Alternator or starter issue?

[Boater71]

I changed my batteries after my second outing this season. they were 3 year old and can still hold a charge. Reason i changed was because the starboard engine would shut down. After changing the batteries, everything was good for a couple of weeks until yesterday and today. Last night, after going out for a couple of hours and while docking, the starboard engine shutdown. Today, as we are leaving, the and few minute into the ride, the starboard shutdown. restarted and spend the whole day boating (3-4 hours). coming to start the engines, low voltage message showed up. the voltage read 8 VDC - 10 VDC.

How do i trouble shoot the issue and where do I start?

 

[northern]

If your alternator is gone the voltage when the engine is running will slowly drop. Measure voltage at a 12 volt location in the cabin with engine off. Start engine measure voltage again. If same probably alternator. Rev engine to 2000RPM if voltage does not go up alternator is gone. Take it off go to NAPA and they will test it for you. If gone NAPA can order you a new one. I have bought marine starters and alternators from NAPA they seem to be fine.

 

[Boater71]

Thanks for your input. I figured cabins lights are all powered by port batteries. STBD powers all the external stuff. So i connected a voltmeter to the dc at the helm, i got 13.7. I switched lights on and off and voltage dropped a bit. I turned key on and the voltage on the vesselview was 9 v to 10 v while the DC was still 13.7. I rev'ed up the engine to 2000 rpm and i got 14.2 to 13.7u on the vesselview (more like consistent 14.2). what am I missing here? what do i trace?

 

[northern]

If you get 14 or so with engine rev to 2000 your alternators are probably fine.
Next wash all battery terminals with baking soda and water then rinse then take off each set of wires one at a time and clean with brass wire brush. I do this once a year and all seems fine. If that does not help you need a real mechanic

 

[Bt Doctur]

put the meter on stbd battery, read the voltage, start the engine , at 2000 rpm the reading MST be higher if the alt is charging. Then do the same for the other battery.
This is assuming your electrical systems are NOT combined

 

[Boater71]

So i went out again today, few minutes away from the dock, I put the gear in neutral, the STBD engine shutdown as the battery volt reading at the vesselview was only 8.8 V (key off and back on, restarted and no more issues the whole trip). when under load, the battery is between 14.2 to 13.7v. however, when idle, it goes to around 10 V. The port engine when idle is on 12 v. STBD is less by a couple of volts.

When i replaced the batteries earlier in May, I used electric grease.

I'm at a loss here. what should i do next to troubleshoot.

 

[northern]

Beyond my mechanical ability. I would pull an alternator and get it tested. Could be worn brushes and only works at high RPM.

 

[ZZ13]

So i went out again today, few minutes away from the dock, I put the gear in neutral, the STBD engine shutdown as the battery volt reading at the vesselview was only 8.8 V (key off and back on, restarted and no more issues the whole trip). when under load, the battery is between 14.2 to 13.7v. however, when idle, it goes to around 10 V. The port engine when idle is on 12 v. STBD is less by a couple of volts.

When i replaced the batteries earlier in May, I used electric grease.

I'm at a loss here. what should i do next to troubleshoot.

First off all do what BT Doctor said. Take all your voltage readings at the battery and ignore the vessel view or any other helm voltage displays (maybe you did, I couldn't tell from your post).

Second, dielectric grease is an insulator. It should only be applied onto already joined connections as a protection against the elements. If you put it on the battery post, then put the clamp on the greased post, you need to disconnect the battery cables and clean it all up so you have bare metal to bare metal.

 

[Boater71]

First off all do what BT Doctor said. Take all your voltage readings at the battery and ignore the vessel view or any other helm voltage displays (maybe you did, I couldn't tell from your post).

Second, dielectric grease is an insulator. It should only be applied onto already joined connections as a protection against the elements. If you put it on the battery post, then put the clamp on the greased post, you need to disconnect the battery cables and clean it all up so you have bare metal to bare metal.

will do tomorrow and report back.

 

[frosh coach]

One thing is to check the wires on the back of the ALT.

I had one engine that wasn't charging (12.3v when at 2000 rpm). I decided to order a new ALT since the engines are from 94. When I pulled the alt off, the nut holding the pos wire was missing and the wire wasn't hooked up... NOT GOOD!

I decided to install the new alt, and have will have the old one rebuilt (if possible).