Volt meter low on 4.3 merc

[s2ntburd]

I have a 1998 4.3 mercruiser I have 2 batteries in. We left a switch on so last time we went out it was dead. I jumped it and it ran ok volts stayed right about 10v. Came back next week boat started weak and volts are still around 11v. Does this sound like the alternator is not charging them now? My thought was after a full day out running they should of been charged. What is a good alternator for it? Thanks

 

[alldodge]

My thought was after a full day out running they should of been charged

Not really.
The lead acid battery cannot be drained dead without damage. In most cases there is no amount of charging that will fix a battery that has been drained to below 10V. You probably need new batteries

To check the theory, remove the battery and place it on a charger for several hours. Afterwards remove from charger and let it sit for an hour. Now measure the volts in the battery. If it is below 12V it's bad, should be 12.5 to 12.6V

 

[s2ntburd]

Thanks for advice. One more question, i just put 2 new batteries in it and the volts went up to 12. Im not sure but i thought before it was reading almost 14 volts but i cant remember. With 2 batteries hooked up should it be reading 12 volts or around 14 when running? At idle or high rpm? Im just taking the boat on vacation and dont want to run into any problems. Thanks

 

[Silvertip]

Better use a known accurate hand held voltmeter and take readings at the battery ONE AT A TIME. At 1500 rpm you should see 13.5 - 14.5 volts. Anything less says the alternator is not working or there are some wiring issues.

 

[s2ntburd]

At 1500 rpm its 13.0 is that ok or should i replace?

 

[mjf55]

s2ntburd, as Silvertip just said "At 1500 rpm you should see 13.5 - 14.5 volts. Anything less says the alternator is not working or there are some wiring issues."
So 13.0 volts @ 1500 rpms is not good.
Did you measure with only 1 battery in the circuit at a time?
Can you measure the voltage at the back of the alternator. Are the cables corroded? What is the wire routing for the alternator + wire ( usually Orange).
How many terminals on the back of the alternator?

 

[s2ntburd]

So new question, i found out it was the orange wire going to starter. It looks like it broke about 2 inches from starter. My question is can i run a new wire right from the alternator to the battery or does it have to go to starter? Just looks hard to rewire it to the starter. Thanks

 

[alldodge]

So new question, i found out it was the orange wire going to starter. It looks like it broke about 2 inches from starter. My question is can i run a new wire right from the alternator to the battery or does it have to go to starter? Just looks hard to rewire it to the starter. Thanks

Yes, but....... may not be the best idea. The reason it goes to the starter is there is a 90 amp fuse between it and the starter bolt. So if you do run it straight to the battery (8 AWG) you should also install a 90 fuse inline. This way if you alternator shorts out, it will not cause the battery to overheat and or burn the boat down

 

[s2ntburd]

Ok sweet thats what i will do. Thank you guys for all the help.

 

[Silvertip]

Not sweet! You are adding three more potential points of failure. Do it like the manufacturer designed it. Easy is not always best.