RV water pump not working: how to find the problem and fix it

How an RV water pump works

Most RVs use a 12V diaphragm pump mounted near the fresh water tank. When you open a tap, pressure in the lines drops, a built-in pressure switch closes, and the pump runs until pressure builds back up, then switches off. Water is drawn from the tank through a strainer (a small clear or screen filter) and pushed through one-way check valves inside the pump head.

Knowing that sequence helps: the pump needs 12V power, a supply of water, an airtight suction line, and healthy check valves and switch. Each symptom points to one of these.

First, identify your symptom

Safety: The pump is low voltage, but it lives near water. Turn the pump switch off before taking anything apart, and don't run a pump dry for long. Many pumps tolerate a short dry run, but extended running without water can damage the diaphragm.

Step 1: Check power to the pump

1. Is the battery healthy? With everything off, a 12V lead-acid battery near full charge reads roughly 12.6V or a bit more. If it's down near 12V or lower, the pump may run slowly or not at all. Plug into shore power or charge the battery and try again. See RV battery not charging: how to troubleshoot the converter and 12V system if it won't hold a charge.
2. Check the fuse. Find the 12V fuse panel (often near the converter) and locate the fuse labelled "pump" or "water pump". Pull it and look at the metal strip, or better, test both sides with a 12V test light while it's installed. Replace it only with the same amperage. If it blows again immediately, there's a short. Don't fit a bigger fuse.
3. Check the switch. Many RVs have two or more pump switches (kitchen, bathroom, control panel). Make sure at least one is on. Switches do fail. If you have a Digital multimeter (auto-ranging, with DC amps), check for 12V on the output side of the switch.
4. Check voltage at the pump. With the switch on and a tap open, measure across the pump's positive and negative wires right at the pump. You should see close to battery voltage. No voltage means a wiring problem between the panel and pump, often a corroded connector or a broken ground. Voltage present but the pump doesn't run means the pump itself (motor or pressure switch) is suspect.

Quick motor test: If you're comfortable, you can briefly power the pump directly with fused jumper leads from the battery. If it runs this way, the problem is upstream (switch, wiring, fuse). If it doesn't, the motor or internal switch has failed.

Step 2: Check the water supply

It sounds obvious, but confirm the fresh tank actually has water. Tank gauges are notoriously unreliable, so look or listen. Then check:

• Winterizing valve position. Many RVs have a pump bypass or "winterize" valve with a short suction hose for drawing antifreeze. If it's left in the winterize position, the pump sucks air. Make sure it's set to draw from the fresh tank.
• Tank vs. city water valve. Some rigs have a valve that selects between tank fill and pump supply. Check your manual.
• Frozen lines. In shoulder seasons around Calgary, overnight lows can freeze an exposed pump or the line from the tank, even if the days are warm. If it's been below zero, warm the compartment gently (a space heater at a safe distance, not a torch) and wait.

Step 3: Clean the strainer

The inlet strainer catches sediment, sealant flakes and debris from the tank. A clogged screen starves the pump, causing noisy running and weak flow.

1. Turn off the pump and open a tap to relieve pressure.
2. Put a towel and small container under the strainer (water from the tank side will drain).
3. Unscrew the clear bowl, rinse the screen, check the O-ring and refit it hand-tight. Overtightening cracks the bowl. A light film of food-grade silicone grease on the O-ring helps it seal.

If your RV doesn't have a strainer, adding an inline pump strainer is cheap insurance.

Step 4: Find air leaks on the suction side

A pump that runs but won't prime is often sucking air rather than water. Any loose fitting, cracked hose or bad O-ring between the tank and the pump lets air in. Check:

• The strainer bowl O-ring (the most common one).
• Hose clamps and threaded fittings at the pump inlet.
• The winterizing siphon hose and its cap.
• The pump head itself. Freeze damage often cracks the plastic pump housing, especially if a rig wasn't fully winterized. Look for hairline cracks or a weep of water when it runs.

To prime a stubborn pump, open the tap closest to the pump and let it run until the water flows steadily, then close it and open the others one at a time.

Step 5: Pump cycles on and off by itself

If you hear the pump kick on briefly every few minutes with nothing open, pressure is leaking away somewhere:

1. Look for external leaks at every tap, the shower head, the toilet valve (watch the bowl for slowly rising water), the outside shower and the water heater pressure-relief valve. Check under sinks and in the basement for damp spots. A moisture meter helps with hidden ones.
2. Isolate the pump. If there's a valve right after the pump, close it. If the cycling stops, the leak is downstream in the plumbing. If it continues, the pump's internal check valve is leaking back, and the pump head or check-valve assembly needs service or replacement.

Rapid cycling while a tap is open at low flow is often normal on older pumps. An accumulator tank smooths this out.

Step 6: Pump runs but won't shut off

If there are no leaks and the tank has water, the pressure switch may need adjustment or replacement. Many pumps have a small adjustment screw on the switch. Follow the manufacturer's instructions carefully, as turning it the wrong way can push pressures higher than your plumbing likes. If the motor runs but output is weak, the diaphragm or valves are probably worn.

Replacing the pump

If the motor is dead or the head is cracked, replacement is usually a straightforward job:

1. Turn off the pump switch and disconnect the battery (or pull the fuse).
2. Photograph the wiring and hose layout before you touch anything.
3. Drain the lines by opening the lowest drains.
4. Note the old pump's model, flow rate (litres or gallons per minute), pressure rating and port size. Buy a replacement pump with matching specs. A higher-pressure pump isn't automatically better.
5. Mount the new pump on its rubber isolators to cut noise. Use the provided fittings and avoid overtightening plastic threads.
6. Reconnect the wiring with proper crimp connectors and heat shrink, restore power and prime the system. Check every connection for drips.

Prevent future pump problems

• Winterize properly. Blow out the lines and run RV antifreeze through the pump. Freeze damage is one of the most common causes of a cracked pump head in Canada. See How to winterize an RV for a Canadian winter (step by step).
• Clean the strainer each spring during de-winterizing.
• Fill your tank through a filter and don't let it sit stagnant for months.
• Turn the pump off when you leave the RV or use city water, so a burst fitting can't empty the tank onto the floor.

When to call a pro

Book an RV technician if you find damaged wiring you can't trace, water damage in floors or walls, or a leak inside a wall you can't reach. Also call one if the fuse keeps blowing after you've replaced the pump, since that suggests a short somewhere in the circuit.