The direct answer: To test a capacitor on an electric motor, set your digital multimeter to the capacitance (µF) setting, safely discharge the capacitor using a high-wattage resistor, and place the probes directly across the terminals. A good motor capacitor will display a numerical reading within ±5% to ±6% of its printed microfarad (µF) rating. If the meter reads 'OL' (open) or '0.00' (short), the capacitor has failed internally and must be replaced.
Motor capacitors—whether the black cylindrical start capacitors or the silver metal-cased run capacitors found on HVAC compressors, pool pumps, and well pumps—are the most common failure point in single-phase AC motor circuits. Testing them takes less than two minutes, but doing it incorrectly can destroy your meter, give you false hope, or expose you to lethal stored energy.
Safety First: CAT Ratings and Lethal Stored Charge
Motor capacitors are connected directly across 120V, 208V, or 240V AC mains circuits. A charged run capacitor can hold a lethal DC voltage long after the breaker is turned off. Always de-energize the circuit at the breaker, verify the circuit is dead with a non-contact voltage tester, and discharge the capacitor before touching the terminals. Furthermore, because you are working on equipment tied to the building's main electrical panel, your multimeter must be rated CAT III (minimum 600V) or CAT IV. Using a cheap, unrated electronics meter on a 240V HVAC disconnect can result in an arc flash explosion inside the meter if a transient voltage spike occurs. For authoritative safety practices on electrical measurement categories, refer to the Fluke guide on capacitor testing and safety.
Meter Setup and Capacitor Discharge Procedure
Before you touch the motor or the capacitor, you need to prepare your test equipment and safely bleed off the stored electrical charge. Never use a flathead screwdriver to short the terminals; the massive instantaneous current spike can weld the screwdriver to the terminals, damage the capacitor's internal dielectric, and shower you with molten metal.
Meter Setup Block
- Lead Jacks: Plug the black lead into the COM (common) jack. Plug the red lead into the VΩ or mF/µF jack (check your specific meter's manual, as some dedicate a specific jack for capacitance).
- Dial Position: Turn the rotary dial to the capacitance symbol (usually µF, mF, or a symbol resembling a capacitor plate -||-).
- Range: If your meter is not auto-ranging, set it to the highest µF range available (e.g., 2000µF or 20mF) to prevent an 'OL' overload error on large start capacitors, then step down for resolution.
- Zeroing: Touch the red and black probe tips together. The display should read 0.00 or close to it. If it reads a small residual value (like 0.05µF), note this number or use the meter's 'REL' (relative) button to zero it out. This accounts for the internal capacitance of your test leads.
Safe Discharge Steps
- Turn off the breaker or pull the disconnect switch feeding the motor.
- Verify zero voltage across the motor's line terminals using your CAT III multimeter in AC Voltage mode.
- Using an insulated discharge tool (a 20kΩ, 5-watt resistor wired to insulated alligator clips), connect the resistor across the capacitor terminals for 5 to 10 seconds.
- For a dual run capacitor (three terminals), discharge from the Common (C) terminal to the HERM terminal, and then from the Common (C) terminal to the FAN terminal.
Probe Placement and Expected Reading Table
With the capacitor fully discharged and the wires disconnected from the spade terminals (testing in-circuit will yield false readings due to parallel motor windings), you are ready to measure. Hold the probes by the insulated grips and press the metal tips firmly against the metal spade terminals.
The table below outlines what a good reading looks like numerically, based on standard industry tolerances. Run capacitors (silver) typically have a tight ±5% or ±6% tolerance, while start capacitors (black) have a wider ±20% tolerance. For a deeper understanding of capacitor dielectric tolerances and aging, consult the engineering resources at All About Circuits.
| Capacitor Type | Printed Rating | Tolerance | Acceptable Range (Good) | Bad Reading (Replace) |
|---|---|---|---|---|
| Run (Compressor) | 45 µF | ± 5% | 42.75 µF to 47.25 µF | < 42.7 µF, > 47.3 µF, or OL |
| Run (Fan) | 5 µF | ± 5% | 4.75 µF to 5.25 µF | < 4.7 µF, > 5.3 µF, or OL |
| Start (High Torque) | 300 µF | ± 20% | 240 µF to 360 µF | < 240 µF, OL, or 0.00 (Short) |
Dual Run Capacitor Probe Placement
If you are testing a dual run capacitor (which houses both the compressor and fan capacitors in one silver can with three terminals labeled C, HERM, and FAN):
- To test the compressor side: Place one probe on C (Common) and the other on HERM. Compare to the larger µF number printed on the label (e.g., 45µF).
