During our tests we have noticed that in certain models of engines of the Optimax series dating before 2008, the engine guardian alarm may ring nonstop when the key is set to the on position even if the engine has no failure.
This behavior is essentially due to a magnetic field buildup that damages the driver circuit of the alarm.
With a digital multi meter check the resistance between pin 11 (central connector of the control unit) and pins 15, 16 and 24. If the test gives you a low resistance (less than 150 Ohm) in both measurement senses the circuit is shorted and the engine control unit must be replaced.
Before replacing the control unit we recommend you to, in any case, contact an authorized Mercury center.