AC Diagnostics & Repair

Why AC Compressor Fails to Start: Capacitor & Winding Diagnosis

Published on 2026-06-19 · 5 min read · By ACServiceOS Team

When the indoor AC fan runs fine but the outdoor compressor makes a faint humming buzz and trips the MCB, technicians must test components in logical order to avoid misdiagnosing a simple capacitor failure as a dead compressor.

Technician testing dual run capacitor microfarads with digital multimeter on outdoor unit

1. Testing the Run Capacitor Microfarad (MFD) Rating

Over 65% of compressor non-starts are caused by degraded run capacitors. Discharge the capacitor safely with a 20K resistor. Use a multimeter capacitance setting: A 50 MFD capacitor reading below 45 MFD (more than 10% drop) must be replaced.

2. Testing Compressor Motor Windings (Common, Start, Run)

Disconnect terminals. Measure resistance between Common-to-Start (Rcs), Common-to-Run (Rcr), and Start-to-Run (Rsr). The formula must hold true: Rcs + Rcr = Rsr. Any open circuit reading indicates blown internal winding.

3. Checking for Ground Fault (Short to Body)

Test resistance between each terminal (C, S, R) and a clean scraped bare metal spot on the copper suction pipe. Any reading other than infinity indicates an electrical short-to-ground, meaning the compressor is condemned.

4. Internal Thermal Overload Protector (TOP) Tripping

If the compressor is boiling hot to the touch and winding reads open-circuit, the internal thermal protector has tripped. Allow the compressor to cool down with water rags for 45 minutes before re-testing.

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