Yamaha 150 Overheat alarm goes off randomly

I have twin 1996 S & L 150 txru's. The overheat alarm on the stbd engine goes off randomly and drops the rpms. I have replaced the waterpumps, thermo switches and thermostats in the last two weeks. I have temp gun and port engine will be at 120 and stbd engine will be about 130 when the alarm goes off after running 10-15 minutes at 4000 rpms. Stbd gauge arrow points to the overheat symbol. Alarm sound is not steady or even intermittent, sort of like a warble before it finally goes full steady sound. It will happen when I first start off, after making a hard turn or whenever, no rhyme or reason.

Main power wires to the engine were replaced last season after a meltdown within the tube running to the outboards. Positive wire melted through the insulation to the ground wire.

I am thinking it is electrical, so I have checked the wiring connection at the back of the ignition plate and all seems clean and tight. Could that key switch assembly be the culprit? Also, load tested both batteries and they both checked out fine.


This problem has not come up suddenly, but been gradually getting worse over last season.

Re: Yamaha 150 Overheat alarm goes off randomly

The overheat is triggered by a simple system. Either of the 2 sensors will supply a ground setting off the alarm and put the motor in the limp home mode. Is possible that there is a frayed wire on the motor touching a ground.
My thought goes to something different. You saying that new wires were pulled thru the gutter, trought, conduit after the melt down might be a clue. No matter how experienced and professional a person is, it can be a bear getting anything thru that raceway. Can take minutes or hours with the steering hose ( or cable ) harnesses, extra wires stuffed in there and the installer is happy when he get the item thru, sometimes with a lot of tugging force. The yamaha harness is seperate, one item unit covered with sheathing, pretty well protected but I am thinking something damaged the harness and frayed a wire or two enough to provide a ground off another metal sheathed item or the ground wire in the harness its self.
Not a fun job, no one likes to do it but suggest you remove the harness and inspect/ repair if necessary.
Please let us know what the problem was if you eventually get it fixed, thats how we all learn. To many people never post what the fix was. They forget about us until they have another problem. regards FRED

Re: Yamaha 150 Overheat alarm goes off randomly

That would be my thoughts also, maybe run a external harness for that motor just to see if that is the problem before removing the old one or switching the two to see if the problem follows the harness or motor.

These motors can end up with hot spots in them due to corrosion or salt blocking just some passages, and if that spot is at the tempature switch then that maybe the problem.
You can pull and test the temp switches to see if they are responding at the proper temperatures.
could be one is going bad. But I would look had at the harness 1st

Re: Yamaha 150 Overheat alarm goes off randomly

I like that swapping harness idea. Can all be done under the console, just keeping in mind what motor you are looking at, would not be that confusing. FRED