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Troubleshooter: Shake Hands with Solenoids

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. Knowing how these simple devices work may save your cruise.
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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.

Instead 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. 

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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.


Each solenoid application has its own variation on a theme, and the following troubleshooting procedure applies to most of them. Let’s imagine that you attempt to start some device (engine, windlass, etc.) and nothing happens. After checking the obvious (circuit breaker and/or fuse OK, main disconnect switch on), you wonder if current is making its way to the device. Remember, the current must pass through the solenoid in order to complete the circuit.

Step #1: Is power getting from the source to the solenoid?

With a voltmeter or test light, check for power on the line side large terminal of the solenoid (one test lead on the stud, the other lead to ground). If there is no power, then the problem lies elsewhere—blown fuse, tripped breaker, battery disconnect turned off. If you do have power on the line side, then you need to determine if the solenoid is working.

Step #2: Is the solenoid working?

Now it’s time to check the control circuit, which are the smaller terminals. If there are two terminals, the test leads can be placed on each one. If there is only one terminal, one lead will have to be attached to ground. With the test device in place, have someone turn the key or flip the switch that normally starts whatever you are troubleshooting.

If the test shows no voltage while the switch is held on, the problem lies elsewhere, perhaps in the key switch or foot switch, or in the power supply to that switch. In other words, the solenoid is not the problem. The problem lies somewhere in the control circuit that leads to the solenoid.

If you do not have the time or the ability to track down the cause, you can bypass the control circuit and trigger the solenoid manually. For these situations it is good to have aboard a simple and inexpensive device known as a mechanic’s remote start switch (available at NAPA, Sears, etc.) A pair of wires with spring clips will also work and in many configurations the same can be accomplished using an insulated screwdriver. The remote start switch is the safest method.

triggering slave solenoids with a remote starter

To bypass the failed control circuit, attach one lead of the remote starter switch to the larger terminal on the line (hot) side of the larger terminals and the other lead to the single small terminal, or if there are two, the small terminal with the red wire attached (See opening photo). Now when you press the button on the remote starter switch it will serve the same function as the failed control circuit. This action provides a momentary connection to trigger the solenoid, which will then power the device until you release the button.

 A mechanic is using a remote starter switch to bypass the failed control circuit.

A mechanic is using a remote starter switch to bypass the failed control circuit.

If the test does show proper voltage while the switch is held on, then the solenoid likely has failed and a different solution is needed.

Step #3: If the solenoid has failed, is there a temporary work-around?

The first step is the simplest one. While your shipmate activates the control circuit by turning the key or stepping on the foot switch, tap on the solenoid using a plastic mallet or the wooden handle of a hammer. Sometimes a solenoid will stick, and tapping on it might do the trick. If the mallet does not free it, you will have to tackle the problem a different way.

The following procedure applies to most solenoid situations, but does not apply to the starter solenoid (we will address that shortly). Begin by turning off all power to the device. Confirm that the power has been disconnected by testing for voltage on the line side of the solenoid’s larger terminal posts. Remove the wires from the two large studs, and attach both wires to the same large stud (either one will do). You have now fully bypassed the solenoid. Remember—as soon as you supply power, the device will run, so exercise caution. Tape over the power-disconnect switch or breaker so that you do not accidentally engage the motor.

The same can be accomplished by simply holding a large insulated screwdriver across the two main terminals. This method is riskier than the one described above: Be prepared for some sparks and make sure you are clear of all moving parts. This basic procedure works for most solenoids.

Many windlasses have two solenoids, one for raising, and one for lowering. If the down solenoid has failed, you are in luck, since you can lower the anchor manually. If the up solenoid has failed, you can follow the procedure described above, or switch the control leads from the two solenoids. After switching the control leads, when you press the down switch the windlass will raise the anchor. You might have to refer to the owner’s manual to identify the correct solenoid. Label the switches so that you do not forget they are reversed.


A remote starter in action.

A remote starter in action.

Most marine diesel engines utilize two solenoids. The starter solenoid will be attached to the starter. If you follow the control wires coming to the starter solenoid, you will soon locate the slave solenoid. In our experience the slave solenoids fail more often that the starter solenoids. You should begin the troubleshooting process at the slave solenoid, and the procedures described above apply, with one important exception: Instead of connecting two wires on one of the larger studs, use a pair of heavy-duty jumper cables to make a temporary connection between the two large terminals. This difference is important, because as soon as the engine starts, you will have to remove the jumper cables quickly. Also remember that the ignition switch must be on.

If the slave solenoid is not the problem, repeat the tests on the starter solenoid. If the tests indicate that the starter solenoid has failed, you should stop there. On modern diesels there is no point in jumping the main solenoid terminals, since the pinion gear function will not be activated. The starter will spin but will not engage the engine’s ring gear.


Sailors should learn to reef their sails before the wind starts to howl and the same principle applies here. Given the number of solenoids on a cruising powerboat and the variety of applications and types, it makes sense to familiarize yourself with them before the need arises. At a minimum, learn about your starter solenoid and its slave solenoid, since these are the most critical and often the most difficult to access. If you can’t identify the components on your own, it will be well worth the cost of a mechanic’s time to show you. Once you understand the fundamentals, you will be well prepared to deal with a variety of situations.

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.