Pool Pump Capacitor: How to Test and Replace It (the $30 Fix for a Humming Pump)
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Key takeaways
- A pump that hums but will not spin, or spins only after you nudge the shaft, almost always has a failed start capacitor.
- A replacement capacitor usually costs $15 to $40. A pro charges roughly $100 to $250 for the same job, parts and trip included.
- Match the new part to the old one exactly: same microfarad (MFD or µF) rating and the same or higher voltage.
- Capacitors hold a charge after the power is off. Shut off the breaker and discharge it before you touch the terminals.
The capacitor is the small cylinder, usually black or silver, mounted on the back or top of the pump motor under a metal cover. It gives the motor a burst of energy to get the impeller turning. When it weakens, the motor still gets power but cannot start, so it sits there humming and heating up until the thermal overload trips. It is the most common pool pump failure and the cheapest one to fix.
Signs your pool pump capacitor is bad
- The motor hums but the shaft does not turn.
- The pump starts if you give the shaft a spin with a screwdriver (power off first), then runs fine until the next start.
- The breaker or the motor’s thermal overload trips a few seconds after start-up.
- The capacitor case is bulged, cracked, leaking oil, or has a domed top.
- The pump starts slowly, or only on some days, often worse in hot weather.
If the motor makes no sound at all, the capacitor is probably not your problem. Check the breaker, the timer and the wiring first. If you hear loud grinding or screeching, that points to motor bearings instead.
Start capacitor vs run capacitor
Most single-speed pool pump motors use a start capacitor: a short, round cylinder rated in a range, such as 161 to 193 MFD at 125 or 250 volts. It only works for the first second of start-up. Some motors also have a run capacitor, usually an oval or rectangular metal can with a single rating like 25 or 30 µF at 370 volts, that stays in the circuit while the motor runs. Replace like for like. Variable-speed pumps use electronic drives instead and generally have no user-replaceable capacitor.
How to test a pool pump capacitor
- Kill the power at the breaker, not just the timer. Confirm the pump will not start.
- Remove the capacitor cover on the motor (usually one or two screws).
- Look first. Bulging, leaking or burn marks mean it is bad. Replace it.
- Discharge it. Using an insulated screwdriver, touch both terminals at the same time. Better: use a 20,000-ohm, 2-watt resistor across the terminals for a few seconds.
- Disconnect the wires and take a photo of how they were connected.
- Test with a multimeter that has a capacitance (µF or MFD) setting. Touch the probes to the two terminals. A healthy start capacitor reads inside its printed range. A run capacitor should read within about 6 percent of its rating. A reading of zero, or well below the rating, means replace it.
No capacitance meter? At $20 to $40 for the part, many owners just replace a suspect capacitor, especially if the pump hums and the case looks swollen.
How to replace it
- Read the label on the old capacitor: MFD or µF rating, voltage, and size.
- Buy a replacement with the same MFD rating and the same or higher voltage. Size matters only so it fits under the cover.
- With the power off and the old capacitor discharged, move the wires to the new one, one at a time, matching your photo.
- Seat the capacitor, replace the cover, restore power and start the pump.
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New capacitor and it still hums?
- Seized impeller or bearings: with the power off, try turning the shaft by hand. If it is stiff or gritty, the bearings or the impeller are the issue.
- Bad centrifugal switch: the switch inside the motor that cuts the start capacitor out can stick. A technician can test it.
- Low voltage: a 230-volt motor wired to 115 volts, or a loose connection, will hum and stall.
- Worn-out motor: on an older single-speed pump, a new motor costs about as much as a new variable-speed pump that uses half the electricity. Run your numbers in the pool pump calculator before you buy.
If it is past a DIY fix, see what each repair usually costs and get a quote from a local pro, or check what a full replacement costs.
What to check before you replace, how to size it right, and how to avoid overpaying. One page, sent to your inbox.
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FAQ
How much does a pool pump capacitor cost?
Usually $15 to $40 for the part. A service call to replace it runs roughly $100 to $250.
Can I use a capacitor with a higher voltage rating?
Yes. Higher voltage is fine. The MFD (µF) rating must match the old one.
How long does a pool pump capacitor last?
Often 3 to 7 years. Heat, frequent starts and voltage problems shorten that.
Can a bad capacitor damage the motor?
Yes. A motor that sits humming without starting overheats. Repeated stalls can burn out the windings, so fix a humming pump quickly.
The 60-second call
Humming but not spinning means test or replace the start capacitor first. Breaker off, discharge it, match the MFD rating, swap the wires. It is a $30 repair that saves most “dead” pumps. If the shaft is seized or the motor is old, compare a repair with a variable-speed replacement before you spend money.