ServiceNomad RV · Knowledge
RV Water Pump Pulsing On and Off
A water pump that rapidly pulses on and off (cycling every few seconds) even when no faucets are open indicates a pressure issue in the plumbing system. This behavior is often called 'short cycling' and is caused by the pump trying to maintain system pressure but losing it faster than expected. The three main causes are a water leak somewhere in the system, a failed pressure switch or accumulator tank, or a check valve that's stuck open.
How RV Water Pumps Work
RV water pumps are demand pumps—they monitor system pressure with an internal pressure switch (usually set to 40-55 PSI). When you open a faucet, pressure drops, the switch closes, and the pump runs to restore pressure. When you close the faucet, pressure builds, the switch opens, and the pump shuts off. If the pump is pulsing when no faucets are open, it's because pressure is dropping for some reason, triggering the pump to run repeatedly. The pulsing pattern is distinctive: pump runs for 2-5 seconds, shuts off for 3-10 seconds, runs again, shuts off, and repeats. This differs from a pump that runs continuously (indicating a major leak or failed switch) or a pump that won't run at all.
Finding and Repairing Leaks
The most common cause of pulsing is a small leak in the plumbing system. Water is escaping slowly, causing pressure to drop gradually until it crosses the threshold that triggers the pump. The leak may be at a faucet connection, toilet valve, water heater relief valve, or an unseen crack in a PEX line hidden behind a wall or under the floor. To locate the leak, systematically inspect all visible plumbing connections while the pump is pulsing. Look for drips, wetness, or water stains. Turn off the water pump and watch the pressure gauge (if equipped)—if pressure drops even with the pump off, you definitely have a leak. For hidden leaks, a technician may need to pressurize the system with air and listen for hissing sounds.
Accumulator Tank and Pressure Switch Issues
Many RV water systems include a small accumulator tank (usually 1-2 quarts capacity) that stores water under pressure and dampens pump cycling. If the accumulator's internal bladder has failed, it can no longer hold pressure and the pump will pulse rapidly. Replacing the accumulator often solves chronic pulsing issues. The pressure switch inside the pump can also fail or become contaminated with debris, causing erratic switching. A technician can test the switch by monitoring voltage at the pump terminals while manually adjusting pressure—if the switch is chattering or not switching cleanly, the pump may need internal service or replacement.
Check Valve Failures
Some water pumps have an internal check valve that prevents water from flowing backward when the pump is off. If this check valve fails or gets stuck open, water drains backward through the pump, causing pressure loss. The pump senses the drop and kicks on to restore pressure, but as soon as it shuts off, water drains back again—creating the pulsing cycle. Check valve failures are often accompanied by a 'water hammer' sound or the sound of water draining back through the pump when you shut off a faucet. Replacing the pump or servicing the internal check valve resolves this issue.
💡 Prevention Tips
- Winterize properly and use compressed air to blow out lines—freezing can crack PEX lines and create slow leaks.
- Install a pressure gauge on your system to monitor trends—gradual pressure loss indicates developing leaks.
- Replace the water pump strainer screen annually—debris can cause check valve failures.
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