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Troubleshooter: Shake Hands with Solenoids

Courtesy of PassageMaker Magazine

Like the proverbial tail wagging the dog, a $50 solenoid can stop a $3,000 windlass from raising the anchor or prevent the engine from starting. This simple device also figures in the operation of the propane stove and the windshield washer. If you understand how a solenoid works and how to test one, you can often come up with a work-around that will enable you to continue a cruise or at least make it back to the dock.

Solenoids allow a small amount of electricity to trigger a greater action, such as connecting a high-amperage switch, or actuating a valve. A starter motor, or a windlass, demands a lot of current. This high draw requires heavy-gauge wire of the type you see connected to a battery. Without a solenoid, this cable would have to be run all the way from the battery to the switch that starts the motor, and then down to the motor itself. The switch would have to be large and robust so that it could handle the load. This arrangement would create a loss of voltage due to the long run of wire, additional weight and cost due to the extra wire runs, and the cost and complexity of a switch that can handle such high loads.

Screen shot 2015-10-21 at 12.16.12 PMInstead of running the heavy gauge wire all the way to the switch, the solenoid permits a much lighter gauge to be used. When you press the start button or step on the windlass foot switch a small amount of control current flows to the solenoid which then causes the two high-amperage terminals to connect, allowing a high current flow directly to the windlass motor.

Most solenoids consist of two larger terminals, or studs, and one or two smaller ones. The large terminals receive the heavy gauge wire and are connected to each other only when the solenoid has been activated by a key switch, toggle, or foot switch (the control circuit). One of the large terminals is connected to a wire that is hot, while the other remains isolated until the connection is made. We refer to the terminal that receives the incoming voltage as the line side, and the terminal that feeds the device (starter, windlass, etc.) as the load side.

The small terminals handle the control circuit. In some cases, there are two small terminals, one for the DC positive wire and the other for the negative. When the solenoid is bolted to an engine, these bolts connect the solenoid to ground, and therefore only one wire is needed for the control circuit.

A boat’s propane system relies on the same principle. In this case, however, instead of providing a way to remotely make an electrical connection, the solenoid allows you to remotely stop the flow of propane gas into the boat. The switch in the galley sends a control current to a solenoid mounted at the LP cylinder, causing a piston to move out of the way and allowing the gas to flow. Windshield washers with spray nozzles work in the same manner as a propane solenoid, allowing or stopping the flow of water by using a low amperage control circuit to open or close a valve.

A remote starter, such as this one, can allow you to trigger a solenoid manually.

A remote starter, such as this one, can allow you to trigger a solenoid manually.

The starter on your engine presents the most demanding application of a solenoid. The starter on a marine diesel engine draws a high amount of current for a very brief time. A typical 300hp engine will demand several hundred amps during normal start-up (by comparison, all of the cabin lights turned on at once might draw around 25 amps). In addition to making the electrical connections, modern starter solenoids also contain a mechanism that thrusts a pinion gear into position to engage the starter with the engine ring gear.

The demands placed on the starter solenoid require a significant supply of current to the solenoid control circuit. For this reason, most modern diesels rely upon a smaller solenoid, often referred to as a slave solenoid, to trip the larger starter solenoid.

Click here for the full article and some troubleshooting tips.

Steve Zimmerman is the president of Zimmerman Marine, which operates four boatyards in Maryland, Virginia and North Carolina. Zimmermanhas been repairing and building boats for almost four decades. He is also PassageMaker’s Technical Editor. 

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