OMC Alarm Horn

Electrical and electronic topics for small boats
dbiscayne
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Joined: Mon Nov 02, 2020 2:44 pm

OMC Alarm Horn

Postby dbiscayne » Mon Nov 02, 2020 3:06 pm

[I] [NOTA BENE: the moderator has added a sentence SUBJECT to most of your your SENTENCE FRAGMENTS] have a 1996 Johnson 225 Ocean Runner. [The engine] has been a reliable engine [in 2018 and 2019], [but on the] last trip out [in 2020] the warning horn was faintly chirping[, that is, the horn was] on for a second then off for a second[.] I throttled back to idle speed, [and the warning] horn quit[,] but [it] would come on again (faintly) at a fast idle, [amd] then get louder as I increased speed.

[There was] plenty oil in the tank, and the engine seems to be putting out enough blue smoke to make me believe it is still getting oil.

[I c]hecked out the horn in the binnacle: it had two [violet] wires connected together, one tan [wire], and one black [wire] to ground. The epoxy potting was melted [and] trashed to the point when I pulled on one of the wires it came out of the horn fitting[amd] all.

[I r]eplaced the horn, all I could find was a two-wire setup [with one violet and one tan wires]. so left the black [wire] hanging there.

As soon as I had everything hooked up the horn sounded, but this time a steady BEEEEEEP. Then I disconnected all the sensors--overheat and oil level--to start testing.

[I t]ested the overheat sensors at the cylinder head[. T]hey both seem to do as they should. As soon as I connect the oil level sensor the alarm sounds with a steady BEEEEEEEP.

[Does this description] sound like the oil level sensor has given up?

One thing I don't understand: I did a continuity check from the tan wire going into the oil sensor and the black coming back from the sensor, [and I] got no continuity. These wires eventually get bundled into the harness and disappear under the flywheel somewhere. Could it be something has gone bad under the flywheel, fusing these wires together somewhere and triggering the horn?

jimh
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Re: OMC Alarm Horn

Postby jimh » Mon Nov 02, 2020 5:49 pm

The OMC Alarm System is described in some detail in an article in the REFERENCE section. See

http://continuouswave.com/whaler/reference/warningHornSignals.html

I will have to carefully review your narrative and compare it to the information in that article in order to arrive at a possible cause of the alarm behavior you describe. You should do the same; we can compare our conclusions here.

jimh
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Re: OMC Alarm Horn

Postby jimh » Thu Nov 05, 2020 12:48 pm

I have not had time to completely compare all your postulates with what is normal. I will leave that task to you for now.

I do note the following: the most likely cause of a warning aural alert that occurs only with higher engine speed is due to a problem with restriction in the fuel line. A vacuum switch monitors fuel line suction and signals an alarm when fuel line suction exceeds the threshold for the vacuum switch. Generally fuel line vacuum increases as the engine demands more fuel.

Also, note that the usual wiring for the aural alert is to have a 12-Volt signal provided continuously to the device, and then second lead from the device (usually TAN color insulation) is pulled to ground by a sensor. All the sensors are usually wired in parallel in what is known electrically as a wired-OR circuit, meaning that the alarm sounds when any of the wired-OR sensors goes to ground.

The OIL TANK LEVEL sensor may have some circuitry incorporated into the sensor housing so that it is not just a simple mechanical switch. This circuitry creates a distinctive CADENCE for the alarm so it can be identified and the operator can associated the alarm to the oil tank level being low. Because there is circuitry in the sensor, your notion that you should be able to measure a resistive value or check for DC continuity is not founded on a reasonable assumption.

I recommend you re-read the article I linked for you earlier and read it carefully and for content. It will help you understand how the OMC Alarm System works, and you will be able to diagnose the source of the problem causing the aural alert to sound.

I am not familiar with an aural alert or horn or buzzer that has multiple wires. Usually there are just two wires, RED and BLACK or perhaps VIOLET and TAN.

An article I wrote in the REFERENCE section lists wire COLOR CODES for Evinrude/Johnson engines. See

http://continuouswave.com/whaler/refere ... rCode.html

Please read the section on Evinrude E-TEC outboard engines, and see the subheading under SYSTEM CHECK.

These wire colors are used in the MWS (Modular Wiring System) harness that might have been used in c.1996 on your Johnson engine. You can identify the SENSOR functions from the wire color codes. Note that the change to the MWS harness was around 1997, so your 1996 engine may still have had the older wiring method.

Using the wire color code information I provide (see linked article above) may help you understand how the aural alert sounder should be wired. In the MWS system the SYSTEM CHECK device (the tachometer or the gauge) runs the aural alert, and the aural alert is not in a wired-OR arrangement with the sounders.

To diagnose why the aural alert sounds continuously, just disconnect all the sensors. If the aural alert still sounds there is something wrong in your wiring of it. If the aural alert does not sound continuously with all sensors disconnected, then re-connect the sensors one at a time until you discover which one is causing the aural alert to sound.

The reason the older aural alert sounder failed might have been it was being asked to run all the time and it burned out.