Compressor Diagnostics: Open, Grounded, Shorted
A compressor is the most expensive part on the truck to guess wrong about. Before you condemn one, three quick electrical tests tell you whether the motor is actually dead — or whether a $12 capacitor is taking the blame.
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In This Guide
The Three Electrical Failures
When a hermetic compressor dies electrically, it fails in one of three ways. Every one of them shows up on a meter, and each points somewhere different:
- Open — a winding (or the internal overload) has lost continuity. The circuit is broken, so the motor can't energize. You read infinite resistance where you should read a few ohms.
- Grounded — the winding insulation has broken down and a conductor is touching the steel shell. This trips breakers, blows fuses, and is a shock hazard. You read continuity from a motor terminal to bare metal.
- Shorted — turns inside a winding have fused together, bypassing part of the coil. Resistance reads lower than spec. Shorted compressors draw high amps, run hot, and trip on overload.
A healthy compressor is none of the above: solid continuity through both windings, no path to ground, and resistances that match the nameplate. Your job is to prove which bucket the unit falls into before you cut a line set.
Reading the C, S, R Terminals
A single-phase hermetic compressor has three terminals in a triangle: Common (C), Start (S), and Run (R). Two windings share the Common terminal — the run winding (C to R) and the start winding (C to S). The relationship between the three readings is the whole trick:
The winding rule
The reading across Start and Run is the sum of the two windings in series, because the meter's current has to travel S → C → R. The terminal that is not part of the largest reading is Common.
Typical resistances for a residential 1.5–5 ton compressor:
- Run winding (C–R): the lowest reading, often 0.5–3 Ω — heavy wire, few turns.
- Start winding (C–S): a higher reading, often 2–8 Ω — finer wire, more turns.
- Across the two (R–S): the sum of the other two.
If you don't know which terminal is which, take all three readings. The largest value is R–S; whichever terminal is left out of that pair is Common. Confirm with the math above. Scroll-type compressors and larger equipment run lower resistances, so always sanity-check against the data plate.
Before You Touch a Terminal
Power off, capacitor discharged, wires off
Resistance and insulation tests are done dead. Open the disconnect, then verify zero volts line-to-line and line-to-ground at the contactor with your meter — don't trust the handle.
A run capacitor holds a lethal charge. Discharge it through a 20k Ω, 2-watt resistor (or an insulated-handle screwdriver as a last resort) across the terminals before handling any leads.
Then remove the wires from C, S, and R. If you ohm the compressor with the capacitor and contactor still landed, the capacitor charges through your meter and gives you a drifting, meaningless number. Reading the compressor by itself is the only way to trust the result.
EPA 608 / A2L note: None of these are refrigerant-side tests, but if the diagnosis leads to a compressor changeout, recover the charge properly. On R-454B, R-32, and other A2L systems, no torches near a charged system, keep a leak detector running, and follow the manufacturer's service fittings and ventilation procedure.
Testing for an Open Winding
Set your meter to the lowest ohms range (or auto-range). Read all three pairs: C–R, C–S, and R–S. A good compressor gives you three real, low numbers. An open shows up as OL / infinite resistance on any pair that includes the broken winding.
| C–R | C–S | R–S | Diagnosis |
|---|---|---|---|
| 1.6 Ω | 4.9 Ω | 6.5 Ω | Good — sum checks out |
| OL | 4.9 Ω | OL | Open run winding |
| 1.6 Ω | OL | OL | Open start winding |
| OL | OL | OL | Open internal overload — let it cool |
Don't condemn a hot compressor
Most compressors have an internal overload wired in series with Common. When the motor overheats, that overload opens and every reading goes to OL — looking exactly like a dead compressor. Let the unit cool for 30–90 minutes (a fan on the shell speeds it up) and re-test. If the readings come back, the compressor is fine and you're chasing the reason it overheated: low charge, high head, a weak capacitor, or a stuck contactor.
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Testing for a Grounded Compressor
A grounded winding means the insulation has failed and copper is touching the steel shell. Symptom on the truck: the compressor trips the breaker instantly, blows the fuses, or you smell that burnt-motor odor when you crack the terminal box. This is the failure that most often burns a compressor to the ground.
