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Old 17 March 2014, 17:24   #1
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Early 90's Mercury "classic' 40

My outboard is sounding it's over heating alarm and I can't figure out why. I just replaced the water pump (I triple checked the impeller is facing the correct way) I also changed the temp sensor sender. all were quicksilver replacement parts. The engine seems to be in very good shape but I am not sure of its service history.

water streams out pretty nicely at idle and POURS out at WOT.

The water is always warmish out of the stream

The alarm comes and goes totally randomly.... there seems to be no rime or reason as to why.

am I overlooking something?

any thoughts??
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Old 17 March 2014, 18:24   #2
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I can't remember that engine too well but there may be a thermostat and or water pressure relief valve diaphragm which may be causing an overheat.
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Old 17 March 2014, 20:35   #3
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Quote:
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I can't remember that engine too well but there may be a thermostat and or water pressure relief valve diaphragm which may be causing an overheat.
from what I understand, this motor has neither
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Old 18 March 2014, 04:11   #4
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Definitely overheat and not low oil?
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Old 18 March 2014, 04:36   #5
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Could be a frayed wire "earthing".

The temp sender effectively connects to earth during an overheat. A frayed wire "shorting" to any metal part could cause these symptoms.
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Old 18 March 2014, 08:30   #6
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Definitely overheat and not low oil?
Yes, as I understand it the overheat alarm is a constant noise were the low oil is an intermitent beep. I have disconnected the low oil sensor to be sure, and the problem remains

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Could be a frayed wire "earthing".

The temp sender effectively connects to earth during an overheat. A frayed wire "shorting" to any metal part could cause these symptoms.
This was going to be my next thing to ty and rule out.
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Old 18 March 2014, 09:04   #7
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My overheat alarm would come on after the ignition had been on for a few minutes, regardless of the engine running or not.


Turned out to be dodgy IGN switch.
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Old 18 March 2014, 09:11   #8
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My overheat alarm would come on after the ignition had been on for a few minutes, regardless off the engine running or not.


Turned out to be dodgy IGN switch.
really? was that on a Merc?
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Old 18 March 2014, 09:13   #9
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really? was that on a Merc?
Nope, it was on a Mariner but a Mercury Mariner.
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Old 19 March 2014, 20:03   #10
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How "classic" is it?

As said above a lot of these alarms are simply connecting the buzzer gnd to earth.

Worst case you might need to go through the entire loom totally randomly would hint to me at an earthing problem, BUT what else might fire that buzzer? (as mentioned low oil if it is an automixer).

I know this may sound suicidal, but why not disconnect the sensor, wrap the connection in tape & motor off (whilst keeping a close eye on the tell tale! ). If you get no alarms then you probably have a faulty sensor.


Other thing - it is the original sensor? There are a fair few that are physically interchangeable but have subtly different trip temps.
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