No output from new alternators

[efb3]

Due to sub 12 volt readings on my voltmeters checked battery voltage with engine running. Only 12 volts. Getting only 11 volts at alternator output. Removed alternator and shop verified alternator bad. Bought two new alternators since voltage readings on port also low and now have no output from either new alternator. Boat is a 1986 340 Sedan Bridge with crossover charging system. Could the battery isolator be affecting the alternator? Need help.

 

[Mike Blake]

It could very well be the problem. Do you have a test meter? Follow the voltage starting at the Alternators back to the batteries. BTW have you properly tested the battery condition on all of them?

 

[efb3]

Thanks for your reply. Yes, I have a test meter and all batteries are good. Do you know how to test the battery isolator? Thanks.

 

[Mike Blake]

I have had no experience with them how ever I do have one. I would imagine just follow the voltage from post to post. Input and output voltage readings with the engines running. That's how I would go at it. Maybe someone else on here had gone through this before. Check and maybe clean all your grounds to the engine blocks also. I have had that problem before and that will cause havoc with 12V's

 

[ttmott]

Isolators are not an easy check with a simple meter due to the diode gates and breakdown differential voltages needed to trigger current across the diodes. Regardless, a measurement to the battery with engine running should indicate a bit over 13 volts. Ensure the battery charger is off and has been off for at least three hours; this allows the batteries to sink to a stable 12 or 12.5 volts before you do the checks. Due to the age of your boat it's not known here if you have external voltage regulators or not; or has the boat been updated to the newer one wire alternators which have integral regulators.