A normal winding ohms check and an insulation-resistance test ask different questions. In 313A troubleshooting study, confusing the two can turn a useful measurement into an unsupported diagnosis. Fluke's compressor testing guidance distinguishes winding resistance from insulation leakage; use the compressor and instrument manufacturer's procedures for actual service.
What does an ordinary ohmmeter tell you?
With an isolated, de-energized compressor, a low-voltage resistance measurement between winding terminals helps check continuity and compare winding paths. Our common, start and run walkthrough shows a three-reading example. The reading may reveal an open path or an obvious ground fault, but a normal terminal-to-terminal value does not establish that insulation to the shell is healthy.
Before any test, follow an authorized lockout and isolation procedure, verify absence of hazardous energy and address stored energy. CCOHS notes that capacitors may retain charge.
What does an insulation-resistance tester add?
A megohmmeter applies a specified DC test voltage and measures very high resistance through insulation, commonly from isolated winding terminals to the compressor shell. Fluke explains that this can reveal leakage that an ordinary ohmmeter misses. It also warns to isolate connected electronics and wiring before testing; applying test voltage through controls can damage them.
For a hermetic compressor, refrigerant, oil, moisture and winding temperature can affect readings. Fluke recommends recording comparable conditions and following manufacturer guidance. Its discussion of a general motor threshold explicitly says that threshold does not simply transfer to a motor immersed in refrigerant and oil. Avoid declaring a compressor good or bad from one generic megohm number.
How would you interpret two study readings?
Imagine a disconnected compressor with terminal-pair ohms that follow the expected winding pattern, but a low insulation-resistance reading from the joined terminals to clean shell metal. The first result describes continuity between winding points; the second raises an insulation or contamination question. Record conditions, confirm the test setup and consult that compressor's service literature before deciding on a field action. This is an invented reasoning example, not an exam question or a service pass criterion.
Fluke cautions against insulation testing a compressor under vacuum. For exam preparation, explain what each meter is measuring before memorizing a number. Link this concept to your study plan, the current exam outline and the original, unofficial free sample, whose draft questions await certified-mechanic review.
Sources
- Fluke: Insulation resistance and electrical testing for HVAC compressor motorsTest purpose, environmental caveats and vacuum warning; checked 2026-10-06
- Fluke: High-efficiency motor installationIsolate electronics before insulation testing; checked 2026-10-06
- CCOHS: Lockout/Tag outEnergy isolation and stored capacitor energy; checked 2026-10-06
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer exam standard; checked 2026-10-06
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.