Air Conditioning Repair Electrical Components and Controls 4 — Questions and Answers
Question 1: A variable-speed ECM blower motor is not responding to a speed signal from the control board. After confirming 24V supply to the module, what is the next step?
- Replace the capacitor
- Check the PWM or DC analog signal wire from the board to the motor module (Correct answer)
- Measure discharge air temperature
- Check refrigerant charge
Correct answer: Check the PWM or DC analog signal wire from the board to the motor module
ECM motors receive a speed-control signal (PWM or 0–10V DC) from the board; loss of this signal causes the motor to default to a fixed speed or stop.
Question 2: What is the correct procedure for discharging a capacitor before handling it?
- Short the terminals with a screwdriver blade rapidly
- Use a 10,000-ohm, 10-watt resistor across the terminals (Correct answer)
- Allow the unit to sit unpowered for 10 seconds
- Use needle-nose pliers to short the terminals
Correct answer: Use a 10,000-ohm, 10-watt resistor across the terminals
A resistor safely dissipates the stored charge without the arc that a direct short causes, which can damage terminals and capacitor internals.
Question 3: Which component in a gas furnace/AC system directly controls when the indoor blower comes on after heating ignition?
- Thermostat R terminal
- Fan limit switch or control board fan delay (Correct answer)
- Contactor coil
- Pressure switch
Correct answer: Fan limit switch or control board fan delay
A fan limit switch or control board fan-delay timer delays blower startup until the heat exchanger warms up, preventing cold air distribution.
Question 4: If a compressor's internal overload protects it from overheating but the unit starts immediately after being reset manually, what concern exists?
- The overload is too sensitive
- The underlying cause of overheating was not corrected and the compressor will trip again (Correct answer)
- The refrigerant charge is too high
- The run capacitor is oversized
Correct answer: The underlying cause of overheating was not corrected and the compressor will trip again
Resetting an overload without diagnosing and correcting the root cause (e.g., low refrigerant, dirty coils, high ambient) will result in repeated tripping.
Question 5: What is the purpose of the C (common) terminal on a 24V thermostat circuit?
- It carries the high-voltage power to the compressor
- It provides the return path for 24V AC from the transformer secondary (Correct answer)
- It signals the reversing valve
- It connects to the outdoor unit high-pressure switch
Correct answer: It provides the return path for 24V AC from the transformer secondary
The C wire completes the 24V control circuit by returning current to the transformer's secondary common, allowing smart thermostats and other loads to operate continuously.
Question 6: A technician reads 5 ohms between the compressor's common and run terminals and OL between common and start. What does this indicate?
- The start winding is open (Correct answer)
- The run winding is shorted to ground
- The compressor has normal windings
- The capacitor is faulty
Correct answer: The start winding is open
OL (overload/no continuity) between common and start indicates an open start winding, meaning the compressor motor needs replacement.
Question 7: Which wiring configuration connects the thermostat's Y terminal output back through the equipment to energize the contactor coil?
- Y → condenser fan motor → C
- Y → contactor coil → C (Correct answer)
- Y → compressor run terminal → GND
- Y → high-pressure switch → transformer primary
Correct answer: Y → contactor coil → C
The Y terminal signal travels through any safety switches in series, then through the contactor coil, and returns to the transformer via the C wire.
Air Conditioning Repair Electrical Components and Controls 4
A variable-speed ECM blower motor is not responding to a speed signal from the control board.
After confirming 24V supply to the module, what is the next step?