Mercury Tach Problems - is rectifier or ?

[Sharla]

Mercury Big Foot 60 HP EFI motor

Back History
We have an amp and sound system on boat.
Everything is ran off of one battery (we have bought another battery just need to run the extras to it.
We have light all along the bottom of the pontoon boat and through the Bimini top.

We kept getting the low voltage beeps when anchored cant leave any lights on etc. or it runs it dead.
Replaced with new battery and determined we just need to run a second battery for extras.

So a few evenings ago we noticed that the RPM tach gauge was showing extremely high RPM like 5000-6000 and we were barely moving. Motor sounded completely fine and super quiet as always (not revved up etc.)

After further research I determined we probably had a spun hub, and since it had been two years since prop was replaced we went ahead and did that. we use the Mercury Spit Fire pontoon prop number 8M8026570, the one we currently had was all ate up with nicks and not smooth. So it needed to be done.
took it out the next day ran fine actually amazing with the new prop LOL wow what a difference.

Next day we take the boat out go across to the beach area Half a block distance beach it and turn everything off swim for 1.5 hours push off start engine and by the time we got back to the dock 1/2 block distance the battery beep warning did it again.
Husband turned off radio and went on fishing... but said that the tach was started acting erratic again same as before...

I am thinking maybe it is the rectifier on it, that would be causing the tach to act crazy and also be the voltage issue we are having...

Any help appreciated

 

[Sharla]

Well I went ahead and ordered a voltage regulator, and hopefully that will solve the issue.

 

[Sharla]

Well that didn’t work

 

[Texasmark]

Not familiar with the 4 stroke 60 but the 2 stroke has a 16 amp charging circuit. So, assuming you were at full rated rpms causing the alternator (stator in these engines) to put out a full 16 amps...not designed for such for any length of time however......you have a recharge source of 16 amps x around 13 volts (under loaded conditions) or 78 watts.

If you turn the engine off and pump the watts out of the battery, and crank back up and continue to consume the watts the charging circuit could easily have a hard time keeping up with the power demand to recharge the battery while supplying power for the stereo and lights and whatever else electrical is being used.

Overheating would be expected which would lead to damage of the rectifier and possibly the stator also since there is no cooling mechanism under the flywheel for it....the rectifier/regulator module is heat sunk to an aluminum plate in the engine compartment.

I just bought a used boat and it had an 80 watt stereo Marine CD player and didn't seem to be anything special. You didn't specify the size of your boom boom box nor your light wattage.

I assume you replaced with the correct part number rectifier/rectiifier assembly. Were the yellow bayonet connectors running from the stator to the regulator charred? If so, an example of excessive current and possible damage to the stator.

Time to run some tests and see what's what. Test procedures are in the service manual for your engine. They sell them here and ebay has them too.