Water pumps in Class B RVs, or pretty much all RVs for that matter, are just about the same. The basic design was developed back in the middle Cretaceous and hasn't changed within living memory, because it doesn't have to – it does the job. What I would like to do is demystify this device so that the average RV owner will know what's going on and how to troubleshoot any water supply problems that arise.
Above you have your basic water pump. The metal part is a big 12 volt DC electric motor, and the black plastic part is the pump head, where the water is pressurized and delivered to your RV's water outlets.
There's nothing fancy about the electric motor part – it's a 5 amp (60 watt) low-RPM electric motor, like you would find in your windshield wipers or any other automotive application. Eventually they get old and tired and the brushes and bearings wear out. If your pump doesn't pump, but will run if you thump the motor housing with a blunt object, the motor is in the process of giving up the ghost. You can replace the motor part separately if the head is still good.
The motor shaft has a flat spot on it, and transmits rotary motion to the base of the head. Notice that it's off-center? That's on purpose, and how the pump works. As the off-center part rotates around, it pushes on each of the three diaphragms below it in turn, which is what changes the chamber size and pumps the water.
Here are the three diaphragms on the other side of the pump head base. These get damaged if you winterize by blowing compressed air through your water system, because the high speed airflow stresses them in ways that they aren't used to flexing. It's cheaper and better for your system to winterize with the pink RV antifreeze, and save the compressed air for your tires.
This is the top of the pump head, showing the three chambers above the diaphragms. One-way check valves allow the water to go out of the chamber as it's compressed, and bring in more water as it's expanded. I know it's simple, but don't laugh – this system has been reliably supplying RVers with their water since World War II.
The mechanism for maintaining a steady pressure is also mind-numbingly simple. There's a pressure switch, which is another diaphragm at the top of the three-lobed chamber. There's an adjustable spring in this pressure switch, and when the water pressure is high enough, the spring is compressed and the switch opens, turning the pump off. Use some water, the pressure drops, the switch closes, and the pump comes back on until the pressure builds back up to the set point. That's why your pump turns off and on when you run water slowly – it's cycling up to the set point, turning off, and turning back on when the pressure drops. In the wiring to your pump, 12 volt power comes to this pressure switch first, and then to the pump motor. There are two spade terminals on this switch you need to check for tightness if you get a no-water situation.
Here's a pump as installed, and on the suction side (the side coming from your fresh water tank) I'm pointing to the filter housing. Once a year or so, unscrew this housing and clean out the debris that has accumulated. Low flow problems are often due to debris blocking this filter. If the housing is not properly secured after cleaning, the pump will suck air through the gap around the housing edge instead of water from the tank, and it will act as if you're running out of water when your tank is full – bubbles and low flow.
RV water pumps get used frequently, and do wear out, but they are surprisingly easy to change. It's just the two water fittings and two wires. Consider carrying a spare if you're going to be on a long trip far from home and repair facilities. Online, they're maybe $100 or so, and a bad pump can bring your trip to a screeching halt if you don't have a way to fix it. I carry around an extra motor and two heads, as swap parts around as needed until I'm running again.
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