Boat Repair - Momentary Switches

By Tom Neale

Momentary switches make generators and engines start at the push of a button. But when they don't start, consider the switch rather than the component itself.

Photo of a ship's hornsIf your horn won't blow, or blows erratically, it's probably the switch that's the problem.

A momentary switch can come in numerous forms. You're probably most familiar with the little push button on your panel at the helm. Let's use the button for the horn as an example. You push it in and the horn blows. You take your finger off, it pops back out, and the horn stops blowing. Suppose the horn doesn't blow, or blows erratically, when you push the button in. The issue may be corroded connections at the switch terminals or elsewhere. This is easy to fix so check connections at the switch, horn, and power sources, before going further. If you see corrosion or other impairments such as a loose connection, fix that first. This may be as simple as tightening a screw or disconnecting the connection, cleaning it with an abrasive, adding a squirt of moisture-disbursing oil, and reassembling. If you don't see a problem with the wiring and connections, suspect the switch.

Finding The Fault

The basics of a momentary switch are fairly simple. The button is essentially a plunger held in the out position within its casing by a spring. At the other end of the plunger (inside the switch housing) is a contact surface. This is a metal conductor attached to the plunger, or pushed by the plunger, so that it contacts (jumps) and thus connects the back ends of the two wire terminals. The circuit is completed and the horn blows. When you remove your finger, the spring pushes the plunger back out and the contact is broken.

You can usually determine quickly if the switch itself is the culprit by carefully jumping the wires, which are connected to the terminals behind the switch. Simply remove one wire from its terminal and touch it to the other. This normally causes a spark, and sparks cause explosions and fire unless the area where you are working is completely free of combustibles, including gases. You can also get a small shock if you're not careful. Some mechanics use an improvised jumper wire, which is a short piece of well-insulated wire with terminals at each end and a positive on/off switch in the middle. You turn the wire switch off, connect the wire ends to the momentary switch terminals, turn the wire switch on, and this more safely jumps the terminals. This wire and its on/off switch must be heavy duty enough to safely conduct the current involved. (If you have any doubt, get a qualified professional to do this simple check.) If the horn blows when you jump the wires, your issue is probably in the switch.

You can also test the switch, in theory, by removing both wires and using a volt-ohm meter to test for continuity by touching the probes to the switch terminals. There should be absolute continuity when you push the button. If there is not continuity, or if it is intermittent, you've located the problem. But volt-ohm meters are sometimes difficult to interpret unless you're familiar with them. (Notice I use words such as "probably" and "generally" a lot. This is because, with boat work, even simple boat work, few things are certain and it's important to always expect the unexpected.)

The Inner Workings

Given how simple momentary switches are, what can go wrong?

Usually the contact points become pitted or corroded to the extent that they pass either no current or current inadequate to do the job well. The fix may be to simply sand or file the contacts. You must disconnect the wiring and remove the switch from the panel first and open the housing at that end of the switch. This may be easy or not worth the effort, depending on the switch, but in an emergency situation you may be glad you know how to solve the problem at least temporarily.

Another type of problem is that the contact component shifts or becomes dislodged from its assembly and therefore doesn't firmly seat against the backside of the wire terminals when you push the button. This is usually caused by age and wear, and is seldom worth fixing. (A well-made switch will typically last a long time before this happens.)

A third issue can occur if there is obstruction between the plunger and the shaft walls in which it moves. This could occur from problems such as too much lubricant that has hardened from dirt, or from corrosion. Depending on the build of your switch, this may be easy or difficult to access and clean using very light emery polishing paper or perhaps just a rough clean rag. It can be a particularly worrisome problem because often the plunger gets stuck in the down position, meaning that the horn won't turn off. This problem is far worse if the momentary switch is being used to start an engine. I've seen these switches stick down in this application, causing the starter solenoid to remain activated, and thus continuing to run the starter after the engine has fired off. This can quickly ruin a very expensive starter and related components and is a good reason to replace these relatively inexpensive switches regularly.

When One Switch Activates Another

The solenoid on an engine starter illustrates a multifaceted application of this type of switch. Instead of utilizing the muscle power from your finger to push the plunger, you utilize 12-volt current by turning the key or pushing the starter button, which sends the current to wires in the wall of the solenoid, creating magnetism. The "finger" in the engine solenoid is the heavy metal shaft in the open end of the solenoid. It is held in position by springs. The magnetism from the coil overcomes the springs' resistance, pulling the shaft in. This does two things in a typical starter arrangement. The shaft, as it is pulled to the end of its chamber, pulls the starter's gear out so that it engages the engine. But the other end of the shaft, as it is thrust forward, contacts and pushes a "button" which is essentially another momentary switch.

This button is a much smaller shaft, also normally held out by springs. It moves forward to move a contact, typically a large copper contact disk (it looks like a washer), to close the circuit between the heavy-duty hot wire from the battery, to the positive wire going into the starter, which then causes the starter to begin spinning. So with this system, you are utilizing two "momentary switches." Some boats have a relay solenoid between the starter button and the starter solenoid.

With age, the interior wall of the housing for the main plunger can become rusty, clogged with dust and grease, or otherwise impaired. If this happens, the plunger may get stuck in the engaged position, forcing the starter's gearing to remain engaged with the engine (unless the speed of the engine overcomes this engagement) and holding down the momentary switch at the opposite end, continuously powering the starter. Big trouble! The cure is to regularly pull the solenoid (often you can do this without removing the starter), inspect the inside of the cylinder, and clean if necessary. Also, if you suspect a problem there, carefully remove the cap over the opposite end and inspect the contact points. Arcing from normal operation will eventually pit the contacts to the extent that they hang up or don't make sufficient contact to carry the amount of current required. Sometimes removing this cap (usually plastic) is more of a problem than most people want and replacement of the solenoid is the preferred solution.

There are many variations of this type of switch, and yours may be considerably different from what we've discussed here, but if you understand the principle, you can probably deal with the problem. All of this may sound complex, but it's not and you may save yourself a lot of money or a ruined weekend if you have a general understanding of what's going on when you push that button. 

Published: December 2012

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