Slowing down pump during a Fresh Water Flush



The below method is typically used to change the pump speed during a flush. This sometimes needs to be adjusted if your feed pump is running too fast and drawing in seawater during a flush. If this is happening, many times the system will alarm 'Service Prefilter' during your flush.


Under the top of the feed pump module, under the MPC box, is the pump 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.


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 Fresh Water Flush button.

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 alarms Service Prefilters.


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 the pump speed during the specified mode in step 2.