A standard ohmmeter is too coarse to trust here — it might read OL on a winding that's leaking to ground at 100,000 Ω. Use a megohmmeter (megger) at 500V DC. Put one lead on a compressor terminal and the other on clean, bare copper — scrape a spot on the shell or a suction line stub, because paint and rust will fool the test.
Good insulation
A serviceable compressor reads well above 1 megohm to ground — a healthy one is often 20 MΩ or higher. New compressors can read hundreds of megohms.
Grounded / failing
Below about 1 MΩ cold (or 0.5 MΩ hot) the winding is going to ground. A dead short — a few ohms or less to the shell — is a condemned compressor.
Rule of thumb some techs use: at least 1,000 ohms per volt of rating, so a 240V compressor should read well over 240,000 Ω to ground. Whatever the threshold, a compressor trending down toward 1 MΩ over successive visits is on its way out even if it still runs today.
Testing for Shorted Windings
Shorted turns are the sneakiest failure. When turns fuse together inside a winding, they short out part of the coil, so the resistance reads lower than the nameplate value. The motor still runs — but it pulls high amps, overheats, and cycles on its internal overload. It often reads fine cold and fails once it's hot.
A cheap meter can't reliably resolve a fraction of an ohm, so pair your ohm reading with real-world evidence:
- Resistance measurably below the data plate / spec value on a winding.
- R–S no longer equals C–R + C–S — the series math breaks because part of a winding is shorted out.
- Compressor draws near or above RLA (rated load amps) and climbs as it heats, then trips on overload.
- A drawing of LRA (locked-rotor amps) with no start indicates a locked/seized rotor or a dead start component — a related but distinct mechanical failure.
Amp-draw benchmark
A healthy compressor runs below its nameplate RLA. A reading above RLA points to a shorted winding, high head pressure, or a failing capacitor; a reading below 50% of RLA usually means it's not pumping — valves or a low charge. Clamp the common leg and compare to the plate every time.
Worked Example: The No-Start Call
The call:
3-ton condenser humming, not cooling. Outdoor fan spins. Compressor hums for a couple seconds then clicks off. Data plate: RLA 16.4, LRA 88, 40/5 MFD dual run cap.
Step 1: Kill power, verify 0V, discharge the run cap.
Step 2: Pull the leads and ohm the compressor.
C–R = 1.5 Ω, C–S = 3.8 Ω, R–S = 5.3 Ω → sum checks. No open.
Step 3: Megger each terminal to bare copper.
All three > 50 MΩ → not grounded.
Step 4: Windings are good, so test the parts around it. Check the run capacitor.
Herm terminal reads 21 MFD on a 40 MFD rating → more than 6% low. Failed.
Diagnosis: weak capacitor, not the compressor
The compressor tested perfect — good windings, no ground. The hum-and-click was the motor trying to start without enough phase shift because the run capacitor lost more than half its capacitance. Replace the 40/5 dual cap (match MFD, equal-or-higher voltage rating), confirm the compressor starts and pulls below 16.4 A, and you saved a $1,200 changeout on a $15 part.
Rule Out the Cheap Stuff First
A compressor that ohms good and meggers good is almost never the problem on a no-start. Work outward from the terminal box before you condemn it:
- Run capacitor — the single most common condenser failure. Test MFD under load; replace if more than 6% off rating.
- Contactor — pitted or welded contacts, or a dropped-out coil, stops power at the compressor. Verify line voltage on both load legs while it's pulled in.
- Hard-start kit / start capacitor and potential relay — on systems that use one, a failed start assembly mimics a bad compressor exactly.
- Incoming power — confirm both legs of 240V at the disconnect; a lost leg or low voltage causes hum-and-trip.
- Overload cause — if the internal overload opened, find out why: low charge, dirty condenser, high head, or a weak cap.
The three-test summary
Open = infinite ohms across a winding (or a tripped internal overload — let it cool). Grounded = low resistance from a terminal to the shell; confirm with a megger under 1 MΩ. Shorted = resistance below spec, the winding math fails, and amps run high. Prove one of the three — and clear the capacitor, contactor, and start components — before the compressor gets condemned.
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