posted 05-01-2010 12:43 PM ET (US)
I am sorry--I did not carefully read your narrative where you exclude the level sensor from being based on a float or on some basis of buoyancy of the sensor in the oil.I am not familiar with the design of the level sensor used by Mercury for the under-cowling oil reservoir tank as you describe, but we can apply some of the same logic to diagnosis of the problem of the false alarm.
We have an alarm indicating low-oil level but we see visually the oil level is fine. This implies a false alarm. The likely causes of a false alarm are:
--bad sensor
--bad wiring between sensor and alarm module
--bad alarm module
You should first investigate the sensor. Perhaps you can deduce the principle of its operation from inspection of the device. If there is not floatation or buoyancy involved in sensing of the tank level, the operation of the sensor must be based on some other method. One method of detection of oil level might be to immerse two electrical contacts in the oil and monitor for a change in resistance between them. This method does not seem likely, as most oils are not very good conductors of electricity, and the change in resistance (from oil to air) would be hard to detect. Another basis for a sensor might be optical. The sensor reacts to a change in the transmission of light between a source of light and a sensor of light. But this also seems unlikely, as oil is known to cling to surfaces and could make reliable detection of a level drop difficult. This brings us back to buoyancy as being the most common sensor technique with regard to tank level indicators.
I am sorry to say I am not familiar with this level indictor, but I would very much like to learn about it. If you discover the basis on how it detects tank level, please let us know.
To test the sensor you might be able to monitor its electrical properties while changing the sensor input to correspond to different tank levels. You should observe a change in the electrical properties of the sensor. Typically the OEM service manual for your Mercury engine will include a procedure for testing the sensor to establish if it is operating properly.