- To test the fan side: Place one probe on C and the other on FAN. Compare to the smaller µF number on the label (e.g., 5µF).
- Note: Polarity does not matter for AC motor capacitors; the red and black probes can go on either terminal in the pair.
Common Mistakes That Give Misleading Readings
Even with a high-end Fluke 117 or Klein Tools MM600, operator error can make a dead capacitor look good, or a good capacitor look dead. Avoid these bench and jobsite pitfalls:
- The 'Finger Capacitance' Error: If you pinch both metal probe tips between your bare fingers while touching the terminals, your body acts as a dielectric. This can add 50pF to 200pF of stray capacitance to the circuit. On a large 45µF compressor cap, you won't notice. On a small 2µF blower motor cap, this will skew the reading enough to make you misdiagnose the part.
- Testing In-Circuit: Leaving the spade connectors attached to the capacitor means you are measuring the capacitor in parallel with the motor's start or run windings. The inductance and resistance of the copper windings will completely corrupt the capacitance reading. Always pull the spades off first.
- Incomplete Discharge: If a capacitor still holds even 2 or 3 volts of DC charge, many auto-ranging digital multimeters will refuse to calculate capacitance and will simply display 'OL' or an error code. If you get an immediate 'OL' on a known-good meter, re-discharge the capacitor with your resistor and try again.
- Ignoring the Swollen Case: If the top dome of a silver run capacitor is bulged upward (like a swollen 18650 lithium cell), the internal pressure switch has tripped or the dielectric fluid has vaporized due to heat. Do not test it. Throw it away. A swollen capacitor is mechanically failed, even if a meter somehow picks up a residual µF reading.
Frequently Asked Questions
How to test a dual run capacitor on an HVAC compressor?
Testing a dual run capacitor requires two separate measurements using the three available terminals: C (Common), HERM (Hermetic compressor), and FAN. First, ensure the power is off and the capacitor is discharged. To test the compressor circuit, place your multimeter probes across C and HERM; the reading must match the higher microfarad (µF) rating printed on the label (typically 30µF to 60µF) within a ±5% tolerance. Next, move the HERM probe to the FAN terminal (keeping the other probe on C) to test the fan circuit; this reading must match the lower µF rating (typically 3µF to 10µF). If either reading falls outside the 5% tolerance band, the entire dual capacitor must be replaced, as they are sealed in a single housing.
Can I test a motor capacitor without a capacitance meter?
Yes, but you can only test for catastrophic failures (dead shorts or complete opens), not for a loss of capacitance. Using an analog multimeter (or a digital meter with a fast-updating bar graph) set to the highest Ohms (Ω) range, touch the probes to the discharged capacitor terminals. A healthy capacitor will cause the resistance reading to swing rapidly from near zero up to infinite resistance (OL) as the meter's internal battery charges the capacitor plates. If the meter immediately reads 'OL' and never swings, the capacitor is internally open. If it reads near zero ohms and stays there, the capacitor is internally shorted. However, this 'sweep test' cannot tell you if a 45µF capacitor has degraded to 30µF—a condition that will cause the motor to overheat and trip the thermal overload. For accurate diagnostics, a dedicated capacitance meter is mandatory.
Why does my capacitor read the correct µF but the motor still won't start?
A standard multimeter measures capacitance using a very low test voltage (usually under 3V). A capacitor can pass this low-voltage bench test but fail under the 240V AC load due to high Equivalent Series Resistance (ESR) or internal arcing across a degraded dielectric. Standard multimeters cannot measure ESR. If your µF reading is perfect but the motor just hums and trips the breaker, the capacitor has high ESR and cannot deliver the necessary phase-shifted current surge. Additionally, check the motor's mechanical components: a seized bearing, a failed centrifugal start switch (on split-phase motors), or an open thermal overload switch inside the motor housing will mimic a bad capacitor symptom. If the capacitor tests good, use an ESR meter or substitute a known-good capacitor of the exact same µF and voltage rating to isolate the fault.
