Boat Repair - Momentary Switches

By Tom Neale
Published: December 2012

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 horns
If 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.

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Photo of monetary switch from a horn
Monetary switch from a horn - the wires attach to the terminals at the base.

Photo of opening switch by bending back tabs
Carefully opening switch by bending back tabs.

Photo of removing terminal plate from switch box
Removing terminal plate from switch box.

 

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.)

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