Bilge Pump Runs Constantly — Causes and How to Stop It

My bilge pump never shuts off even when the bilge looks dry — what's wrong?

A bilge pump that runs with a dry bilge is almost always a stuck or miswired float switch, not water — the pump is being told "high water" when there's none. Either the float is mechanically jammed up (debris, oil, hose, a tilted mount), the switch contacts have failed closed, or the wiring/panel is sending constant power. Confirm by lifting/lowering the float by hand: if the pump obeys, the switch is fine and the issue is mechanical or wiring; if it ignores you, the switch or its circuit has failed. Running a dry centrifugal bilge pump continuously overheats it and will burn out the motor and drain the battery, so stop it and diagnose rather than ignore it.

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

💵 $15–$45 DIY for a marine-rated float switch plus connectors; $150–$350 at a marine shop for diagnosis and switch/wiring replacement. A leaking thru-hull or shaft-seal repair runs higher and may require a haul-out. ⏱ 15–30 minutes to check the switch and free a stuck float; 1–2 hours to replace a float switch and re-test; longer if wiring must be traced or a leak source repaired. ● Use caution

Safety: Work with the engine off and the blower run / bilge ventilated before reaching in — bilges collect fuel vapors and a spark or hot motor near gasoline fumes can cause an explosion. A bilge pump dry-running for hours can overheat and is a fire/odor risk and will flatten your battery, leaving you with no pump when you actually need one. Treat any water returning from a thru-hull, shaft seal, or stuffing box below the waterline seriously — an unaddressed leak can sink the boat at the dock. Before doing any electrical work in or near the bilge with the boat in the water, disconnect AC shore power first — a fault in the boat's AC system around water creates an electrocution and electric-shock-drowning hazard.

Common causes

Float switch stuck in the ON (up) position — jammed by oil, debris, a loose hose, or grit in the pivot, or a tilted/loose mount that won't let the float drop. (most common)
Quick check:
Failed float switch (contacts welded/stuck closed, or water intrusion corroded it) so it never opens the circuit even when the float is down. (common)
Quick check:
Wiring or panel fault — pump wired to a constant 'ON' bilge switch, a manual/auto switch left in MANUAL or ON, or a short feeding the pump motor directly. (common)
Quick check:
A small steady leak keeping a tiny but real water level the pump keeps chasing — slow stuffing-box drip, raw-water hose weep, leaking thru-hull or shaft seal, or rain/washdown — bilge looks 'dry' at a glance but isn't. (less common)
Quick check:
Electronic/field-sense pump (no float) with a fouled or failed sensor stuck in test-cycle, or water draining back down the discharge hose (no vented loop) making it re-cycle every few seconds. (rare)
Quick check:

How to fix it

Cut power first. At the helm, turn the bilge pump to OFF, or pull the bilge pump fuse/breaker so a hot, dry-running motor stops cooking itself. Note whether the pump stops — if it keeps running with the switch off, you have a wiring fault feeding it directly (skip to step 6).
Check the switch position and mode. Many panels have a 3-way OFF / AUTO / MANUAL (ON) switch. If it's in MANUAL/ON, that alone explains constant running. Set it to AUTO and see if the pump stops with a dry bilge.
Inspect the float switch by hand. With power restored to AUTO, and only after the engine is off and the bilge has been ventilated (run the blower on a gas boat), gently lift then lower the float. If the pump starts when raised and stops when lowered, the switch works — the problem is that something is holding it up. Look for oil/sludge, a hose or wire lying on it, debris in the hinge, or a mount that has tilted so the float can't fall.
Clean and re-seat the float. Wipe oil and grime off the float and its pivot, clear any debris, and remount it level and low in the bilge where nothing can pin it up. Secure stray hoses and wires away from it. Re-test by hand. Use only marine-rated float switches; replace, don't bend, a damaged one.
Test the switch electrically if it ignores the float. Disconnect the float switch leads and use a multimeter on continuity: a good switch reads open with the float down and closed with it up. If it stays closed (or reads erratic) with the float down, the switch has failed — replace it with a marine-rated float or solid-state switch wired to ABYC standards.
Trace the wiring. Confirm the pump's positive feed runs through the float switch (and panel) and not a constant hot. Check for a chafed wire shorting to the motor lead, corroded butt connectors, or a switch wired backward. All connections in the bilge must be above the highest water level, made with marine-grade tinned wire, adhesive-lined heat-shrink crimp connectors, and no wire nuts or electrical tape — per ABYC E-11. On a gasoline boat, any electrical device in the engine or fuel compartment — including the bilge pump and its float switch — should be ignition-protected (or use a sealed/mercury float), per USCG 33 CFR 183 and ABYC; a non-ignition-protected pump or switch that arcs near fuel vapor is an explosion risk.
Rule out a real leak. If the switch and wiring are good but the pump keeps cycling, dry the bilge fully with a sponge and watch where water returns: a steady stuffing-box drip beyond a drop every few seconds, a weeping raw-water hose clamp, a seeping thru-hull, or water draining back down the discharge with no vented loop. Fix the source. A thru-hull or shaft-seal leak below the waterline is not a 'let it ride' item — address it or haul out.
For float-less electronic pumps, clean the sensor per the maker's instructions and confirm the discharge hose has a vented (anti-siphon) loop above the waterline so water isn't draining back and re-triggering it. Use a vented loop rather than a check valve where possible — check valves in bilge discharge lines are prone to clogging and can disable the pump when you need it. Replace the sensor module if it won't reset.

DIY or call a pro?

DIY for most owners: checking the switch position, freeing or cleaning a stuck float, and swapping a float switch are straightforward with basic tools and a multimeter. Call a pro if the pump runs even with all power off (hidden short), if you find a leaking thru-hull, shaft seal, or stuffing box below the waterline, or if the wiring is a corroded mess that needs to be redone to ABYC standards.

Tools & parts

Multimeter (continuity/DC volts)
Marine-rated automatic float switch or solid-state bilge switch (ignition-protected if in a gas engine/fuel space)
Marine-grade tinned copper wire (correct gauge for the pump)
Adhesive-lined heat-shrink butt connectors and crimp tool
Sponge and rags to dry the bilge
Bilge cleaner/degreaser to clear oil from the float
Screwdrivers and mounting screws or marine sealant
Headlamp
Spare inline fuse for the pump circuit

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Based on: BoatUS / BoatUS Foundation; ABYC (American Boat & Yacht Council) Standard E-11, AC & DC Electrical Systems; USCG / USCG Auxiliary (33 CFR 183 ignition protection); NMMA (National Marine Manufacturers Association); Rule / Johnson Pump (Xylem) bilge pump and float switch installation guidance

General marine-maintenance guidance, not a substitute for a qualified marine technician or surveyor. Boats and conditions vary; for fuel, electrical, fire, or structural issues — or anything safety-critical — consult a professional. Always follow your engine and equipment manuals.