Checked fire engine last night all good.
Dop in at ramp this morning... Turn key, buzzer doesnt do the normal 1/2  check warning... goes on stays on...the lights go thru there check  all good....buzzer still on...engine fires stays on.. kill engine  re trailer bring home.
So sit down with manual and nothing .. other than over heating, the buzzer continuous...not fired not hot.. thinking possible dud head sensor or ground in sensor circuit.
Study the cicuit diagrams..
Worked out, instead disconnecting all the sensors.. disturbing stuff , possible grounds... if pull the connector from the back of the tach...then check continuity (resistance) of each of the tan coded sensor circuits back from there....all check good...
Check continuity of the tan wire from that connector back to the buzzer connector... good....
Check the buzzer power feed, and operation ..all good

 Therefore It SEEMS that the fault is inside the tach warning system.
So unplug all sensors on motor oil tank etc.. turn key on  buzz stuck on.. lights go thru check fine.
Check continuity of each sensor and wire  plus from tach thru loop to sensor plugs again... all good
Power feed to buzzer good.. jump to the buzzer.. without tan wire from tach connected .. no buzz, connect the tan wire.. buzzer on

Engine runs fine. Oil / fuel pump feed check out fine etc.. oil tank level sender fine...

Still comes back to the warning system circuit inside the tach...
I ASSUME the following...
I dont have a circuit diagram of the circuit board inside the back of the tach
Now the tach system has purple wire feed, and so does the buzzer have its own... therefore I conclude the buzzer works by power feed into it, then into warning system, thru a couple switches./ relays, maybe a diode to make the 2 second test buzz when ign turned on  and different buzzes for different sensors .. then out t be grounded at the sensor that is activated

So
1/ Am right or wrong and if wrong where?
2/ Anyone have a johnson/ evi/nrude warning system tach I could loan for a couple hrs.. including pick up drop back?.. maybe buy?  mid 90s and above.
or
3/ A buzzer gauge I could borrow, maybe by....and do the buzzer warning gauges still use the buzzer? and the std Johnson / evinrude plug connectors

Sth Auckland area
Cheers
Steps

PS I know I can run fine, no buzzer, just warning lights... but in a boat dash idiot lights are not  in your face like sitting behind the wheel on a race track.

what size is your engine?

Sound like the buzzer circuit warning system in tacho steps - you seem to have covered all the bases.  Like you I would prefer to have a warning system working - mechanic was telling me that one customer had float in oil tank fail so he disconnected and kept an eye on oil level 2 years later he ran out of oil and blew motor.   

If your starboard sensor has only got 1 wire going into it & the port side 2 & 1 wire goes through a diode this tells you the bodies of the sensors are the negative there fore if the alarm sounds when you turn the key on that one or either the sensors are faulty being the bodies the negative.

I have been experiencing similar issues with my "2000 Johnson 150hp. this is a carb'd engine with optical ignition.

try this

I have not found your issue on google..  or in the full manual while looking to sort mine.....
So much of the above is just my own thoughts from off the circuit diagrams and checking continuities  etc

Thu .. further thought... u say over 4000 rpms.....to me that does say fuel pressure / supply...does it happen as soon as u go over, or delayed.. enough time for the carb bowel fuel level drop?

And if leaning out... the engine runs hotter... not over heat depending on lean, and heat sensor could just be on the verge of cutting in and out????
Un plug the head sensors in the engine one at a time...and see if buzzer still does it...
This would also indicate a crook sensor
U can also pull a heat sensor, hang it in a jug water carefully , with a therometer in the jug....Ohm meter across.. turn on the jug and find out what temp the sensor goes off at.
The tan/ blue wire   ...closed 116 C +/- 3 deg  open 97 C +/- 6deg   
The White/ black wire  closed 105 C +/-3          open  32 C +/-2 deg

Similar method to check the engine thermostat opens and closes (another SERVICE check)
opens 62 deg C   fully open  170 deg C.

 While looking up the head thermostat specs found this also
 V6 engines... underneath where the fuel line connects into the engine is a wire to a vaccuum switch....this should turn on the engine warning light and buzzer.
Activates between 6.6 and 7.5 inches  and releases at 5"
Check that is operating ok to.

Sorted...just got to strap the battery and oil tank back in.
Tested with a known good buzzer... not the buzzer as all the experts recon.
Not the diode or loom or sensors.
Swapped in a known good tach/ warning system gauge....
All good.

Its in the system warning part of the tach... was we suspected in the 1st place.

So who is good at fixing these things... a new gauge is over $600

From sensors (Ground).. to the self warning system in the tach...
The buzzer feed is from the tach to power
And believe the switching for the buzzer self check is in the buzzer...is what have been told, but none of those ppl have actually dismantled a buzzer.

The tach self check system I believe (maybe) is the same/ interchangeable for merc and Suzi.

The schematic for the loom circuits are above attachments, but can not find any internal circuit diagrams of the tach self warming or the buzzer anywhere.

The other thing, how do they open up the tach to get inside?

Thanks for the contact John..I was also thinking maybe Robinsons... they sorted a few vintage gauges 30 odd yrs ago for me...
And somewhere in these forums, cant find it, there is someone  highly recommending a guy 6/ 12 months ago.

Yep checked out E bay...and there are some after market ones on trademe also.

Yes checked the retifier before and when finished... all good.
The tach its self works off the rectifier circuits...generally if the tach is doing stuff it shouldnt and pole selector on the back correct, it will be the rectifier.
The workshop/ service manual is excellent for this.