Engine Safety Shut-off Switch Replacement

Electrical and electronic topics for small boats
marctroy
Posts: 1
Joined: Wed May 26, 2021 9:59 am

Engine Safety Shut-off Switch Replacement

Postby marctroy » Wed May 26, 2021 10:01 am

I finally got [the propulsion engine in] my newly acquired 15-foot Jet drive boat running right thanks to a new capacitor discharge ignition module, but now there's a new problem; the engine won't shut down.

The [safety engine shut-off lanyard] switch was disconnected and removed.

[Left in place are three] wires:

--two wires with insulation color BLACK with YELLOW stripe
--one wire with BLACK insulation.

Also, a new OEM replacement [ [safety engine shut-off lanyard] switch is not available from the boat builder.
[Aftermarket safety engine shut-off lanyard] switch replacements I can find have four terminals.

Help!

jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Engine Safety Shut-off Switch Replacement

Postby jimh » Wed May 26, 2021 11:53 am

If the propulsion engine is an OMC Jet Drive, the safety engine shut-off lanyard switch may be wired in a manner similar to the ignition key switch with integral engine shut-off switch as shown in an article and pictorial diagram in the REFERENCE section. See

https://continuouswave.com/whaler/refer ... witch.html

The circuit carried on the two conductors with insulation color BLACK with YELLOW stripe is part of the engine ignition system. The switch terminal is used as a wiring point to connect two elements of the circuit

The circuit carried on the conductor with BLACK insulation is the battery negative and engine chassis circuit.

The behavior of the switch is as follows:

There is a NORMALLY CLOSED circuit between the first M terminal and the second terminal M, so the BLACK with YELLOW circuit is connected to the BLACK circuit. This causes the engine to stop because its ignition primary voltage is shorted to ground. (This voltage can be as high as 300-VAC, so be careful working with this wiring when the engine is running. On some switches the M terminal intended for the BLACK with YELLOW stripe conductor is elevated above the other M terminal because of the high voltage on that conductor) Also, the M is for magneto. An engine with a magneto ignition can run without the battery connected, and that is why the engine keeps running in your situation; the BLACK with YELLOW conductor is never shorted to ground.

When the lanyard is in place, the switch is operated and the NORMALLY CLOSED circuit is opened, so the engine can run (because its ignition is no longer shorted to ground).

When the lanyard is pulled away, the switch reverts to the NORMALLY CLOSED state, and the ignition is shorted.

As you have discovered, without the BLACK with YELLOW circuit being wired to the engine safety shut-off lanyard switch, the engine won't shut off.

On the ignition key switch the OFF position also shorts the BLACK with YELLOW to ground (BLACK) to shut off a running engine.

Regarding finding aftermarket switches with four terminals, I suspect that those products have a set of NORMALLY OPEN contacts and a set of NORMALLY CLOSED contacts. This allows them to work with different brands of engines, as not all engines use the same method of suppressing engine spark.

jimh
Posts: 11670
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Engine Safety Shut-off Switch Replacement

Postby jimh » Wed May 26, 2021 3:01 pm

I have inferred that in your boat there are two switches: a key-operated engine OFF-RUN-START switch and a lanyard-operated ENGINE SHUT-OFF SAFETY switch. On many OMC engines the two were combined into one switch assembly.