Servicing Your Boat's Potable Water Pump

By Tom Neale

If the water doesn't flow, here's how to find and fix two common causes.

 Crevalle 25 Bay

When you turn on the faucet serving that sink or shower in your boat, you want the water to flow. When you turn it off, you want it to stop. Too many times this doesn't happen, and usually the cause is your boat's potable water pump. The good news is that most of the problems are easily fixed with common sense or with "plug-and-play" parts readily available.

There are different brands of pumps but we're going to concentrate on the Shurflo types because the basic issues and fixes across brands are somewhat similar, and so many of us have this brand. The problems addressed here are hardly exclusive of all the possible issues, but these are frequently the culprits. Whatever the symptoms, consult the manual that came with your pump or should be available online.

Pressure Switch

Pressure switch illustration

Common Symptoms

If your water doesn't flow, your pump motor will either be running or not. If it runs very fast, you're probably out of water and you're going to have to get to a dock. If it doesn't run at all, odds are that you have to replace the pressure switch.

How It Works

The pressure switch closes the circuit from the boat's power source to the motor when there is no or low water pressure on the downstream side of the switch. It opens the circuit when there is the set amount of water pressure. It typically does this by the water in the system pushing against a small diaphragm, which flexes against a small spring and pushes a piston connected to what is probably a microswitch. The switch opens and closes rapidly and many times as you run water. Over time there will be malfunction from arcing or fatigue of parts.

How To Fix It

1. First, check to see that there is the proper amount of DC voltage (usually 12) at the DC incoming wires on the back of the motor. If there is no current, fix the wiring and you will probably have water.

2. Remove your pump from its space to where you can easily work with it, such as in the cockpit or on a bench. To do this, turn off the pump's circuit breaker and disconnect the two wires leading from the boat's wiring system to the pump. (You should have a quick disconnect in the wiring near the pump. If not, you may want to create one for future convenience.) Disconnect the output and input hoses. Typically you either loosen hose clamps or disconnect proprietary fittings that have come with the pump.

3. The switch assembly is usually located within a small raised plastic module on top of the pump head (the larger plastic housing on the front of the motor) and has two wires running from it to the back of the pump motor.

4. Disconnect those two wires. There may be a quick disconnect in the vicinity of the switch. If not, cut the wires leaving room to splice them. Note which wire goes to which.

5. The pressure switch assembly is held against the body of the pump head by several screws. (These are metal screws threaded into plastic and should be reinstalled carefully to avoid stripping the plastic threads.) Remove them and then remove the switch assembly. Install the new switch assembly in reverse order. Replacements are usually readily available at West Marine and other boating stores, as well as online. Typically, the very small switch itself will be within the assembly, and you may not have to deal with it directly. Also, sometimes the actual switch assembly will be included as part of a kit.

6. The switch kit will likely include other components relative to the switch, such as the check valve and switch diaphragm. Since you are in there anyway, replace these, too. Again, this is usually a plug-and-play replacement. Take care that all gaskets are in place and their seating surfaces are clean.

7. Sometimes your issue may be due to debris in the switch's diaphragm, portal, or check valve into the switch. Disassembly, examination, and clearing of these parts may solve the problem without needing to replace anything.

Valve Assembly

Valves and diaphragms illustration

Common Symptoms

In the night you hear your pump cycling on and off and chattering. It may mean water is leaking from plumbing downstream of the pump. But often, this is because of leaking or obstructed valves in the pump itself.

When the valves lose a tight seal, as they do sometimes from debris or from stiffening due to aging, the water on the pressurized side of the pump pushes back into the tank, the pressure switch senses loss of pressure, and the pump cycles on and off, usually rapidly. Even a check valve may not prevent this problem because of the small amount of water seepage. These valves often manifest this behavior before failing to work at all, which is good. It's easy to fix.

How It Works

The valves and diaphragm(s) in most newer pumps are in a plastic housing attached to the front of the motor. These pumps utilize what are sometimes known as wobble plates. The motor shaft drives an eccentric component, which turns its circular motion into a reciprocal motion, which makes the diaphragm go up and down rapidly. The valves open and close, sucking water in and pushing it out. But you normally don't have to worry much about the theory and complexity to correct your problem.

How To Fix It

1. There are frequently three or four valves, depending on the pump's capacity, and they come assembled in a valve kit.

2. Remove the valve-housing chamber. Usually this is the rounded or sometimes flat plastic part attached at the front of the motor with the switch housing on top of it. The chamber is usually secured by four or more metal screws threaded into plastic. You should see the valve set inside. Remove the old valve set housing with its valves, and insert the new housing with new valves. Reinstall the screws carefully to avoid stripping any plastic threads.

3. Be sure that all gasket surfaces are clean and mate evenly.

4. Before replacing the valves, check for debris in the old ones. Often debris will be caught between the rubber valve and the metal webbing in which it sits. Simply removing this may fix your problem without the need of a new part.

5. Inspect the diaphragms in the drive assembly for wear, debris or stiffness. You can replace this just as easily, and some prefer to do it anyway. 

— Published: April/May 2018


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