To test a capacitor on an electric motor, set your multimeter to capacitance (F), safely discharge the terminals, isolate the component by removing all wires, and place the probes across the C and FAN/HERM terminals. A good reading falls within ±5% or ±6% of the microfarad (µF) rating printed on the label. If it reads outside this window, shows infinite (OL), or reads zero, the capacitor is dead and must be replaced.
Safety First: Discharging and CAT Ratings
Motor capacitors store lethal electrical energy long after the power is disconnected. A failed start capacitor on a 240V compressor can hold a 400V DC charge capable of causing severe shock or stopping a heart. Before touching any terminals, you must verify the circuit is de-energized and safely bleed off the stored charge.
Because you are working on equipment tied to mains branch circuits (120V/240V AC), your multimeter must carry a CAT III 600V or CAT IV 600V safety rating. Meters like the Fluke 117 or Klein Tools MM700 are standard for this work. Using a cheap, unrated meter on a motor circuit risks an arc-flash event inside the meter casing if a transient voltage spike occurs while you are probing.
Meter Setup and Probe Placement
Capacitance is measured by charging the component with a known current and calculating the time it takes to reach a specific voltage (the RC time constant). To get an accurate reading, your meter must be configured correctly and the capacitor must be completely isolated from the motor windings.
| Setting | Required Configuration |
|---|---|
| Dial Position | Capacitance (F, µF, or the ⊣⊢ symbol). If shared with another function, press the SELECT/FUNC button. |
| Lead Jacks | Black lead to COM. Red lead to VΩF (or the dedicated capacitance jack if your meter has one). |
| Range | Auto-ranging is preferred. For manual meters, set to a range higher than the capacitor's label (e.g., 200µF range for a 45µF run cap). |
| Zeroing | Short the red and black probe tips together and press the REL/Zero button to subtract the test leads' inherent capacitance (usually 0.1µF to 0.3µF). |
Probe Placement: For a standard single round capacitor, place one probe on each of the two spade terminals. For a dual run capacitor (common in HVAC compressors and blower motors with three terminals labeled C, FAN, and HERM), you must test the two internal capacitors separately. Place probes across C and FAN to test the fan circuit, then across C and HERM to test the compressor circuit. Never measure across FAN and HERM directly; that measures the series combination and will yield a confusing, useless reading.
Step-by-Step Testing Procedure
- Kill the power: Turn off the disconnect switch or trip the breaker. Verify zero voltage at the contactor using your meter's AC voltage setting.
- Discharge the capacitor: Connect your 20kΩ bleeder resistor across the terminals for 5 to 10 seconds. Verify discharge by switching your meter to DC Volts and checking across the terminals (should read < 1V).
- Isolate the component: Pull the spade connectors off the capacitor terminals. Do not skip this step. Leaving the motor windings connected will create a parallel DC path that completely invalidates the capacitance reading.
- Inspect visually: If the capacitor's top dome is bulged, the casing is cracked, or dielectric oil (smells like rotting fish) is leaking, stop testing. It is physically destroyed. Replace it immediately.
- Apply probes: Touch the red and black probes to the bare metal spade terminals. Polarity does not matter for standard AC motor capacitors.
- Read and wait: Large start capacitors (e.g., 400µF) can take up to 15 seconds for the meter's internal charging circuit to stabilize and display the final value. Wait for the reading to stop climbing.
Expected Readings and Decision Tree
Motor run capacitors typically carry a tolerance of ±5% or ±6% (sometimes ±10% on older or cheaper models). Start capacitors usually have a wider tolerance of -0% to +20%. You must do the math on the label rating to determine your acceptable window. For example, a 45µF ±5% run capacitor must read between 42.75µF and 47.25µF.
| Multimeter Reading | Diagnosis | Action Required |
|---|---|---|
| Within ±5% / ±6% of label | Good. Dielectric is intact. | Reconnect wires and restore power. |
| >10% below label rating | Weak. Dielectric degradation or internal open sections. | Replace immediately. Motor will overheat and trip thermal overload. |
| OL (Out of Limits / Infinite) | Open circuit. Internal fuse or foil trace has burned open. | Replace. Motor will not start (will just hum). |
| 0.00µF or near zero | Short circuit. Dielectric layer has punctured. | Replace. Check motor windings for collateral short-circuit damage. |
| Reading constantly fluctuates | Internal arcing or loose internal connection. | Replace. Unstable starting torque will damage the motor shaft. |
Common Mistakes That Give Misleading Readings
When troubleshooting motor circuits in the field, techs often condemn a perfectly good capacitor or pass a bad one due to measurement errors. Avoid these three specific traps:
1. Failing to Disconnect the Wires
This is the most common error. A multimeter measures capacitance by applying a small test voltage and measuring the current flow over time. If the motor windings are still connected to the capacitor terminals, the meter's test current flows through the low-resistance copper windings instead of charging the capacitor plates. The meter will either read wildly high, read zero, or throw an error. Always pull the spade connectors off the metal terminals before testing.
2. Touching the Metal Probe Tips
The human body has a small amount of inherent capacitance and high resistance. If your fingers touch the bare metal of the probe tips while measuring a small run capacitor (like a 2µF blower cap), your body creates a parallel circuit. This can skew the reading by 0.5µF or more, which is a massive 25% error on a small component. Hold only the insulated plastic grips.
3. Ignoring Equivalent Series Resistance (ESR)
A standard multimeter measures bulk capacitance (µF), but it cannot easily measure Equivalent Series Resistance (ESR). A capacitor can read a perfect 45.0µF on your meter but still fail under load if its internal ESR has spiked due to dried-out electrolyte (in start caps) or degraded metallized film (in run caps). If the capacitor tests perfectly on the bench but the motor still struggles to start and draws high locked-rotor amperage (LRA), swap the capacitor anyway. For deep bench diagnostics, an ESR meter or a dedicated component tester like the Peak Atlas ESR70 is required to catch this hidden failure mode.
By isolating the component, using a properly rated CAT III meter, and strictly following the tolerance math on the label, you eliminate the guesswork. If the microfarad reading falls outside the ±5% window, the part is trash—install a 440V-rated replacement and put the system back online.
