Alternator Working?

skidrow

Hi all,

A few weeks ago I noticed my voltmeter showing below 13 v as well as 13.1-13.0v digitally on
my digital guages.
I took the alternator in to get rebuilt. The guy said it was working ok and he cleaned it up.
I put it back in and was still getting a maximum of 13.1 to possibly at times, 13.3 v.
This is while underway at 3800rpm.
I changed both batteries to new agm batteries.
I replaced the serpentine belt as well as replacing the battery isolator.
I took the battery into the alternator guy again.
He put it on his machine and ran it for about 25- 30 mintues. I saw that it was putting out
almost 14v and the max. amperage of 70amps.
I took it back and re-installed it, started the boat and it showed 14 v. digitallly and
on the dash voltmeter.
I then went out for a ride.
The voltmeter and digital meter showed 13,4-13.5 volts. This is right from the battery posts.
Could it be possible that one of the diodes or transistors in the alternator are working, then not working?

Any thoughts or comments welcomed...

youstolemybeer

your alt is working if the battery voltage while running is at least 1 volt over the battery voltage when not running. So if the battery shows 12.4 v with engine off and 13.4 with engine on then I would say you have a good charging system.   Start the boat and turn EVERYTHING on and check battery voltage. Battery not dash board volts. If your battery voltage is over 13 with everything on your ok

Handymans342

Here is a simple test. Correct me if I am wrong. Unhook positive lines on batteries while engines are running. If they die, its the alternator. If they stay running, its the batteries.

raybo3

Wow Handy that's old school. I am not sure if that is a wise thing to do with computers on cars nowadays. But I am not sure.....

MarkB

I thought you still need the batteries even with alternator on? Doesn't the spark ignition power come directly from the battery, so alternator to battery, battery to spark. Disconnect the battery and the alternator could charge all day long, but the problem is you lose the spark, so alternator quits.

Handymans342

Battery is for starting only. Alternator runs everything.

MarkB

On my Dodge caravan I had a b battery go bad and wouldn't hold charge. Car stalled because battery wouldn't hold charge. If it was only the alternator why would the car stall? 

MarkB

That's what I though Al. Thanks for the clarification.

rasbury

As mentioned, get a volt meter...if your not familiar with its use, you soon Wii be anyway!

howardrams

Agreeing with Al, here.  Nobody should expect the electronics of a modern boat to like it if you disconnect your battery while running.  Not any way to test an alternator.  Skidrow sounds like his alternator is fine, putting out 14 under no load, and dropping down around 13 or 12.5 under load?