T&T: Alternator Charging

[Ownyacht at aol.com]

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



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