Beeper/alarm on 90 horse mercury stays on

I think on those 2 stroke mercs the constant "BEEEEEEEEEEEEEEEEEEP" is a temperature alarm.

What model (efi/optimax/carburated) and what year?

Lots of possibilities here.

Or low block pressure, or low voltage, or some other things.

Really need the engine model

On my carb 175. Constant beep was overheat and or oil pump not pumping. Intermittent beep was low oil on the motor reservoir. Mine had been played with by the previous owner though.

Now that it is deleted and tank mixed, overtemp is wired straight to the beeper. I did have a temp sensor fail closed one time and it did exactly what you say.

If I remember right, mine used to give you a few beeps or a long beep when you first turned on the key to let you know the module was working. I kinda remember that maybe it beeped until the engine was started.

Temp sensor should be easy to test. It should be open to ground when not tripped. And located on the head. I think the oil reservoir is the same, open to ground when the float is up. Not sure about the pump sensor though. I wonder if it’s pressure based or based on the pump shaft rotating and that might be why I kinda remember mine beeping until the engine was running.

Alright, I have a 2005 exlpto 115, exact same motor except a long shaft and different calibration

Constant beeping is either oil system failure, overheat, low voltage (triggers at about 10 volts), or low cooling water pressure, or a hot short to the buzzer. Download a service manual online and start checking those sensors, make sure it is oiling correctly. You can jump out normally closed switches or unplug sensors to find the culprit.

Optimax engines are electrical hogs and if you have bad batteries or a conductor problem itll get you. I had to troll to the camp in Toledo bend because I blew the fusible link off the alternator and the engine drained the battery in about 5 minutes.

If you unplugged the engine oil tank level sensor and the alarm stopped, replace the engine mounted oil tank. The float in the tank goes bad, not the sensor that’s easy to replace.