ASE G1 Auto Maintenance and Light RepairHeating and Air Conditioning (HVAC)Hard
A technician is troubleshooting an automotive AC system where the blower motor operates at all speeds except the highest speed. All other blower speeds work correctly. Which component is MOST likely at fault?
- ABlower motor itself
- BBlower motor relay
- CBlower motor resistor block
- DHVAC control unit
Show answer & explanationAnswer & explanation
Correct answer: B. Blower motor relay
In most blower motor circuits, the highest speed bypasses the resistor block and receives full battery voltage directly through a relay. If only the highest speed fails, the relay for that speed is the most probable cause, assuming other speeds (which use the resistor) function correctly.
Why the other options are wrong
- A. If the blower motor itself were faulty, it would likely affect all speeds or not work at all, not just the highest speed.
- C. The blower motor resistor block controls the lower and medium speeds by adding resistance. If it were faulty, usually some or all lower speeds would fail, not just the highest speed.
- D. A faulty HVAC control unit could cause various issues, but if only the highest speed is affected while others work, a component in the high-speed circuit (like its relay) is more specific.
Blower Motor High Speed Circuit
The electrical path that allows an automotive blower motor to operate at its maximum speed, often bypassing the resistor block and using a dedicated relay.
- Highest speed typically bypasses the resistor block.
- Often controlled by a separate, dedicated relay.
- Failure of this relay results in loss of only the highest speed.
Memory trick: Resistor for Rests, Relay for Rush.