An RV water pump is a diaphragm pump with a pressure switch. It runs until line pressure reaches the cut-off point, then stops and holds pressure against a check valve until you open a tap. When it cycles, something is letting that pressure escape or the switch is not reading it correctly. There are three distinct symptoms, and each points to a different set of causes. Identify yours first.
Symptom A: pump runs briefly every few minutes with every tap closed
Pressure is leaking away somewhere between the pump outlet and the faucets. The pump does exactly what it should: it tops the pressure back up. The interval tells you how big the leak is; a cycle every 30 seconds is a real leak, every 20 minutes is a slow drip.
Check in this order:
- Every faucet, including the outside shower and the outdoor kitchen. A shower head with a shut-off valve on the handset, like the water-saving RV shower head, leaves the mixer open; a slow weep past the handset valve is easy to miss.
- The toilet. The flush valve on RV toilets is the most common hidden leak. Water seeps into the bowl silently. Dry the bowl and look for a wet ring after ten minutes. The RV toilet flush valve ($19.99) is a cheap fix.
- The water heater. A tankless heater's relief or drain valve, or a tank heater's pressure relief, weeping outside where you cannot hear it.
- The city water inlet. The check valve in the inlet stops pump pressure escaping out the hose fitting. When it fails, you will see a drip at the inlet with the cap off. Replace with an inlet with brass check valve ($12.99) or the OEM Black Series inlet.
- Fittings under the sinks, behind the shower, and at the pump. Dry paper towel at each joint. Push-fit and compression fittings on RVs loosen with vibration. Snug, or replace the fitting.
- The pump's internal check valve. If nothing above is wet, the pump is letting water flow backward into the tank. Test: with the pump off and the tank low, open a tap; if the line depressurizes and stays flat but no leak is found, the valve is the culprit. Some pumps have a serviceable check valve; on most, replacement is the practical fix. The 12 V water pump ($99.00) is the Black Series standard, and the replacement guide takes about an hour.
Symptom B: pump pulses or hammers while a tap is open
The pump is switching on and off rapidly because flow is low enough that pressure hits the cut-off almost immediately. This is annoying rather than damaging, but it shortens switch life and makes showers unpleasant.
- Restriction downstream. A clogged aerator, a partially closed valve, or a spent inline water filter. Pull the aerator; if flow improves, clean or replace it. A clogged filter is the most common cause on campers that fill from creeks.
- No accumulator. An accumulator tank gives the pump somewhere to push against. Many RVs have none. Adding a small one on the pump outlet smooths flow and reduces cycling substantially.
- Pressure switch set too tight. Some pumps have an adjustable switch; a small adjustment widens the gap between cut-in and cut-out. Consult the pump's manual before turning anything.
- Kinked or undersized line. Check the flexible hose at the pump inlet and outlet for kinks; a replacement fitting ($18.00) with a straight run is often the fix.
Symptom C: pump runs continuously and will not shut off
Pressure never reaches the cut-off.
- Empty tank or closed tank valve. Obvious, but check. A stuck tank level sender can read full when it is not.
- Air leak on the suction side. A loose fitting or cracked hose between tank and pump lets the pump suck air instead of water. The pump runs, sounds normal, and delivers a sputter. Tighten the inlet fitting and check the strainer bowl seal.
- Clogged strainer. Remove the clear strainer bowl at the pump inlet and clean the screen.
- Failed diaphragm. The pump runs, gets warm, and moves little water. Replace the pump.
- Pressure switch failed. Rare, but it happens. If the pump builds obvious pressure at the tap and still does not stop, the switch is stuck closed.
- Large leak. A burst line under the floor or a split tank. Look under the camper for water.
Winter damage
If the cycling started after the first freeze, assume a cracked fitting, faucet cartridge, or pump housing. Ice splits things along seams that then weep under pressure. Check the pump body itself for a hairline crack; they are hard to see and easy to feel as a damp patch. The winterizing checklist and the Winterization Service exist for exactly this reason.
A note on pump pressure and tankless heaters
Tankless water heaters need a minimum flow to ignite, and a cycling pump delivers flow in bursts. If the heater lights and goes out repeatedly, fix the pump cycling first, then revisit the tankless troubleshooting guide if the heater is still misbehaving.
Parts to carry
A spare pump, a strainer, two of each fitting size you use, a water hose quick-release connector ($9.99), and a bag of stainless hose clamps ($8.99). With those, every fault above is fixable at a campsite.
Shop this
- Water Pump 12V — $99.00
- RV Toilet Flush Valve — $19.99
- RV City Water Inlet with Brass Check Valve — $12.99
- Fitting for Water Pump — $18.00
- RV Water System Kit (collection)