Slowing Down Pump During a Fresh Water Flush



The below method is typically used during system commissioning to set the vane pump/motor speed during a freshwater flush cycle. The factory setting is a default parameter that may need to be fine tuned depending on install specifics such as length of hose runs, strength of freshwater pump, etc. Often times, it must be adjusted to a lower speed if your feed pump is running too fast and calling for more flow than the house pump can deliver. If this is happening, many times the system will draw in seawater to compensate, or can alarm 'Low Pressure, System Stopped' or 'Service Prefilter' during your flush.


On DC models, inside the feed pump module is the motor speed controller. On the speed controller board are two magnetic reed switches for adjusting the pump motor speed. The switches are narrow black bars, 5/8” (16mm) long. The increase speed switch is labeled S2; the decrease speed switch is labeled S3. Each time a small magnet is placed near the switch with the pump is running, the speed will change slightly. This can be confirmed by placing a voltmeter on the outer studs labeled MOT+ and MOT -.

In the front are 4 posts. The inner posts two should always show battery power (a good place to measure the ship's power supply). The outer posts show are connected directly to the motor. The green plug has 3 wires which act as signal wires. They come from the circuit board as well as the toggle switch. Depending on which signal is engaged (with 12V) the motor will receive different voltages; ~12.5 for run high, ~7.7 for run low and ~5.25 for a flush. You can use experimental testing to see which wire corresponds to which speed or refer to a wiring diagram.

Adjusting the speed:

1. Open the thru-hull for the raw water inlet. (this step may be skipped if in an area with poor quality water such as a harbor or marina)

2. Push the Freshwater Flush button ("Auto Store" on MPC units)

3. Gently tap a magnet on the S3 Reed Switch (Decrease Speed Switch). Wait 3 seconds between each pass of the magnet.

4. Gently tap the magnet 3 times, then go back to step 3. Repeat this process until the system no longer draws in seawater during a flush or trips a fault alarm.


Once you’ve adjusted the speed correctly, the speed controller will stay programmed for this speed during freshwater flushes. You can also use this method to increase or decrease the pump speed during the mode specified in step 2.

Along with setting the pump speed, the flush duration should also be calibrated so that the system is sufficiently flushed but no additional freshwater is wasted. See Optimizing Flush Duration for a write-up on this process.