To test a motor capacitor with a multimeter, set your meter to the capacitance (µF) setting, discharge the capacitor safely, and place the probes across the C and FAN/HERM terminals. A good capacitor reads within ±5% to ±10% of its printed microfarad (µF) rating. If it reads 'OL' (open), '0.00' (shorted), or drifts wildly, the capacitor is dead and must be replaced.
Safety First: Discharging and CAT Ratings
Motor capacitors store a dangerous electrical charge long after the power is disconnected. A fully charged 45µF run capacitor on a 240V circuit can deliver a lethal shock or cause severe burns. Always de-energize the circuit, lock out the breaker, and verify zero voltage before touching any terminals.
When working on HVAC compressors, well pumps, or industrial motor circuits, your multimeter must be rated for the environment. Because these circuits are hardwired into building power systems, you need a meter with a CAT III 600V or CAT IV 600V safety rating. Using a cheap, unrated meter on a 240V motor circuit risks an internal arc flash if a transient voltage spike occurs while you are probing.
The Discharge Procedure:
Never short a capacitor with a bare metal screwdriver. The instantaneous current spike can vaporize the internal foil connections, ruining a capacitor that might have otherwise been good, and it can weld the screwdriver to the terminal. Instead, use a dedicated capacitor discharge tool, or build a bleed resistor using a 20kΩ, 5-watt wirewound resistor attached to insulated alligator clips. Place the resistor across the terminals for 10 to 15 seconds to safely bleed off the stored energy.
Multimeter Setup and Probe Placement
Before testing, you must configure your meter correctly to avoid reading lead capacitance or blowing the internal fuse.
Meter Configuration
- Dial Position: Set to Capacitance (marked with the -| |- symbol or µF).
- Lead Jacks: Black lead to COM. Red lead to the V/Ω/Cap jack (Note: On some advanced meters like the Fluke 87V, there is a dedicated mA/µF jack; consult your manual).
- Range: Auto-ranging is preferred. If your meter is manual-ranging, select the range just above the capacitor's printed rating (e.g., select the 200µF range for a 45µF capacitor).
Nulling the Leads: Test leads themselves have a small amount of inherent capacitance, usually between 0.1µF and 0.5µF. Touch the red and black probe tips together, wait for the reading to stabilize, and press the REL (Relative) or Zero button. This subtracts the lead capacitance from your final measurement, which is critical when testing small start capacitors under 10µF.
Testing for Shorts, Opens, and Ground Faults
Before measuring exact capacitance, perform a quick resistance check to ensure the capacitor hasn't suffered a catastrophic internal short or a ground fault to the casing.
- Set your multimeter dial to Resistance (Ω), preferably the highest range or auto-range.
- Place one probe on the C (Common) terminal and the other on the HERM or FAN terminal.
- Observe the display. A healthy capacitor will briefly show a low resistance spike as it charges from the meter's internal battery, then quickly climb to 'OL' (Open Loop / Infinite resistance).
- Reverse the probes and repeat. You should see the same charging spike and return to 'OL'.
Ground Fault Check: Place one probe firmly on a bare metal spot on the capacitor's casing (scrape away a little paint if necessary) and touch the other probe to each terminal. The meter must read 'OL'. If you read any continuity or resistance between a terminal and the metal casing, the internal dielectric has breached. The capacitor is a shock hazard and must be replaced immediately.
Measuring Capacitance: Expected Readings vs. Failures
Capacitance degradation is the most common failure mode for motor run capacitors. Over time, the metallized film inside degrades, losing its ability to store a charge. According to Fluke's official testing guidelines, a capacitor that has dropped more than 6% from its rated microfarad value will cause the motor to run hot, draw excess amperage, and eventually burn out the windings.
| Printed Rating | Acceptable Range (Good) | Failing (Weak / Replace Soon) | Dead (Shorted / Open) |
|---|---|---|---|
| 35 µF | 32.9 µF – 37.1 µF | 29.0 µF – 32.8 µF | 0.00 µF or 'OL' |
| 40 µF | 37.6 µF – 42.4 µF | 33.0 µF – 37.5 µF | 0.00 µF or 'OL' |
| 45 µF | 42.3 µF – 47.7 µF | 38.0 µF – 42.2 µF | 0.00 µF or 'OL' |
| 50 µF | 47.0 µF – 53.0 µF | 42.0 µF – 46.9 µF | 0.00 µF or 'OL' |
| 60 µF | 56.4 µF – 63.6 µF | 50.0 µF – 56.3 µF | 0.00 µF or 'OL' |
Note: Start capacitors (usually black, round, and rated above 70µF) often have a wider tolerance, typically +30% / -10%. Always check the printed tolerance on the label.
