What it is

A single-phase compressor has three terminals: Common (C), Start (S), and Run (R). Inside are two windings — the run (main) winding and the start (auxiliary) winding — that share the common terminal. Being able to identify the three terminals and ohm the windings is a core compressor diagnostic. It tells you whether the motor is electrically whole before you blame the charge, the metering device, or anything else. If the windings are bad, nothing else matters.

How it works

The two windings join at Common. The run winding goes from C to R, and the start winding goes from C to S. The run winding is heavier-gauge with fewer turns, so it has the lowest resistance. The start winding is finer wire with more turns, so it has higher resistance. The reading from S to R is the two windings in series, so it's the sum of the other two — and it's the highest of the three.

That relationship is how you identify unmarked terminals: measure all three pairs, and the math sorts them out. The largest reading is across S–R (start + run in series). Of the remaining two readings, the smaller is C–R (run winding) and the larger is C–S (start winding). The terminal common to both of the smaller readings is C.

In the field

Set up: lock out the disconnect, discharge the run/start capacitors, and pull the wires off all three compressor terminals so you're reading only the windings. Set your meter to ohms (a low resistance range; a meter that resolves tenths of an ohm is ideal).

Identify the terminals: measure all three combinations — terminal-to-terminal three ways. Apply the rule above:

  • Highest reading = the S-to-R pair (so the terminal NOT in that pair is Common).
  • Of the two readings that include Common, the lower one is C–R and the higher is C–S.
  • That confirms R (lower) and S (higher).

Then verify against the factory markings or the wiring diagram if present.

Confirm the windings are good:

  • The three readings should be sensible low-ohm values, and C–R + C–S should roughly equal S–R. If that addition holds, the windings are continuous and balanced.
  • Check each terminal to the compressor shell/ground — this is the ground test (covered fully in its own article): you want infinite resistance (OL) to ground.

Normal values & targets

  • Run winding (C–R): the lowest of the three readings, often in the low single-digit ohms to low tens of ohms depending on compressor size — bigger compressors read lower.
  • Start winding (C–S): higher than C–R.
  • S–R: the highest reading, approximately equal to C–R plus C–S.
  • The addition check: C–R + C–S ≈ S–R within measurement tolerance. If it's way off, suspect a reading error or an internal fault.
  • Terminal-to-shell (ground): infinite / OL. Any continuity to ground is a grounded compressor.
  • Exact ohm values vary widely by model; the relationship between the three readings is the reliable tell, not a universal number.

Common faults & what they mean

  • One pair reads OL (open) → an open winding; the compressor is electrically failed (open start or open run).
  • C–R + C–S does NOT add up to S–R → bad reading, wrong terminals, or shorted turns inside a winding.
  • Any terminal reads continuity to the shell → grounded compressor, condemn it.
  • All three read 0 ohms to each other → shorted windings, condemn it.
  • Readings look fine cold but the compressor trips on overload when running → could be an internal overload open from heat, mechanical, or a high-amp condition — windings being okay cold doesn't clear a hot-failure.

Tech tips & gotchas

The add-up rule (C–R + C–S ≈ S–R) is your friend on an unmarked terminal block. It both identifies the terminals and sanity-checks the windings in one set of measurements. If the numbers don't add up, stop and recheck before condemning — a fat lead or a bad probe contact throws low-ohm readings off.

A compressor that's hot from a recent trip may show an open reading because the internal overload (line-break protector) has opened, not because a winding is bad. Let it cool and re-test before you condemn it — many "open winding" calls are just a tripped internal overload that resets when it cools.

Use a meter that reads down to tenths of an ohm on these. Compressor windings are low resistance, and a cheap meter that only resolves whole ohms can't tell a good run winding from a slightly off one. Test-touch your leads together first to know your lead resistance and subtract it.

Always pull all three wires before ohming. Leave them connected and you're reading the capacitor, the contactor, and any start gear in parallel with the windings — garbage in, wrong diagnosis out.

Safety / code notes

Discharge both capacitors and lock out at the disconnect (required within sight per NEC Article 440) before touching the terminals. Never apply power with the terminal cover off and never bump-start a compressor at the terminals without proper protection — a shorted terminal can arc violently. If a terminal looks burnt or has blown out, treat the compressor as failed and inspect for a terminal-venting hazard before re-energizing anything.