Strange electrical problem

goalie59

I was wondering if anyone had any thoughts on this weird electrical issue . 2005 410. As long as shore power (battery charger) are hooked to the boat the port engine starts ok but notice that the alternator out put is only 10.5 volts. When you turn off the battery charger or remove shore power the port engine will not start. You can get it started by pressing the emergency start switch  on the helm but as soon as you release it the engine stalls. Hook shore power back up and everything is ok with the exception of the 10.5 volt alternator output

raybo3

Alternator is bad

Dutch320

Sounds like your alternator is bad, should be putting out 13.5+ volts. I would disconnect shore power to eliminate the battery charger and check the battery voltage, hopefully it’s reading 11 volts or more otherwise the battery is probably bad also. Most auto parts stores will test your battery for you also. 

goalie59

Thanks for the comments . All new batteries and good voltage 12.6. I was wondering about the isolator. Is it possible. I agree that the alternator does seem to be an issue but you can start the port motor with another battery by using the helm battery sync. 

Handymans342

Where did you measure the alternator output?

LaRea

Check the voltages on the input and output posts of the isolator.  With engines running and charger turned off:  

Input to ground:  around 13.5 volts
Output to ground:  around 12.9 volts
Input to output:  around 0.5 to 0.7 volts

If that's not what you see, maybe the isolator is bad.  We've been seeing a lot of those over the past couple years.  

goalie59

The alternator voltage was on the smart craft and also the gauge. I need to confirm that right at the alternator 

LaRea

Somebody correct me if I'm wrong.  SmartCraft and the helm gauge are not reading the alternator output.  They are reading the voltage of the starter battery, as measured at the starter motor.  If current from the alternator isn't reaching the battery, the engine load will pull the battery voltage down to 10.5 volts.  

goalie59

Hmm . Interesting. When the engine is running port voltage says 10.5. After turning off the engine it goes to 12.6. 

Aqua_Aura

With it running the new variable would be the alternator so maybe it has a voltage leak/bad diode. Either way sounds like a bad alternator as others said. You could check it with a meter or just have it bench tested at an auto parts store. 

goalie59

Not that this maters but the alternator was completely rebuilt by a reputable marine electronics rebuilder. Not ruling out the alternator just saying that it had an overhaul and bench tested ok. I will check it though and also the isolator. 

YYZRC

Maybe swap the alternators between port and starboard? Not sure how much work that is on your motors. 

LaRea

goalie59 said:

Hmm . Interesting. When the engine is running port voltage says 10.5. After turning off the engine it goes to 12.6. 

This is the same behavior I saw a few weeks ago when I had miswired two cables on my isolator.  

You said:
* The voltage is 12.6, then drops to 10.5 when the engine is running.
* Turning on the charger doesn't change anything.  
* The batteries are new and the alternator was serviced.  

Whenever the engine is running, it draws something like 50 amps of current from its starter battery.  Normally, the battery is also receiving 50 amps from the isolator, which gets it from the alternator.  In that case, the voltage stays around 12.6 volts.

With a faulty isolator, no current is reaching the battery.  The engine is drawing 50 amps from the battery, which temporarily pulls the voltage down to 10.6 volts.  If you continue running the engine, it discharges the battery and you can't start the engine.  

I don't think swapping alternators will help.  If the alternator was the problem, you'd still be able to run using current from the battery charger.  But you said the charger doesn't change anything.  

Everything you've said is consistent with a failed isolator. 

goalie59

Thanks guys . Excellent info. One thing to re confirm is that the engine will not start unless the charger is on or I hold the battery sync button down. 

goalie59

Sorry for the late reply, I wanted to get back to the group about this issue. Was kind of a strange problem but it seems to be resolved. I can say that the wiring in this boat seems strange and I am not sure how much of it has been tampered with by the previous owners. Found a blown fuse on engine wiring harness and the port engine ecm was wired on the wrong battery circuit . The blown fuse seemed to be a one time freak situation that along with the ecm wiring caused the starting problem. Isolator checked good, and everything seems to be working okay, keeping my fingers crossed. Thanks again for all your help.