We were idling up the creek the other day and went to punch it and the alarm went off and stayed on constantly. I turned the engine off and checked the oil and it was fine. Water is coming out fine also. The alarm is on even when the engine is not started.  Before I start taking things apart done anyone have any ideas of what this is? Ever happen to you? I'm thinking some sort of sensor is bad. Any info. is appreciated. 

When mine went off like that it was the the thermostat. 

There are usually only a couple of sensors on those engines, over temp and oil.   I have heard of a lot of them having problems with the oil sensor in the tank.  You can disconnect the sensors one at a time from the harness and see which one is causing the alarm.

Its the oil sensor in the tank like the man says above.  i have a 2005 50HP mercury 2 stroke , same problem.  just disconnect the wires for it.  and when you do important thing to note.  on your motor ,  it may seem like you have less than a 1/4 tank left of oil left, because yo ucan stil see oil in the little visual gauge.  thats deceiving, if you hit a 1/4 tank of oil , put more in period. 

 

Good info. Thanks guys.

So ran through it and unplugged the blue wire coming out of the oil tank and it stopped. New tank on order. Can I run it as long as I know there is oil in it? Just not sure if unplugging the sensor will allow this. 

Yes you can, but be careful not to run low on oil as I am sure you already know.  

I get paranoid about disconnecting alarms...I'd run pre-mix until the new tank gets here, even if that ment I'd have to disconnect the oil tank.

Not much can ruin a motor faster than lack of oil.

You need to find out if your motor is getting oil. The alarm is for low oil, no oil, bad sensor. Drain the tank, flush the lines, then fill it back up and see if the alarm still sounds. The tank might not be the issue...

My buddys 100 will sound oil alarm when its cold out, he runs crap oil and it doesnt flow well when we are duck hunting in -0 temps. He pre-mixes during winter. 

31 minutes ago, Kevin22 said:

You need to find out if your motor is getting oil. The alarm is for low oil, no oil, bad sensor. Drain the tank, flush the lines, then fill it back up and see if the alarm still sounds. The tank might not be the issue...

My buddys 100 will sound oil alarm when its cold out, he runs crap oil and it doesnt flow well when we are duck hunting in -0 temps. He pre-mixes during winter. 

I like your thinking because it is on the safe side of things.  However I don't believe that the alarm would sound with only the ignition on if it were anything other than a sensor.

I had the exact same problem on my 2004 Mercury 90 HP 2 stroke engine. If you disconnect the light blue oil tank sensor wire and the alarm goes away, you can be certain the tank is bad. Mine was, and a new one fixed it. 

I ran with the wire disconnected for a few weeks before I got the green light from my wife to buy the new tank for around $90 shipped to the house. I just knew to top the tank off every time I went out and never got below 3/4 full at the end of a trip.

Bottom line: if disconnecting the light blue wire coming from the tank makes this alarm go away, you can be certain the float internal to the tank is bad. If you disconnect it and make sure you always have oil, you can still run it. My engine has no oil flow sensors or anything of the sort. It is a simple sensor for oil level in the tank and that's it. 

Good luck and tight lines,

Pete

In the process of fixing that exact issue in mine.

Just had this issue 

disconnected the blue wire from the oil tank 

 

Alarm still going off at low speeds

 

I took the thermostat off and cleaned it. ( wasn’t really dirty), and it’s still going off at low speeds 

 

i can run full out with no issue 

 

any thoughts ?

I have the same problem but I have replaced thermostat/temp sender sensor, unplugged blue wire from oil tank still goes off. Here the thing it goes off at 3000rpm and stay on until you drop down under 1000rpm. Most of the time it will just shut off after two minutes or so. I have no idea... doesn’t run bad at all.

One word of caution when screwing around with the oil injection system.  If you take hoses loose, drain hoses, or do anything that removes the oil from the system.  Premix about five gallons of gas, and run that when you first start running the motor after doing your work.   Yes, that's double oiling the motor, hopefully, but if the oil injection has an air pocket or the pump cavitates in a pocket of air, or something is still not right, it saves your motor until the system has run long enough to purge any air pockets out. 

I don't care how much or how good you bleed the system after working on it, It's better to be safe than sorry.

I have a 97 Mercury 115 with a screeching alarm problem. Idles fine but under throttle about 3500 rpm starts this very loud screeching noise it still runs but the rpms go up and down very slightly. Has oil and does not seem overheated. This is the exact noise that is on the yutube video "what is this alarm" Any ideas?

On 7/27/2020 at 4:51 PM, Brent R said:

Just had this issue 

disconnected the blue wire from the oil tank 

 

Alarm still going off at low speeds

 

I took the thermostat off and cleaned it. ( wasn’t really dirty), and it’s still going off at low speeds 

 

i can run full out with no issue 

 

any thoughts ?

I’ve had that one, it was the intake tube cocked sideways a bit 

I'm the first to admit, I'm not a Mercury guy.  However, I just happened to have just recently rebuilding this same motor. If I remember right, there are only two sensors that make the alarm buzzer go off.  One is the temp sensor and the other is the Oil level sensor.  I don't remember there being a sensor for the oil injection.   

Doing a little research, I found the Oil level sensor is notorious for causing this problem.  The float separates from then sensor so the sensor stays at the bottom, making it look like it's out of oil.  

There is a guy on youtube that has a bush fix that works and seems to be a fairly permanent fix.  He cuts it loose from the tank, using a fine strand of wire from stranded wire and wires the float to the sensor and glues it back on.  Might be worth check it out.  

Before doing this, you might want to take an ohm meter and check the sensor to make sure that is the problem.  There's always the possibility of a short or the temp sensor being bad.  Again, and ohm meter will easily let you check those.