I would suggest you take you engine to a qualified mechanic. Obviously the alarm is faulty if it sounds before you even start the engine. Disabling it is a bad idea as the possibility of an actual overheating situation could occur in the future and you wouldn't know it until damage has occurred to the engine. The other thing of concern is that you added oil to your gas tank as well. It may be a good idea to drain the gas and dispose of it as too much oil could cause problems down the road. You could consult the mechanic for his opinion on that.

1988 Mercury 90hp outboard 2 stroke.

High temp alarm will not off.

Turn the key to the On position before cranking the motor over and the alarm goes off, stays on until the key goes off. Alarm stays on while motor is running, regardless of RPM or speed.

Tattletale drain is strong. New waterpump and thermastat replaced last year. Still had problems then.

Oil mixture is correct. Oil pump works. I even mix some oil into the gas to make sure.

When I touch the cylinder head for all three cylinders, the temperature is even, cool to the touch and not hot.

I would like to disconnect this sensor but not sure where it is located. The alarm is in the throttle/trim tab control unit someplace. I would like to get rid of this...is so annoying.