For deeper diagnostics, advanced meters can measure Equivalent Series Resistance (ESR). As detailed in All About Circuits' capacitor testing guides, a high ESR reading indicates the internal electrolyte or film is drying out, even if the microfarad reading still appears normal. If your meter supports ESR, a reading above 3Ω on a motor run cap is a strong indicator of impending failure.
Common Mistakes That Give Misleading Readings
If your readings look wrong, do not immediately throw the capacitor away. Bench and jobsite experience shows that testing errors are more common than actual component failures. Watch out for these specific traps:
- Leaving Wires Connected: This is the #1 mistake. If you leave the compressor or fan wires attached to the terminals, your meter will measure the capacitor in parallel with the motor windings. The inductance and resistance of the motor will cause the meter to display erratic, wildly inflated, or completely invalid microfarad readings. Always pull the spade connectors off the terminals before testing.
- Touching the Metal Probe Tips: The human body has a capacitance of roughly 100pF to 300pF. If you hold the metal tips of the probes with your bare fingers while testing a small 5µF start capacitor, your body will skew the reading. Hold only the insulated plastic grips.
- Dielectric Absorption (The 'Ghost' Charge): If you short a capacitor to discharge it, the internal dielectric material can 'absorb' some of the charge and slowly release it back into the terminals over a few minutes. If you test it immediately after a hard short, the meter might show a slow, drifting climb in microfarads. Wait 2 to 3 minutes after discharging, or use a high-value bleed resistor, to let the absorption settle.
- Dirty or Oxidized Terminals: Motor capacitors live in harsh environments. A layer of corrosion or dirt on the spade terminal adds series resistance, which can prevent the meter from locking onto a stable capacitance reading. Scrape the terminal clean with a small flathead screwdriver or sandpaper before applying the probe.
Frequently Asked Questions
Can I test a motor capacitor without a capacitance setting?
You can use the Ohms (resistance) setting to check for catastrophic failures like dead shorts, open circuits, or ground faults to the casing. However, you cannot verify if the capacitor has lost its microfarad capacity using only a resistance test. Since gradual microfarad drift is the most common failure mode in aging HVAC run capacitors, a dedicated capacitance meter (or a multimeter with a µF setting) is strictly required to confirm the component is actually healthy.
Why does my multimeter reading keep drifting up and down?
A drifting or 'hunting' reading usually indicates one of three things: high Equivalent Series Resistance (ESR) due to a drying internal dielectric, residual charge from incomplete discharging, or a poor physical connection between the probe tip and a corroded terminal. If the numbers refuse to lock in after 10 seconds of firm probe pressure on a clean terminal, the internal film is degraded. Treat the capacitor as failed and replace it.
What is the acceptable tolerance for a run capacitor microfarad rating?
Standard HVAC and motor run capacitors (the silver, oval or round metal cans) typically carry a ±6% tolerance. For example, a 40µF capacitor is considered good anywhere between 37.6µF and 42.4µF. Start capacitors (often black, cylindrical, and used only for a few seconds during motor startup) usually have a much wider tolerance, frequently marked as +30% / -10% or even ±20%. Always defer to the tolerance printed on the specific component's label.
Do I need to disconnect the wires from the capacitor before testing?
Yes, absolutely. You must physically remove all spade connectors and wires from the capacitor terminals before testing. Leaving wires connected creates a parallel circuit with the motor windings, contactor coils, or relay switches. This parallel path will completely invalidate both your resistance and microfarad readings, often causing the meter to display an 'OL' error or a massively inflated capacitance value that does not belong to the capacitor itself.
