No Cooling After Capacitor Swap Wrong MFD Vs Wiring Decision Tree

Why this matters

A capacitor swap is the most common condenser repair, which makes it the most common place to introduce a new fault while fixing an old one. When a unit will not cool after a fresh dual-run capacitor, the failure splits into two families that present identically: the wrong microfarad value was installed, or a wire landed on the wrong terminal. Both leave the fan and compressor either dead, humming, or cycling on overload, and both will burn the new capacitor or the compressor windings if left running. The cost of guessing is a comeback plus a possible compressor. This tree separates a value mismatch from a wiring error using terminal voltage, run-cap microfarad measurement under load, and a methodical HERM-FAN-C check.

Symptom presentation

The unit was diagnosed with a failed capacitor, the part was replaced, and now the condenser fan will not start, the compressor hums and trips on internal overload, or the unit blows the breaker. Often the fan spins if hand-started but stalls on its own, the classic weak-or-wrong run-cap signature. The customer hears the contactor pull in (a click) but no motor comes up to speed. On a dual-run capacitor the three terminals are marked HERM (compressor), FAN, and C (common). Any swap among these three, or a value outside the nameplate tolerance, produces no-cool.

Quick checks

• Confirm 24 VAC at the contactor coil on a cooling call and confirm the contactor is pulled in. No pull-in is a control problem, not the capacitor.
• Confirm line voltage across the contactor load side, both legs to ground and leg to leg (208 to 240 VAC).
• Read the new capacitor nameplate against the unit data plate. A 45/5 unit needs a 45/5; a 40/5 or 35/5 substituted will not start the compressor or fan reliably.
• With power off and the capacitor discharged, measure microfarads on each section with a meter. Compare to the printed rating and the plus-or-minus 6 percent tolerance.
• Verify which wire lands on HERM, FAN, and C. The C terminal carries the line-side leg to both windings; HERM feeds the compressor start winding, FAN feeds the condenser-fan start winding.

Isolation tree

Branch A, measured microfarads off-spec: the wrong part was installed or the part is defective out of the box. A 5 mfd FAN section reading 2 mfd will not start the fan; a 45 mfd HERM section reading 30 mfd starves the compressor start torque. Replace with the exact nameplate value. Generic "universal" capacitors that round the value are a frequent cause of this branch.

Branch B, microfarads correct but fan dead, compressor runs: FAN and a wire are crossed, or the FAN wire is off entirely. Trace the brown fan-motor wire to FAN and the brown-with-white-stripe to the line side. A fan wire mistakenly on HERM leaves the compressor double-fed and the fan dead.

Branch C, microfarads correct, fan runs, compressor hums and trips: the HERM lead is on FAN, or HERM is open. The compressor start winding is getting the small FAN-section microfarads instead of the large HERM section, so it lacks start torque and trips on the internal overload within seconds. Move the compressor start lead to HERM.

Branch D, microfarads correct, wiring correct, still no start: the capacitor swap was not the original fault. Check for a failed compressor (open or shorted windings by resistance and to ground), a seized compressor (high locked-rotor amps, trips instantly), a bad contactor with pitted contacts dropping a leg, or a hard-start kit that was removed during the swap and is now needed on a tired compressor.

Confirming diagnosis

Confirm a value mismatch by measuring microfarads on an isolated, discharged capacitor, not by trusting the label. Confirm wiring by ringing out each motor lead back to its winding with the capacitor disconnected, then matching it to the HERM, FAN, C silkscreen. Confirm a wiring-induced overload trip by reading compressor amps at start: a winding fed by the wrong (small) section draws locked-rotor amps and the overload opens in two to five seconds. Confirm the capacitor is not the fault at all by reading run amps on a correctly wired, correctly valued unit; if amps are near nameplate RLA and the unit cools, the swap was good and any remaining complaint is elsewhere (low charge, dirty condenser).

Remediation

Install the exact nameplate microfarad and voltage rating; never round the value or drop the voltage rating below the data plate. A 440 VAC capacitor may substitute for a 370 VAC part, but never the reverse, and the microfarad must match within tolerance. Land HERM, FAN, and C per the unit wiring diagram, not from memory; photograph the original wiring before removing the old part on every job to prevent this branch. If the compressor tripped repeatedly on a wrong-value or wrong-terminal feed, let it cool, then verify windings by resistance and megohm before declaring it good; repeated locked-rotor events degrade the start winding and a winding shorted to ground after the event means the swap revealed, or caused, a failed compressor. Where a compressor with a marginal start signature needed the original capacitor plus age allowance, add a correctly sized hard-start kit rather than oversizing the run capacitor, which causes its own bearing wear and overheating. After any correction, run the unit and confirm both fan and compressor amps sit near their nameplate values and the unit cools before leaving.

References

• ACCA Standard 4, Maintenance of Residential HVAC Systems
• AHRI Standard 210/240-2023, Performance Rating of Unitary Equipment
• UL 810, Standard for Capacitors (run-capacitor construction and tolerance)
• NFPA 70 (National Electrical Code), Article 440, Air-Conditioning and Refrigerating Equipment