Q&A with Tom Neale - What are solenoids, and what do they do?

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Q: What are solenoids, and what do they do?

A:Solenoids are used in many applications on boats, as well as elsewhere. Unfortunately, because of how they work, they are particularly susceptible to slow-developing damage in the marine environment.

Stated simply, the solenoids commonly used on boats usually consist of wire wrapped around a metal cylinder with a metal piston inside the cylinder. The piston is normally held partially out of the cylinder by a spring. When electricity is introduced to the wrapping of wires, it magnetizes the wires and that pulls the piston into the solenoid, overcoming the spring.

While there are many types of solenoids and various uses of the term, our solenoids on board usually perform one or more of three functions. They may convert electric energy into linear motion to operate an external device, act as a switch or relay, and/or serve as remotely operated valves. For the first function, the piston is typically attached to a lever, such as a fuel stop control or a choke. For example, when you push the stop switch (probably a normally open momentary switch) at the steering station, it closes a DC circuit sending the electricity to the wire wrappings of a solenoid and magnetizing them. This pulls in the piston, which is attached to the fuel shut-off lever on the fuel pump (usually the injector pump on a diesel). The fuel is shut off, and the engine stops.

Your starter probably has a more complicated solenoid, performing two of those functions. When you depress the momentary starting switch at the helm, the current flowing through those contacts isn’t the current that actually turns the starter. This is the current that passes through the coil in the starter solenoid. When the piston is drawn into the cylinder, it pulls a lever (linear motion) that pushes forward the starter’s gear until it engages with the teeth on the engine flywheel. But the piston also presses a switch at the base of the cylinder. If you look into your solenoid cylinder you’ll see it. It’ll probably look like a button. It’s normally held in the out position by a spring. When it’s depressed by the piston, the heavy-duty contact connected to it in the head of the solenoid (you normally can’t see this unless you disassemble the head) touches the contacts to which the starting battery cable and starter motor cable are attached. This allows high-amperage current to flow into the starter motor, causing it to spin so that it can turn over the engine.

If you have a propane stove, you should have a switch near the stove that closes the propane line at the tank. When you flip this switch it sends current to a specially designed solenoid valve at the tank. Typically the piston doesn’t open and close the fuel line by pulling or pushing an external lever. The piston itself acts as an internal valve. Depending on its position, the piston either blocks or opens the fuel passage. These solenoid valves must be ignition-proof and closed to the atmosphere, so they are usually much more expensive than many other solenoids.

Problems typically occur when the piston’s movement is impaired by a buildup of rust, rust dust, dirt or grease. Solenoids should be protected by rubber boots or similar devices. Some are sealed and can’t be serviced. If it’s easy and safe to do so, removing the piston from the cylinder and cleaning both with a clean, lint-free cotton rag will fix a sticking solenoid, at least for awhile. (You must usually replace a valve solenoid.) Don’t grease the cylinder when you reassemble the unit. Some mechanics lightly spray a very fine mist of a product such as CRC 646. If you don’t have time to do this, try tapping the solenoid with a safe object. Depending on the circumstances, this may be the hard plastic handle of a large screwdriver. This may shake the piston free so that you can, for example, start that engine and get home.

This obviously is a very general, basic discussion of solenoids. Consult a qualified mechanic for any issues you may have.