Adjusting the DC Motor Speed Controller

DC Speed Controller Adjustments

The DC Motor Speed Controller is a simple device once you wrap your head around it.

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 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. Note that 12V systems will run optimally when they receive a minimum of 13.2VDC. You must use the wiring diagram or experimental testing to see which wire corresponds to which speed.

When the systems are built and commissioned the speeds can be adjusted. This is done with a magnet in the field. When the motor is running at a particular speed, simply tap a magnet on the S2 or S3 reed switch and the motor will speed up (S2) or slow down (S3). The outer posts typically change 0.1 V with every tap of the magnet so you will need to tap it a few times before you notice a difference.


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. Operate the system in the mode you intend to change (Run HI, Run LO, or Flush)

3. Gently tap a magnet on the S2 Reed Switch (increase Speed Switch) or S3 Reed Switch (decrease). 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 feed pump operates normally during the specified mode and receives proper voltage.

Once you’ve adjusted the speed correctly, the speed controller will stay programmed for this speed during the specified mode.