T&T: Alternator Charging

[Steve Sipe]

Ownyacht at aol.com wrote:
> My Delco 105 amp alternator is only showing an output of 12.9 volts at the  
> helm. The engine is a 6BTA Cummins. When the key is first turned the preheat  
> mode takes the voltmeter to 10 volts, then to 12 when the engine is ready to  
> start. Then the voltmeter drops down a time or  two as the post heat  kicks in. 
>  Last time I took the boat out it started normally but never got  above 12 
> volts.
>  
> I cleaned the three spade connector on the top of the alternator and it  
> starts normally but now only shows 12.9 volts...but seems to be gradually  
> climbing although I have not taken the boat out for a test run. One odd thing is  
> that if I switch the 110 volt charger on and off it seems to make the  voltmeter 
> at the helm rise quicker.
>  
> I checked the large red wire on the back for tightness and it was  Okay.
>  
> Do alternators have stages of output? Is this one going out? Is it  possible 
> to check the voltage at the alternator with the engine running?
>  
> Thanks
>  
> Lee
>
>   
Lee,
Does your Cummins have a belt shroud? Mine caused the alternator to 
overheat. The PO had replaced the alternator once, I converted it to 
external regulation and it promptly overheated. I since changed to a 140 
amp Amptech and removed the shroud to give the alternator some relief, 
it works OK now. I plan to modify the shroud at some point to provide 
ventilation, for now, it's on the pile of stuff in my shop!

Does your alternator connect with a short lead to the battery connection 
at the starter? Check for bad connections there, and at the start 
battery connections. It may be helpful to clean them all up & coat them 
with dielectric grease. Don't overlook the ground. If you turn the 
charger on, it will add it's output to that of the alternator, so you 
will see that reflected in the voltmeter readings.

Yes, you can check the voltage at the hot terminal on the alternator 
with the engine running, but don't remove the battery lead! Check the 
battery terminal & starter terminals as well. Depending on the length of 
the lead to the instrument panel, number & condition of connections, 
etc. You could easily see a voltage drop at the gauge. I really don't 
put any faith in the voltage values on mine, just the relative position. 
I know it's higher when the alternator is at full load, lower as the 
batteries charge, etc. I use my link monitor to get a picture of what's 
happening.


Good luck!

Steve Sipe