My overheat alarm is staying on constantly even before cranking the motor after sitting for days.  The water pump runs fine just put it in; new spark plugs.  Where should I start because I really don't know much about motors but I am good at fixing things.

I had a 175 mariner (1988) that did the same thing. They replaced the alarm sensor, unfortunately,  not before they replaced the water pump and impeller.

Going off your name of black max 135.  I had one of those motors years ago.  Good motor.  My motor did a similar thing but it was not an over heat alarm it was the oil injection system going out.  From what I was told it was common.  The dealer sold me a few parts to take out the oil injection system and replace with those parts then I had to mix the fuel.  If it was me I'd have them look at it to make sure it is not the oil injection system.

I had the same issue as Mike on my Optimax.  It turned out to be the upper tank causing the alarm.  I replaced it and everything was good to go.

I would look at the water temp sensor first. I'm not sure where it's located on those, but being that you just replaced the plugs, you or the cover could have inadvertently pulled a wire loose.

Going off your name of black max 135.  I had one of those motors years ago.  Good motor.  My motor did a similar thing but it was not an over heat alarm it was the oil injection system going out.  From what I was told it was common.  The dealer sold me a few parts to take out the oil injection system and replace with those parts then I had to mix the fuel.  If it was me I'd have them look at it to make sure it is not the oil injection system.

The oil injection system had already been taken out.  I guess I'm going for the temp sensor cause I can't get my wife to go fishing anymore since it started going off.  Do you know what it looks like and were about it is?

The oil injection system had already been taken out.  I guess I'm going for the temp sensor cause I can't get my wife to go fishing anymore since it started going off.  Do you know what it looks like and were about it is?

 

That I do not know.  Sorry.

I'm guessing, and only guessing, that it's gonna be on the head close to the thermostat at the top. Best thing would be to go to a parts site and pull up your engine's diagram. I find those pretty helpful for figuring out what is where on mine.

I had a similar problem on a 2004 Merc 90 HP carbureted two stroke. All my research pointed to the temp sensor. I disconnected the temp sensor and still had the constant alarm.

I disconnected the oil level sensor in the bottom of the oil tank (bolted to the power head) and the alarm went away. I pulled the tank and discovered the magnetic sensor was external to the tank, but the magnetic float was internal. It turns out the float was sticking.

Fixing it was as simple as ordering a new tank, as the float is not serviceable.

Not sure how different your Mercury is from mine ...