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70HP steady warning horn

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skipbo00

New member
"Greeting All I am new here bu

"Greeting All I am new here but have been reading your forums for some time.... now for the Question.. I have a 1984 ( I think) 70HP 3 cyl Mercury OB.. The warning horn sounds after running it for a little while, maybe after 5 to 10 minutes of running,, horn sounds while running at full throttle or down to idle... while trolling it may sound or not or maybe just a quick beep,, if it does I vary the rpms a couple of hundred rpms it quits some times,, I changed the impeller thinking it was worn out but it wasn't... the water coming out the squirter in the back is basically lake water temp NOT hot???? I took the thermostat out and put it in a pan of water and warrmed it up and it started to open slowly untill fully open..the oil Inj. resivoir is full of oil.. I have a water PSI gauge on the dash it reads 11 psi trolling and 11 to 15 running... do you think it may be the module??? the module is the most expensive of the parts I think... I would think temp related as it waits untill warmed up.... any ideas would be much appreciated..

Thanks
Skip"
 
"Skip, a "constant" ho

"Skip, a "constant" horn usually is an overheat condition - the oil warning bleeps.

With your cooling system apparently working correctly there is a good chance that the temperature sensor/sending unit is starting to give up on you.

These are a pretty simple design and do fail with some regularity.

Basically the way they work is like a switch. They stay off until the temp gets up about 170 degrees and then start to turn on (resistance based). At about 190 degrees they turn on completely (total continuity) and the horn blasts to let you know of the overheat.

Yours may work on slightly different numbers, but that's the general gist of a Merc temp sensor.

They can be tested with an ohm meter, pot of water on the stove and a thermometer. Or can be replaced for about 40 bucks.

Or you could disconnect it and see if the horn issue goes away. But I would only do that long enough to troubleshoot. Replacing a bad part is alot cheaper than cooking your powerhead because of a real overheat..."
 
"graham,,,, is there a way to

"graham,,,, is there a way to put a real water temp gauge on there???? I would rather read a temp than hope the horn is about correct... I was looking on this site and can't figure out what temp switch mine has on it... my serial is #OB344948.. any ideas??
thanks again
Skip"
 
"Not sure, but is this an oil

"Not sure, but is this an oil injected motor? Or is it before oil injection? Someone more familiar with these motors help me out here. If it is an oil injection motor, you can also take a look at your oil alarm module. The test is relatively simple. Had a high temp alarm on my own motor a couple of years ago, but it would go off before the engine had even run...Hope I am not confusing the issue for you..."
 
Oops retract...on the oil alar

Oops retract...on the oil alarm module.....
 
"Lonnie, to start with your se

"Lonnie, to start with your serial makes this motor a 1988 model, so a little newer than what you thought.

To your question, yes, can put on a standard temp gauge. Stop in any Merc dealer with your model number and they will either have, or can get, the sending unit and little harness to hook up to a gauge.

The two parts that go on the engine end are about 30 bucks (and was a standard feature when this motor was included on some package deals), and are separate from the "alarm" system sending unit."
 
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