Water pump
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- What is the pump doing? Fresh tank has water, pump switch on. Open a faucet and listen at the pump. Expect: Shurflo 4008: about 3.0 gpm, cut-off about 55 psi, cut-in about 40 psi, 7 to 8 A running. Aquatec 5.3: about 3.5 gpm.
- 12V at the pump leads with the switch on and a faucet open? Meter right at the pump's two wires. Expect: 12 V or more at the pump
- Suction side Check the tank has water and the tank valve is open. Pull and clean the inlet strainer. Look for a cracked fitting or loose clamp on the inlet hose. Expect: Water in the tank, clean strainer, tight fittings, winterize valve set to tank
- Is water leaving the system? Close every tap. Watch the pump for two minutes. Look under the coach, at the water heater relief valve, the toilet valve, and the outside shower. Expect: Pump shuts off and stays off with every tap closed
Causes and repairs
Pump has power and won't run
The pressure switch on the pump head is open (stuck), or the motor is dead. Tap the pump; if it starts, the switch or brushes are marginal. Meter across the pressure switch terminals with a faucet open: should be near 0 V (closed).
Tools: multimeter
Voltage across the switch with a tap open: replace the switch (sold as a kit) or the pump. 0 V across it and the motor still silent: motor, replace the pump.
No power to the pump
Pump fuse at the panel, the pump switch(es), or the wire. With two switches, a failed switch in parallel still lets the other work; a broken common wire kills both.
Tools: multimeter
Fuse first, then 12V out of the switch when on, then the wire to the pump and the pump's ground.
Tank empty or valve closed
Fill the tank, open the valve, prime with a faucet open.
Tools: none
Steady flow in under a minute.
Inlet strainer clogged
Sediment from the tank. Common after a fill from a rusty hydrant.
Tools: hand tools
Power off, clean the strainer bowl and screen, reinstall, prime. Sanitize the tank if it is bad.
Air leak on the suction side
A cracked fitting, a dry hose clamp, or a hose that popped off the tank outlet behind the panel.
Tools: hand tools
Tighten or replace every inlet fitting and clamp. Replace hardened hose. Prime.
Winterize valve in the wrong position
The pump is drawing from the antifreeze hose, not the tank.
Tools: none
Turn the valve to the tank position. Prime.
Pump worn or internal check valve failed
Good suction and it still won't prime: the diaphragm or the internal check valve. Test with a short hose straight from a bucket.
Tools: bucket, hose
Won't prime from a bucket: replace the pump (or the head on rebuildable models).
Leak in the system
Every drip makes the pump cycle and wears it out. Fix the leak.
Tools: hand tools
Repair the fitting or valve. Then confirm the pump holds for 10 minutes.
Check valve leaking back
Water flows backward through the pump's internal check valve to the tank, or out the city water inlet's check valve (look for a drip at the inlet with the pump on).
Tools: flashlight
Drip at the city inlet with the pump running: replace the inlet check valve. Otherwise replace the pump head or pump.
Slow leak
A weeping fitting behind a wall, a toilet valve seeping, or the water heater relief valve.
Tools: flashlight, paper towel
Pressurize, then walk every fixture with a flashlight and a dry paper towel. Fix what you find.
Pulsing: accumulator or restricted flow
A pump cycling fast while a tap runs is normal on a pump with no accumulator at low flow. Worse pulsing means a clogged strainer, an aerator restriction, or an accumulator that lost its air charge.
Tools: tire gauge, hand tools
Clean the strainer and aerators. Accumulator: pump off, open a tap to zero pressure, set the air charge about 2 psi below the pump's cut-in (on the pump label). Water from the Schrader valve means the bladder failed: replace it.
Pump won't reach cut-off
A big leak, a tap left open (outside shower), the pump can't build pressure (worn), or the pressure switch is stuck closed.
Tools: pressure gauge
Close every tap and the outside shower. Put a gauge on a fixture: should reach 45 to 55 psi. Won't build: pump. Builds and keeps running: pressure switch.
Low pressure everywhere
Clogged aerators and filters, a kinked hose, an inline filter plugged, a pump worn out, or a city regulator stuck if on city water.
Tools: pressure gauge
Aerators and filters first. Then gauge pressure at the pump outlet: 40 to 55 psi. Then the hoses.