Cruisers Forum

Join CruisersForum Today

Thread Tools Rate Thread Display Modes
Old 15-06-2009, 18:21   #1
Registered User

Join Date: Jun 2009
Location: St Pete Beach, Fl
Boat: Hunter 33
Posts: 2
Alternator Drains Batteries - Engine Off

The alternator on my Yanmar 2GM engine is killing my batteries. It may be a generator and not an alternator because it has one small brush visible when I take the back cover off.

What happens is when the engine is off, and the battery selector switch is off as well the alternator gets hot and completely discharges my batteries.

I can't find the source of the current leak. It must be something simple, but I don't know what to look for. Could it be a diode? And if so where I wonder.

Any ideas of where I should start looking, or what to test?

vic26 is offline   Reply With Quote
Old 15-06-2009, 20:38   #2
Commercial Member
CharlieJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: St. Petersburg, FL
Boat: Gulfstar Long Range Trawler; 53'; BearBoat
Posts: 811
I suspect that the regulator (internal or external) is being powered and it in turn is powering the field wire. The regulator may get its power from the ignition switch or from an oil pressure switch. If you have an external regulator, pull the two conductor plug from the back of the alternator (usually has a white and a blue wire) and measure the voltage from the blue wire to ground. If there is a DC voltage there, check the alternator regulator to see why it is being powered. An internal regulator is a bit more difficult to troubleshoot so check what you have and come back with a bit more info.

Hope this helps.

Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
CharlieJ is offline   Reply With Quote
Old 15-06-2009, 21:04   #3
Moderator Emeritus
Ex-Calif's Avatar

Cruisers Forum Supporter

Join Date: May 2007
Location: Singapore
Boat: Maxi 77 - Relax Lah!
Posts: 11,514
Images: 4
Hi vic and welcome to the forum!

Charge the battery, remove the wires from the alternator and see if the problem persists. This will rule out other sources of the current drain.

If your troubleshooting skills are not as up to date as your mechanical skills you could remove the alternator and take it to a shop for testing.

If the alternator checks ok then troubleshooting, with the assistance of a wiring diagram and perhaps a manual is the next step.
Relax Lah! is For Sale <--- Click
Click--> Custom CF Google Search or CF Rules
You're gonna need a bigger boat... - Martin Brody
Ex-Calif is offline   Reply With Quote
Old 16-06-2009, 06:28   #4
Registered User
svHyLyte's Avatar

Join Date: Mar 2006
Location: Tampa Bay area, USA
Boat: Beneteau First 42
Posts: 2,975
Images: 25
With everything apparently "Off" lay a screw-driver or other ferrous metal object against the pulley on your alternator. If it's magnetic, the field current has not been shut off and the Regulator is at fault. We had a similar problem with a Xantrex "smart regulator" last year and replaced it with a Balmar AR5 (and keep a second as a spare) and solved our own similar problem.


s/v HyLyte
"It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
svHyLyte is offline   Reply With Quote
Old 16-06-2009, 09:41   #5
Senior Cruiser
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 5,138
You didn't say how quickly the batteries get discharged, or how hot the alternator. You may have one or more shorted diodes.
Disconnect the fat wire (+) from the alternator while leaving the others connected. If that stops the discharge, you probably have bad diodes in the alternator. If the discharge continues, as previously posted, the field may be
still getting power even though the key switch (bad/miswired?) is off.
Check with a voltmeter on the field winding to see if there's voltage when the key is off.

Steve B.
senormechanico is offline   Reply With Quote
Old 23-06-2009, 13:55   #6
Registered User

Join Date: Jun 2009
Location: St Pete Beach, Fl
Boat: Hunter 33
Posts: 2
Thanks for all the replies everyone, I finally figured it out. Ended up taking the alternator off and bringing it to a starter and alternator place. They tested it and said that the diodes were blown, and the regulator on the back was the wrong one. Apparently someone put the wrong regulator on. They rebuilt it for $175 with new diodes, and regulator. It's set up to be self exciting so all I need to do is hook up the ground and the positive charge wires. Haven't had a chance to put it back on and test yet. I'm not sure why the diodes blew. There was a lightning strike on a tree near the boat a few weeks ago. Might have done it.

Thanks again,
Vic G.
vic26 is offline   Reply With Quote
Old 23-06-2009, 16:03   #7

Cruisers Forum Supporter

Join Date: Apr 2006
Posts: 12,077
"It's set up to be self exciting so all I need to do is hook up the ground and the positive charge wires." That's typically called a "one wire" alternator, as opposed to a conventional "three wire" alternator.

There are some pros and cons to that.

The good thing, is that it is simple.

The bad thing, is that it is crude and has no way to actually sense the battery voltage, so it can never optimally charge the batteries. Never.

With a conventional 3-wire alternator, one lead is a battery voltage "sense" lead. That goes directly to the battery, so the regulator can tell the battery state. The problem is that if you have dual battery banks, one sense lead can't go to two places. So, either you need to switch the sense lead as you switch banks (not impossible, but adds cost) or you fall back to a "one wire" system that abuses the batteries.

Depending on your installation, you may want to consider converting to 3-wire if you can. Your batteries will thank you for it.
hellosailor is offline   Reply With Quote
Old 12-07-2009, 23:02   #8
Registered User

Join Date: Oct 2008
Location: NZ
Boat: Atkinson 14m, Touche
Posts: 7
Had a similar regulator problem from a lightning near miss about 2 months ago. Very carefully check all your electronic gear. At first glance we had no problems, lights came on, electronics powered up etc. But then we noticed things weren't working properly. Eventually replaced our regulator, stereo, SSB, VHF, Autopilot computer and even a handheld GPS. Thankfully our insurance company were great.
drob is offline   Reply With Quote
Old 13-07-2009, 04:31   #9
Registered User

Join Date: Jan 2008
Location: Nova Scotia, Canada
Boat: Seafarer 30 Impetuous
Posts: 426
Images: 45
Hellosailor - Electrical is not one of my strengths, so please bear with me.

Is it possible to have the sense lead go to the common post on the battery selector swith, so the alternator would sense, and more properly charge the bank that was selected at the time.

Northeaster is offline   Reply With Quote

alternator, battery, engine

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Drains spooky alice Construction, Maintenance & Refit 5 27-02-2009 18:12
Switching batteries while engine running hns1934 Electrical: Batteries, Generators & Solar 7 28-09-2008 20:47
AGM batteries in engine space? Beausoleil Electrical: Batteries, Generators & Solar 12 20-03-2008 02:35
Are you pleased with your cockpit drains? Zach Monohull Sailboats 14 21-01-2008 03:13
Sink drains drh1965 Plumbing Systems and Fixtures 3 27-08-2007 08:03

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 11:54.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.