Air Conditioning Repair Electrical Systems and Controls 2 — Questions and Answers
Question 1: A technician measures voltage across a contactor coil and reads 24V AC, but the contactor does not pull in. What is the most likely cause?
- Open circuit in the coil winding (Correct answer)
- Low refrigerant charge
- Faulty capacitor
- Tripped high-pressure switch
Correct answer: Open circuit in the coil winding
If 24V AC is present at the coil terminals but the contactor won't pull in, the coil winding itself is open and must be replaced.
Question 2: Which wire color is used for the common terminal in a standard 24V thermostat wiring system?
- Red
- Yellow
- Green
- Blue or Black (Correct answer)
Correct answer: Blue or Black
The C (common) wire is typically blue or black, completing the 24V control circuit back to the transformer secondary.
Question 3: A single-phase 240V compressor motor draws significantly more current than its nameplate FLA rating. Which component failure would most likely cause this?
- Failed run capacitor (Correct answer)
- Open start relay
- Low supply voltage
- Faulty contactor coil
Correct answer: Failed run capacitor
A failed run capacitor reduces the motor's power factor and efficiency, causing it to draw excess current above its rated FLA.
Question 4: What does a locked rotor amperage (LRA) rating indicate on a compressor nameplate?
- Normal operating current draw
- Current draw when the motor shaft is stationary (Correct answer)
- Maximum allowable current with capacitor assist
- Current draw during low-voltage brownout conditions
Correct answer: Current draw when the motor shaft is stationary
LRA is the inrush current drawn by the compressor motor at startup before the rotor begins to spin.
Question 5: An air handler blower motor runs continuously but does not respond to thermostat fan mode changes. Where should the technician first check?
- Evaporator coil for ice buildup
- Fan relay or control board fan output (Correct answer)
- Blower capacitor value
- Indoor filter restriction
Correct answer: Fan relay or control board fan output
If the blower ignores thermostat commands, the fan relay or control board output signal is likely stuck closed or shorted.
Question 6: A technician finds a burned 5-amp fuse on the control board. After replacing it, the fuse blows immediately. What is the correct next step?
- Install a higher-rated fuse
- Check all 24V control wiring for a short to ground (Correct answer)
- Replace the control board
- Measure high-voltage supply to the board
Correct answer: Check all 24V control wiring for a short to ground
An immediately blown fuse indicates a short in the low-voltage circuit; the wiring must be traced to find the short before replacing the fuse again.
Question 7: Which tool is best suited for verifying that a 24V transformer secondary is producing the correct output voltage?
- Clamp-on ammeter
- Digital multimeter set to AC volts (Correct answer)
- Megohmmeter
- Capacitor tester
Correct answer: Digital multimeter set to AC volts
A digital multimeter set to AC volts accurately measures the transformer secondary output voltage directly at the terminals.
