Locate the owner's manual for the Mercury 90-HP outboard engine. Read the owner's manual section that explains the meaning of the aural alert sounds. Generally, on these older outboard engines using designs from the 1970's, the sounding of an aural alert is only indicative of the existence of a problem. In order to interpret the meaning of the aural alert, the operator must assess the cadence of the aural alert sound and the situations when the aural alert occurs in order to make a reasonable inference about the meaning of the aural alert and exactly what sort of problem it is trying to notify has occurred.
In any alarm or warning system, there are two possible causes for an alarm:
--the warning system is working properly, and the aural alert is sounded to alert you to a real problem;
--the warning system is working improperly, there is a false alarm, and the aural alert sounding is not indicative of a real problem; the problem can be in any component of the system, including sensors, the central processor, or the aural alert sounder itself.
With these older Mercury engines, sounding of an aural alert is often due to one or more failures of system components, sensors, or the alarm itself.
To investigate further, obtain the service manual for your Mercury outboard engine. The service manual will explain in further detail how the aural alert warning system works, what sensors are involved, and what exactly controls the aural alert sounder.
Failure of the aural alert warning system on Mercury outboard engines is a common fault.
Read the 70 prior discussions about these problems with this search:
https://www.google.com/search?hl=en&sou ... gle+SearchIf your Mercury outboard engine has an oil-gasoline mixing system similar to the one described in a c.1998 manual, see a reproduction of a section of the service manual that will explain how the oil system works and what sensors are involved in monitoring its operation in the REFERENCE section article I published many years ago:
http://continuouswave.com/whaler/refere ... ction.html