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Old 07-03-2019, 02:15 AM   #1
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Default 1998 3000SCR alarm Temp switch

Twin 5.0 Merc Cruiser carb engines alarm goes on for a second on starboard engine during high RPMs Temp gauge reads about 175 the sending unit by thermostat housing but where is the temp switch for the alarm?
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Old 07-03-2019, 12:50 PM   #2
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Originally Posted by case1 View Post
Twin 5.0 Merc Cruiser carb engines alarm goes on for a second on starboard engine during high RPMs Temp gauge reads about 175 the sending unit by thermostat housing but where is the temp switch for the alarm?
The alarm temp switch is in the thermostat housing. Have you checked the gear lube reservoir? Common for it to be low and cause an alarm when on plane.
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Old 07-03-2019, 08:56 PM   #3
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Mike thanks for the reply,
Yes both gear lube reservoirs are full, oil pressure gauges read on the high side, but I have not checked oil pressure with a service gauge.
I ran just the starboard engine in the slip and brought the RPM up over 3K and the alarm sounded, temp gauge reading about 175 F.
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Old 07-03-2019, 09:19 PM   #4
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Default IR photo attached

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Old 07-03-2019, 09:25 PM   #5
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This photo was not taken at higher RPM, but warmed up at idle.
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Old 07-03-2019, 09:27 PM   #6
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While 175* is about 10* high it is not high enough to trigger the temp alarm. I believe it is set to sound at 188*.

Have you changed the impellers recently ?
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Old 07-03-2019, 09:44 PM   #7
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A few seasons ago replaced impellers but not many hours on them.
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Old 07-03-2019, 10:42 PM   #8
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If by a 'few seasons' you mean 3 or more years it's time for new ones.... They take a set when the boat is not used for long periods of time and the volume of water they move is diminished.... Worst thing you can do to a boat is not use it.

May not solve your alarm issue but it's time to change them.
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Old 07-04-2019, 12:22 AM   #9
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According to the scale the red looks to be in the 160ís but white is 173 or above and there is something white on the left side of the scan, what is that?

Have you tried disconnecting the alarm temp sensor to see if that shuts the alarm off? If not the next to try is the reservoir wire as sometimes the float sticks.
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Old 07-04-2019, 12:14 PM   #10
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The white on the photo is a tube that runs from the intake manifold to the exhaust manifold. Looks like the hottest in the picture
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