The capacitor is one of the most common parts to fail on a single-phase pump or pool motor. It gives the motor the “kick” it needs to start and, in many designs, helps it run smoothly. When it weakens, you get tell-tale symptoms long before the motor itself dies. Spotting them early can save the whole motor.
Here are the five signs worth knowing.
1. The motor hums but won’t start
This is the classic one. You hear a low hum, maybe feel a slight vibration, but the shaft never turns. After a few seconds a thermal cutout may trip. A dead or badly degraded start capacitor can’t deliver the phase shift the motor needs to begin rotating, so it just sits there straining. Left like this, the windings overheat fast.
2. Slow or laboured start-up
If the pump eventually starts but takes longer than it used to, or sounds like it’s struggling to get up to speed, the capacitor may be losing capacitance. A capacitor that has drifted well below its rated microfarads (µF) gives a weaker push, so the motor takes longer to reach running speed.
3. Pump cuts out or trips the breaker
Intermittent shut-downs, nuisance breaker trips, or the motor stopping under load can all point to a failing capacitor. As capacitance falls, the motor draws more current trying to compensate, which heats things up and trips protection devices.
4. Visible damage to the capacitor
Pop the cover and look at the capacitor itself. Warning signs include:
- A bulging or domed top (especially on electrolytic CD60 start caps)
- Leaking oil or a sticky residue
- A burnt smell or scorch marks
- A cracked or split case
Any of these means replace it. Do not touch it yet — see the safety note below.
5. Weak performance or burning smell
Lower water flow, the motor running hotter than normal, or a faint electrical burning smell are all later-stage symptoms. By this point you want to act quickly, because a failing capacitor that keeps pushing the motor can take the windings out with it.
Safety first — before you touch anything
A capacitor stores a dangerous electrical charge even when the pump is unplugged or switched off at the wall. Treat every capacitor as live until you prove otherwise.
- Isolate power at the breaker — not just the switch.
- Discharge the capacitor: bridge its two terminals with a bleed resistor (around 10–20 kΩ, 5 W) or a proper capacitor discharge tool — a bare insulated-handle screwdriver across the terminals also drains it but sparks violently and can pit the terminals, so use that only as a last resort. This safely drains the stored charge.
- Only then disconnect anything.
In New Zealand, fixed mains wiring legally requires a licensed electrician. If your pump is hard-wired rather than plug-in, or you’re not confident, get a sparky. There’s no shame in it and it’s the law.
How to confirm it’s the capacitor
Once isolated and discharged, you can:
- Visually inspect for the damage above.
- Test with a multimeter that has a capacitance (µF) range. Read the value and compare it to the rating printed on the capacitor body. A reading more than about 5% off the rated µF means it’s out of spec.
Crucially, read the µF and voltage off your own capacitor or the motor nameplate — never guess from a model number online. The wrong capacitance can damage the motor. A replacement should match the µF within roughly ±5%; a higher voltage rating (say 450V in place of 370V) is fine, but the microfarads must match.
Need help reading those numbers? See our guide on how to read a capacitor label, and learn the difference between CBB60, CBB65 and CD60 capacitors before you buy.
What to do next
If two or more of these signs ring true, your capacitor is the most likely (and cheapest) culprit. Confirm the value, then replace like-for-like.
- Browse run and start capacitors on our pump capacitors page.
- Not sure which one you need? Use our find-your-capacitor wizard.
- Shopping for a pool pump? Jump straight to the pool capacitor range.
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CapacitorsNZ — find your exact capacitor with our wizard, then shop the pool range or browse all pump capacitors.
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