Sounds like a bad battery despite what the charger is saying. Try switching the batteries to see if that fixes the problem.

 

Sounds like a bad battery to me as well...

 

http://www.*******.com/merc/Servmanl/13/13E4R2.PDF
replace the *`s with b,o,a,t,f,i,x no comma`s

 

Did you properly test the battery's first ? Before throwing money at it?

 

I suspect the sense or excitation wire may be open but don't no where they go.
They rarely cause a problem unless not connected to the alt.
Do you keep the battery charger on with shore power and do you check the water level if using wet batteries?

 

Both batteries were fully charged before installation and were topped off with distilled water. Switched batteries with a known good one, swapped port to starboard. The original port battery was dead as a door nail, but after charging was 12.6 vdc (at Autozone) and putting in starboard engine the battery charged and operated properly (13.8 vdc with engine at idle). The original starboard battery was charging at 13.5 vdc before moving to port engine, in the port engine the voltmeter reads battery voltage with engine running and continues to drop as the engine runs. The tachometer not working and the alternator not charging on the same engine is too much of a coincidence to not suspect a bad wire somewhere especially since the batteries were purchased last October and both work properly on the starboard engine.

 

A simple test of the charging system is to turn the key to "on" look at the voltmeter, start the engine and see if it goes up to atleast 13.6 .If not your not charging.
The sense will have constant 12v and the excite will have 12v with the key"on"

 

See above post, voltmeter reads battery voltage all the time and actually goes down as engine operates. This indicates no alternator charging. Thanks for the info on the sense and excite connections. That is where I suspect the problem to be.

 

Port engine won’t charge. Voltmeter reads 12 battery voltage 12 vdc and as the motor runs the voltage continues to go lower to 10 vdc over a couple hours of boating and the engine won’t start without tying the batteries. The starboard engine works normal and shows 13.8 vdc with engine running. I swapped the port and starboard batteries with no change in charge output. I replaced the port alternator and there is still no charging of the port battery.

Further troubleshooting:
I removed the port alternator again to check the excitation sense, and batt connection voltages. They were 11.65, 12.74 and 12.45 vdc respectively with the key on engine not running. I made 3 jumpers so I could monitor the Exc, Sense and Batt voltage with the engine running and battery installed. I disconnected all the batteries to try to find out where the wires went, Exc. Connection went to the coil, I couldn’t find where the sense wire went to but the Batt connection went to the Battery Isolator and not the Port Battery. I connected the three wires that go to the port battery and started the port engine.
Battery Voltage reads 12vdc
Sense Voltage at alternator reads 12.6 vdc
Excitation Voltage at alternator reads 11.0 vdc
Batt Voltage at alternator reads 25.6 vdc
This leads me to believe the alternator charge voltage is not getting back to the battery for some reason because the alternator is running wide open at that high a voltage. So now I need to figure out how the isolator plays into the charging system. The isolator is a Newmar model 2-3-70, S/N 02000217
Is there a wiring diagram for the boat wiring itself.

 

Do not run the motor untill you find the cause.
Typically the alt feed goes to the center lug and the batteries are connected to the 2 other lugs.You could join all 3 wires together for testing.
A hi current surge or short circuit could have wiped out the diodes in the isolator
Also look to see if you have a 90A fuse link at the starter. Will be a square looking white block hooked to where the main battery cable is

 

Typical engine wiring system

 

Blue arrow from Alt to Isolator, green arrow returning from isolator to engine wiring

 

Thanks BT Doctor. This has been a great learning experience. My next step is wring out all the wiring at the battery isolator. I assume A1 comes from the port alternator and A2 comes from the starboard alternator and 1, 2, 3 go to the batteries. I should be able to ohm out from the isolator outputs all the way to the battery terminals and from the alternator outputs all the way to the isolator. I have a feeling the A1 input and 1 output are open because the Port alternator doesn’t charge the starboard battery and the starboard alternator doesn’t charge the port battery.

 

Personally I would dedicate the engine charging system and engine only batteries.House service can be done with relays instead of isolators.
Engine starting batteries you need , house batteries are expendable.
Depending how much room you have to work, remove the wire on the alt output and tape up Remove the Orange wire at the starters battery post and tape it up, and run a new #10 from the alt output to the starters battery post

 

Fixed the problem. Wrung out all the wires going to and from the isolator and found the port wiring backwards the port alternator was wired to the #1 output of the isolator and the port battery was wired to the A1 input. Cheap fix but it was a lot of work verifying all the wiring since the isolator was attached to the front bulkhead in the bilge just below the steps going into the galley. It is hard to get a 57 year old body down there. I have only had the boat since Oct and been out a total of three times on it so I am still in the growing pains. I just wonder how long it had been miswired and not charging. I have 12vdc on meter with engine not running and 13vdc with engine running at idle. The splices for the isolator wiring are just below the engine alternators.

 

Time for the Crown Royal!!! And band aids

 

You bet. I feel lucky I didn't burn up an alternator. I am going to test the one I took out to see if it is still good. I may have a good spare alternator. And again thanks for the info from BT Doctor. I was close to wiring straight off alternator to the battery.