A pressure pump (tank/jet pump on a well, bore, or rainwater system) that runs but won’t build pressure is frustrating — taps trickle, the pump cycles oddly, or it never reaches cut-out. There are a few causes, but a weak or failing capacitor is a common one that’s easy to check yourself.
How a tired capacitor causes low pressure
The capacitor helps a single-phase motor reach and hold its proper running speed. When it weakens:
- The motor may start slowly or struggle under load.
- It can run below full speed, so the impeller can’t generate the head needed to build pressure.
- The motor runs hot and may trip the thermal overload after a while.
So a pump that sounds like it’s running but just won’t get the pressure up — especially if it also hums on start or trips out — is worth a capacitor check.
Rule out the non-electrical causes first
Low pressure often isn’t the motor at all. Before opening the terminal box, check:
- Loss of prime / air in the system — jet pumps in particular won’t build pressure if they’ve lost prime.
- Blocked intake, foot valve, or strainer restricting flow.
- Waterlogged or flat pressure tank — a failed bladder or lost air charge causes rapid cycling and poor pressure.
- Leaks on the suction side drawing in air.
- Worn impeller or jet assembly after years of service.
If the water side checks out but the motor still won’t pull full speed, look at the capacitor.
Safety first
The capacitor holds a dangerous charge even when the pump is switched off and unplugged.
- Isolate the power at the breaker before opening anything.
- Find the capacitor (a cylindrical can, often clipped to the motor or under a cover).
- Discharge it by bridging the 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.
- Wear safety glasses and keep one hand clear of the terminals.
- In NZ, fixed mains wiring is work for a licensed electrician. Many pressure pumps are hard-wired — if so, or if you’re unsure, use a sparky.
Confirming the capacitor
- Inspect: bulging, leaking, split, or scorched = failed. Replace it.
- Measure: with a multimeter in capacitance (µF) mode, disconnect one terminal and read the value against the printed rating. A reading well under tolerance, near zero, or “OL” means it’s done. Full walk-through: how to test a pump capacitor.
Choosing the right replacement
Get this right or you risk the motor:
- Read the µF and voltage off your own capacitor (e.g. “16 µF 450V”) or the motor nameplate. Don’t guess from the pump model — the wrong capacitance won’t restore pressure and can damage the windings.
- Keep the µF the same; voltage can be equal or higher.
- Many pressure pumps are PSC motors using a film run capacitor marked CBB60 or CBB65. Higher-torque pumps may also have a CD60 electrolytic start capacitor — replace like-for-like.
- Check terminal type and that the new cap fits the clamp or housing.
For help reading the markings, see the pump capacitors guide.
Still no pressure with a new capacitor?
If a known-good capacitor doesn’t bring the pressure back, focus on prime, the pressure tank/bladder, the foot valve, or a worn impeller. A pump or water specialist can take it from there.
Get the right pressure pump capacitor
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