Core: Troubleshooting and Repair of Domestic Refrigeration and Air Conditioning Systems
Scope
This domain tests systematic fault-finding: verifying the complaint, isolating the fault to electrical, refrigerant, airflow or control, confirming with instruments, then repairing and re-testing.
Symptom-to-measurement logic
- Low suction pressure, high superheat, low subcooling: undercharge or leak - leak-search before charging.
- High subcooling with high discharge pressure: overcharge, restricted liquid line or non-condensables.
- Iced evaporator: airflow (filter, blower, duct) or low charge; check airflow first.
- Compressor hums and trips its overload: capacitor, start relay/PTC, seized compressor or low voltage.
- Breaker trips: insulation failure or short to ground - test with an insulation tester, never power up again to "see".
Electrical tests
Winding check with an ohmmeter (common-to-run plus common-to-start should equal run-to-start within tolerance), insulation resistance typically at least 1 MΩ to ground, capacitor microfarads against the printed rating, and amp draw against nameplate RLA. On inverter units read the error code and use the service manual; never apply a megohmmeter to inverter boards.
Worked example
A window unit trips its overload after 30 seconds. The run capacitor is rated 35 µF at 370 V but measures 19 µF; windings balance and insulation resistance is above 100 MΩ, so the motor is sound. Replace the capacitor with one of at least the same microfarads and voltage (a 440 V replacement is acceptable), then re-check amps and suction pressure before closing the job.
Traps
Replacing parts by guesswork; testing resistance on a live circuit; fitting an undersized capacitor; ignoring supply voltage and loose neutral; overlooking that a dirty condenser mimics an overcharge; failing to re-test after repair.