2400 SCR Intermittent alarms

[Julian]

I have a yr 2000 Merc 5.0 L Alpha , single Carburetor, with enclosed San Juan cooling system.
Eng Serial number 0L692217
The alarm is intermittent and has no pattern, alarms for a few seconds at different speeds etc.
I have eliminated the leg bottle and oil pressure switch, the temp sensor (which is new) is the only thing connected.
The engine is not running hot (checked with other temp gauge and laser temp gun)
I look at the manual wiring diagram and it shows a sensor with only one wire (I am not referring to the temp gauge sender) it shows the sensor grounding through the body of the sensor, however mine has a sensor type 805218t (I see some later models had this) which has 2 wires one yellow and one black, this sensor appears to have the correct ohm readings for the different temperatures when tested.
I see the yellow wire goes to the ECM (pin H) and the black to ground. (ECM PART # 86 1459-1, 5.0L G + ALPHA)
Can anyone please tell me what does the yellow wire do at the ECM (this engine is a Carburetor model) and how does signal eventually reach the alarm buzzer assuming the sensor switch had been closed due to an overheated engine

[mmwjr]

Since you have a carbonated engine you don't have an ECM. Is the yellow wire connected to anything on your boat. The ECM would use this to go into safe mode and limit rpms.

From what you posted you have disconnected the oil and lube sensors, correct? If you disconnect the temp sensor does the alarm remain silent?

[Julian]

Hi Mike,
Sorry its an ICM
The yellow wire goes to the ICM Just like the Tan Blue from the oil and bottle switches
I havent had chance to run it with the temp disconnected yet. Thanks

[mmwjr]

When the switch faults is disables the mean base timing spark advance function to limit rpms.

[Julian]

Thanks Mike,
Thats interesting, the boat engine never falters at over 20 mph or down at around 10 mph. It just gets the audible alarm for a very short period then it stops,. As I mentioned the engine is not hot at all. I wonder if the ICM has a fault that triggers just the alarm . The wiring from the sensor to the ICM looks fine. It seems strange that there is no effect on performance when the alarm sounds. Thanks

[mmwjr]

I cannot say just how noticeable it is but I have heard on EFI engines it goes into what is called safe mode and is quite a limit in power.

[Julian]

Mercruiser, say they have heard of short alarm beeps with no reason, due to the engine to helm connectors on the harness being bad, I will take a look, I wonder if anyone else has had this issue. Still not sure why the boat runs great. Thanks for your help.

[mmwjr]

Interesting, I would think a connector issue would result in an intermittent open. Not sure I understand a scenario that would complete the circuit path to ground.

I have heard of faulty float in the gear lube revisor but you said that disconnected.

[Julian]

I do agree regarding a bad connection, where as the required alarm is subject to a complete connection to ground rather than an intermittent open connection.
Still confused ???

[Julian]

The Gear lube bottle has been replaced with no difference and is now disconnected