What it is

When a compressor dies electrically, it dies one of three ways: grounded (a winding shorted to the metal shell), open (a winding burned in two so current can't flow), or shorted (windings shorted to each other so they bypass turns). Each has a clear electrical signature you can confirm with a meter. Condemning a compressor is an expensive call, so the whole point of this is to be certain — to prove the failure, rule out the cheap stuff, and not replace a good compressor because of a tripped overload or a bad capacitor.

How it works

Inside the compressor are copper windings coated in enamel insulation, sitting in a steel shell. Three things can go wrong with that picture. Grounded: the enamel breaks down and a winding touches the shell — now there's a path to ground that shouldn't exist. Open: a winding overheats and the wire literally melts apart, breaking the circuit — current can't flow through it at all. Shorted: the insulation between adjacent turns fails and current jumps across, shorting out part of the winding — the motor draws high current and makes no useful torque. A standard ohmmeter catches grounds and dead opens and dead shorts; a megohmmeter (megger) catches the early grounds an ohmmeter misses by testing the insulation at high voltage.

In the field

Isolate first. Lock out, discharge the caps, and remove all three terminal wires. You're testing the compressor alone.

Test for OPEN (ohmmeter, winding to winding): measure C–R, C–S, and S–R.

  • All three sensible low-ohm values that add up (C–R + C–S ≈ S–R) → windings continuous.
  • Any pair reads OL → that winding is open. But before condemning, let a hot compressor COOL — an open reading on a hot compressor is often just the internal overload tripped, and it'll read continuous again once cool.

Test for SHORTED (ohmmeter, winding to winding): the add-up check does double duty.

  • C–R + C–S noticeably less than S–R, or readings near zero → shorted turns. A dead short across windings reads ~0 ohms.

Test for GROUNDED (ohmmeter and megohmmeter, winding to shell):

  • With an ohmmeter, measure from any terminal to clean bare metal on the shell. You want OL (infinite). Any continuity, even a few thousand ohms, is a hard ground — condemn.
  • An ohmmeter only catches a dead ground. For a marginal or incipient ground, use a megohmmeter: it applies a high test voltage (commonly 500V) between a winding and the shell and reads insulation resistance in megohms. Strong insulation reads high; a winding starting to break down reads low and is on its way to a dead ground.

Normal values & targets

  • Winding-to-winding: low-ohm, balanced, and C–R + C–S ≈ S–R. (See the C/S/R article for the relationship.)
  • Winding-to-shell with an ohmmeter: OL / infinite. Anything that isn't OL is a ground.
  • Megohmmeter insulation resistance, winding-to-shell: a healthy compressor reads high — generally many megohms (often quoted as a minimum around 1 megohm or much higher; the higher the better). A reading trending down toward the low single-digit megohms or below signals failing insulation even if the ohmmeter still shows OL.
  • A dead short across windings reads ~0 ohms; a dead ground reads near 0 ohms to the shell.

Common faults & what they mean

  • Continuity from a winding to the shell → grounded compressor; condemn it.
  • Megger reads low insulation resistance but ohmmeter shows OL to ground → insulation breaking down; the compressor is failing even though it might still run today.
  • One winding reads OL winding-to-winding, compressor is cool → open winding; condemn.
  • One winding reads OL but the compressor is hot → likely a tripped internal overload, not a failure; cool it and re-test before condemning.
  • Windings read ~0 ohms or the add-up check is badly off → shorted windings; condemn.
  • Breaker trips the instant the compressor energizes → dead short or dead ground pulling massive current; do not keep resetting it.

Tech tips & gotchas

The most expensive mistake in this diagnosis is condemning a hot compressor that only had its internal overload open. An open winding reading on a compressor that just tripped is suspect — give it time to cool (it can take a while), then re-ohm. If it now reads continuous and balanced, the winding was never open; you've got an overload-tripping problem to chase (high head, low charge, weak cap, high ambient), not a dead compressor.

An ohmmeter is a coarse ground tester — it only finds a ground that's already complete. A megohmmeter finds the ground that's coming. On a high-value compressor, or when you suspect a moisture/acid problem, megger it before you decide.

Rule out the cheap external stuff before condemning: a bad run capacitor, burnt contactor contacts, low voltage, or a failed start component can all make a healthy compressor act dead. Prove the windings and the ground are the problem — not the parts in front of them.

Make solid contact on bare shell metal for the ground test. Paint, rust, and grime can fake an OL reading and hide a ground. Scrape to bright metal.

Safety / code notes

A megohmmeter outputs hundreds of volts — make sure the compressor is isolated and nobody's contacting it before you test, and discharge the windings afterward. De-energize and lock out at the disconnect (within sight, per NEC Article 440) for all testing. If you find a grounded or burnt compressor, treat the refrigerant as potentially acid-contaminated and handle cleanup accordingly (see the burnout article). Recover refrigerant per EPA Section 608 before opening the system.