. If only 'HIGH' works, the resistor or its associated control circuit (module) is the most likely culprit.
Why the other options are wrong
- A. A seized bearing would prevent the motor from turning at any speed, or cause it to make loud noises, not just fail on lower settings.
- B. If the blower motor were faulty, it would likely not operate at all, or make unusual noises, not just fail on lower settings.
- C. An open circuit in the 'HIGH' speed wiring would prevent the blower from working on 'HIGH', which contradicts the symptom described.
Blower Motor Resistor Function
The blower motor resistor regulates the current flow to the blower motor to provide different fan speeds, with the 'HIGH' setting often bypassing it.
- Creates resistance to reduce motor speed.
- Usually located in the blower motor housing/ductwork.
- Can fail due to overheating, causing loss of lower speeds.
- 'HIGH' speed typically gets full voltage, bypassing resistor.
Memory trick: Low speeds are gone, resistor's undone, but HIGH keeps on.