Alternator Antics

[Nigel A Veater 6BT-5.9M 210hp]

On my first trip on the boat this Spring going to the new marina, I noticed the voltmeter on the instrument panel wasn’t showing any charge. At the dock, I got out the mutli meter and checked the batteries and sure enough there was no voltage difference after starting the engine. I check the alternator and got a 12.7 voltage reading at the alternator field (the same as the battery) . Connections at the alternator looked good with no evidence of corrosion.  Today, I went to the boat and started the engine this time the voltmeter indicated that it was charging at 12.8. Any thoughts about what else to check. I assume we should see a charge of over 13 0r 14 + volts when the engine is running. I have  cummins 6BT 5.9 (1999 vintage) engine. The PO did replace the alternator in 2019 and I don’t know the type (but I will find out on my next trip to boat) . 

Topic

[Clark Leighs]

A sign of an overheated alternator will be the colour of the windings.     Normally they should look copper coloured , reddish.     with a coat of the special varnish the builder uses.

If the colour is dark and has a burned odour then the alternator has likely overheated.   Although the winding may stand more use as long as steps are taken to prevent this in the future the bridge diodes or the internal regulator may not like the overheat, fail and then either let it run wild for a short time OR quit. 

Most of these alternators will do fine unless a whole lot of battery capacity  has been added  or high draw equipment has been added.  High draw anchor windlass, bow and /or stern thrusters, inverters used a lot are high draw and will run a battery down which then, when the engine is again run demands a long charge time which the alternator may not be up to.  Add in engine compartment heat and that will add to the alternator woes.

There are several things that can be done to monitor the alt. output.

Set up a good digital voltmeter so it can read the voltage at the batteries.   I mean a small digital ,not the typical swing needle dash meter and then monitor it for how long the high output goes on. 

Learn what the readings mean.

Seaboard offers a suitable unit in their offerings.   

Try an ammeter on the alt. output wire for the same idea.   High output over several hours may mean the alternator is not up to the task.

Or consider getting an alternator with a HIGHER temperature rating, maybe higher output,  install a remote regulator with the high temperature sensing ability so the alternator output is controlled by the temp of the alt.  in addition to the voltage developing at the batteries.    It will turn down the alternator output when the alt. reaches a temp  setpoint.

 

Just some things to think about especially if this happens again.

 

1 user thanked author for this post.

Tony Athens

[Neto]

Was it a high quality unit? Many things on a boat “if you dont use it you lose it”.. Things corrode, humidity, heat, etc..

[Nigel A Veater 6BT-5.9M 210hp]

So the alternator was only three years old replaced by the PO. I did pull the unit and took it to an alternator shop where they pronounced it dead on arrival- I got a replacement and had it installed and it seems to be working properly. No obvious cause for the alternator’s demise- I have only run the boat twice since acquisition- the shop was surmising excessive heat and perhaps the PO’s excessive use of the thruster- did check the wiring before install and it seems that its sized appropriately 

[Clark Leighs]

Try jumping the revs up to a bit over 1,000.   Some alternators won’t begin to charge due to weak residual magnetism and need the extra revs to overcome that. If that does not help then get a wooden block and rap the alternator while running.  Don’t whale on it, just a good sharp rap. If a brush has stuck that may loosen it so it begins to charge.     However, this is not a fix.   If this is the case and the rap works even intermittently pull the alternator and take it to a rebuilder.    The brushes need to be replaced.  They are either worn out of sticking.

 

 

[Author]

Replies