How to Test a Run Capacitor: Meter Method and Under-Load Method

Run capacitors are one of the most common failures in air conditioners and heat pumps. A weak one causes hard starts, high amp draw, overheating motors and a compressor that hums without starting.

Safety: A capacitor can hold a dangerous charge even with power off. Lock out and tag out, verify dead, then discharge across the terminals with an insulated resistor tool (not a screwdriver) before handling it.

Know the terminals

• Single capacitor: two terminals, one rating (like 7.5 µF 370 VAC).
• Dual capacitor: three terminals: C (common), HERM (compressor) and FAN. Each side has its own rating, like 45/5 µF.

Method 1: Meter capacitance test (power off)

1. Lock out power, discharge the capacitor and remove the wires. Label them first.
2. Set the meter to capacitance (µF).
3. Measure C to HERM and C to FAN on a dual, or across the two terminals on a single.
4. Compare to the rating and tolerance on the label.

It's quick and simple, but it tests the capacitor at the meter's low test voltage. A capacitor that's starting to fail can still read fine.

Method 2: Under-load test (running)

This tests the capacitor at operating voltage and current, so it catches failures the meter test can miss.

1. With the unit running, clamp the wire from the capacitor to the motor's start (aux) winding. That's the wire from HERM to the compressor's S terminal, or the fan's start lead.
2. Measure voltage across the capacitor, HERM to C (or FAN to C).
3. Calculate: µF = 2652 × amps ÷ volts (at 60 Hz).

Example: 4.0 A on the start wire, 240 V across the capacitor → 2652 × 4.0 ÷ 240 = 44.2 µF. For a 45 µF ±6% capacitor (acceptable range 42.3–47.7 µF), that passes.

The capacitor check calculator does the math and the pass/fail.

When to replace

• Measured value outside the rated tolerance.
• Bulging top, leaking oil, or burn marks.
• Replace with the same µF rating and an equal or higher voltage rating (a 440 V can replace a 370 V, but not the other way around).

FAQ

Can a capacitor test good and still be bad?

Yes. A meter test at low voltage can pass a capacitor that drops off under load. The under-load test is the better check.

What does the 2652 number come from?

It's 1,000,000 ÷ (2π × 60). It converts the capacitor's current and voltage at 60 Hz into microfarads.

Can I use a higher µF capacitor?

No. Match the motor manufacturer's specified microfarads. The wrong value changes the current in the start winding and can overheat the motor.

Sources

• HVAC School: technical tips and diagnostics