Condenser Fan Runs Compressor Does Not Decision Tree

Why this matters

Fan spinning, compressor silent is the opposite failure mode of the fan-down call and the diagnostic logic is different. The compressor side draws ten times the start current of the fan, runs off a dual-run capacitor (HERM terminal) or a separate hard-start kit, and has its own internal overload that can mask a real problem as a "fan-only" presentation. Diagnose the path in order: signal, voltage, capacitor, overload, windings, mechanical. Skipping a step is how techs sell new compressors on systems that needed a $40 capacitor or a stuck-on overload reset.

Symptom presentation

Top condenser fan running smoothly. No compressor hum, no startup current draw, no movement on the suction or discharge gauges. Cabinet stays cool. Thermostat calls for cool, the contactor pulls in. Sometimes the compressor will hum loudly for 1 to 3 seconds then quit (locked rotor / overload trip); sometimes it does nothing at all.

Quick checks (under 2 min)

Step 1: Listen at the contactor. If the contactor is pulled in, you have a control signal. If it's not, the problem is upstream (thermostat, low-voltage transformer, control board, anti-short-cycle delay timer, low-pressure switch open).

Step 2: Hand on the compressor body. If it's hot (well over ambient and uncomfortable to touch), the internal overload has tripped on a previous attempt. Let it sit 30 to 45 minutes. Many "no compressor" calls become "running fine" calls after the overload resets.

Step 3: Visual on the dual-run capacitor. The HERM terminal feeds the compressor start winding. Bulged top, leaking, or burnt - replace before further diagnosis. Read microfarads on the HERM side specifically (not C side, which is the fan).

Step 4: Voltage check at the contactor load side with the contactor pulled in. Should read 230 to 240 VAC (split-phase residential). Below 200 VAC under load = upstream feeder problem; pursue that, not the compressor.

Isolation tree

Branch 1: Contactor not pulled in. Move upstream. Verify 24 VAC at the contactor coil during a call for cool. If absent, the cause is:

• Thermostat not closing (battery, settings, miswiring).
• Anti-short-cycle delay still active (wait 5 minutes, retest).
• Low-pressure switch open (system is in vacuum, no refrigerant - separate refrigerant leak job).
• Float switch or condensate overflow open.
• Outdoor control board fault. Resolve and retest. If 24 VAC present at the contactor coil but contactor does not pull in, replace the contactor.

Branch 2: Contactor pulled in, voltage at line terminals normal, voltage at load terminals normal, but only one of three legs on a 3-phase system. A single-phasing condenser will not start. Burned contactor pad, blown fuse, broken conductor. Inspect, repair, restart.

Branch 3: Contactor pulled in, dual-run capacitor reads bad on HERM side. Replace with exact microfarad match (HERM and FAN sections both - dual-run capacitors fail one side at a time but the other side is rarely far behind). Reapply power. If compressor starts and runs, document and exit.

Branch 4: Compressor hums for 1 to 3 seconds and trips internal overload. Locked rotor or severely degraded start torque. Three causes in field probability:

• Weak run capacitor (read below 90% of rated microfarad even though it visually looks fine).
• No start kit on an older system with degraded starting torque - install a hard-start kit (PTC or potential relay + start cap), retest. If now starts, the compressor is tired but serviceable.
• Mechanical lockup or seized compressor. Run a megger test (compressor lead to ground, 500 VDC, after isolating leads) - a reading below 1 megohm indicates grounded windings, compressor is done. Windings open between terminals - compressor is done.

Branch 5: No hum, no current draw on the compressor leg with confirmed voltage at the compressor terminals. Open windings (internal break) or open internal overload that will not reset.

• Power off, isolate leads, ohm common to run, common to start, run to start. Open reading = winding break = replace compressor.
• All windings show continuity but compressor will not draw current with power applied? Internal overload stuck open. Some compressors recover after a 4-hour cool-down; many do not. Document and recommend replacement; install hard-start as interim only when the customer requests it.

Branch 6: Compressor starts, runs for under a minute, and trips on overload. Different failure mode. Either:

• High head pressure problem - now you're on a different decision tree (see high-pressure-trip article).
• Low voltage at compressor under load (under 200 VAC) - utility or feeder problem.
• Compressor mechanical degradation - amp draw exceeds LRA on data plate; replace.

Confirming the diagnosis

After capacitor or hard-start repair, monitor compressor amp draw across a full cooling cycle:

• Start current should not exceed LRA on data plate.
• Run current should match RLA within tolerance.
• Suction and discharge pressures stable and within expected range for ambient.
• Compressor body temperature normal (not excessively hot) after 15 minutes of runtime.

Repair / remediation

• Dual-run capacitor: replace as a unit, both sections, match microfarads to OEM spec, voltage rating equal or higher than original.
• Hard-start kit: select PTC (cheap, less effective) for marginal cases or potential-relay-and-start-capacitor combinations for tougher cases. Match the start capacitor microfarads to compressor LRA per the start-kit manufacturer's chart.
• Contactor: match coil voltage, pole count, FLA rating; clean or replace burned pad terminals.
• Compressor replacement: weigh in by data plate, install new filter drier (suction-side drier on burnout systems), deep vacuum to 500 microns, hold 10 minutes. Document refrigerant weight on invoice.

References

• AHRI Standard 210/240 - Performance Rating of Unitary Air-Conditioning Equipment
• NEC Article 440 - Air-Conditioning and Refrigerating Equipment
• ASHRAE Handbook - HVAC Systems and Equipment
• OSHA 29 CFR 1910.147 - The Control of Hazardous Energy