Let's talk about Motor Solenoids and the 15059 Diode. Is the Diode a MUST HAVE? No. It is simply an insurance policy of sorts. Technically, there is a slight chance that as the Motor winds down after use, that it can send a voltage spike back to the controller through the Motor Solenoid. IF that was to occur, the Diode would send that spike to ground instead of back to the controller. The Meyer digital controllers (Touchpad and Pistol Grips) have a built in diode, but this is a larger one used on the Motor Solenoid for added first protection. The spike would have to make it past the 15059 first.

The Diode has a fork terminal on one end, that goes on the small post marked "S" under the ring terminal of the white wire (trigger wire), and on the other end, a ring terminal that goes under one of the Motor Solenoid mounting screws (provided that the Motor Solenoid is mounted to a grounded surface). The 3 post and 4 post black and white pictures are from Meyer literature. Picture 3 is an install I did that shows how I install the Diode EVERY time when using a 4 post Motor Solenoid.


The 4 Post Motor Solenoid is a little different than the 3 post Motor Solenoid in ways other than having one less post. The 4 post Motor Solenoid is a "Continuous Duty" rated Solenoid. Many Motor Solenoids are "intermittent Duty" rated, meaning it needs a rest between uses. Theoretically, the Continuous Duty does not need a rest, it can supply continuous power. The case (metal bracket) is insulated, meaning it is not grounded UNITL the post marked I is grounded. THEN the bracket becomes grounded. Because of this fact the 4 post Motor Solenoid does NOT have to be mounted on a surface that provides a ground. So it can be mounted on plastic. There are less and less places to fit the Motor Solenoid under the hood on newer vehicles, and there will only be more and more plastic in the future I am sure. Meyer incorporates a ground wire in the Universal Truck Side Harness (main Harness) that is connected to the ground lug at the battery. THIS is why in picture 3 you see that I connected the 15059 Diode to the S and I terminals. instead of the S and one of the Motor Solenoid mounting screws. You may also notice I cut the large ring terminal off the Diode and put a smaller one on better suited for the small post. I do not put heat shrink on this connection because I am going to coat the entire Motor Solenoid and all wires connected to it liberally with Fluid Film. So I am not worried about corrosion. Another VERY IMPORTANT fact about he 4 post Motor Solenoid is that the power (trigger) wire MUST be connected to the small post marked S. IF you connected power to the I terminal, you would create a dead short and the plow would not work. IF the case of the Motor Solenoid was insulated, it would not matter which (S or I) post got power and which got ground. the Meyer 15370 as I mentioned above is NOT insulated, so S and I are important to take note of.

You also may be wondering which side of the Motor Solenoid you should connect battery power to. It is my experience that IF one of the large posts is NOT marked "BAT" then it does not matter. The Meyer 15370 it does not matter which post battery power is connected to. I can tell you because the white wire is not long enough you almost always have to connect it to the post on the same side as the S post. The black (black with white stripe) is a little longer for some reason.

The 3 post Motor Solenoid MUST be mounted to a surface that will provide a good ground or the Motor Solenoid will not work. The 15059 Diode ring terminal must be mounted under one of the screws. In that case, in my shop I again cut off the large ring terminal and crimp on a smaller one (1/4") that better matches the (#10) self drilling screws I use to mount the Motor Solenoid.

The color (red or black) of the heat shrink on the 15059 Diode does not matter, it is the same part, but for whatever reason they used different colors.