Merc 5.7 30amp alternator upgrade to a 50amp

mstrud

Seaman
Joined
Dec 5, 2007
Messages
72
I recently replaced the alternator on my 5.7 and b/c of part availability had to upgrade from a 30amp to a 50amp alternator. When i took the boat out it seemed to run fine until the motor died. when i tried to start it up again nothing happened. no clicks or cranks. after following some wires I eventually figured out that the fuse relay to the starter solenoid tripped and needed to be reset. that started it up again but the same thing kept happening. it would run but eventually die and need to have the relay reset. I noticed that the relay was a 30amp. My mechanic didn't mention anything about the relay when i asked him about any issues that may come up with the 'upgrade' before hand. I intend to replace the 30amp relay with a 50amp but I would like to find out what other additional surprises lay in-store for me in switching to a 50amp alternator on a 30amp system.

Also on that day the boat seemed to be running fairly sluggish. The motor ran smooth but it seemed to only be running at about 70%; to get to my normal speed @ 3000RPM it took about 3800RPM. Im hoping that the mechanic who installed the alternator pulled a plug wire for accessibility and simply forgot to hook it back up. But given the 30amp/50amp issue I thought i'd mention it and see if anyone can link the two issues.

Mariners, your input please!
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Merc 5.7 30amp alternator upgrade to a 50amp

...Also on that day the boat seemed to be running fairly sluggish. The motor ran smooth but it seemed to only be running at about 70%; to get to my normal speed @ 3000RPM it took about 3800RPM. ...

That sounds more like a prop problem...

Chris.......
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,232
Re: Merc 5.7 30amp alternator upgrade to a 50amp

I don't think there was ever a 30 amp alternator on 350 at anytime. I'll suggest talking to your mechanic again.

Ayuh,... 'ell, I thought 99% of 'em are 55 amp, Stock...
That sounds more like a prop problem...

Or local Wildlife,.....:D
 

Icefire55

Seaman Apprentice
Joined
Jun 22, 2010
Messages
39
Re: Merc 5.7 30amp alternator upgrade to a 50amp

if it's the soleinoid breaker, it need to stay at 30A, it protect it. if you go at 50, you'll fry it.

Probably had the alternator wired wrong? what's the battery voltage while not running and at idle? should be 12.6/14.x
 

mstrud

Seaman
Joined
Dec 5, 2007
Messages
72
Re: Merc 5.7 30amp alternator upgrade to a 50amp

The voltage is running in the appropriate 12.4-14v range. The batteries seem to charge faster. Everything appears correct except for the popping the relay and the sluggishness. But I dont believe the sluggishness is related.
 

lime4x4

Lieutenant Junior Grade
Joined
Apr 25, 2007
Messages
1,040
Re: Merc 5.7 30amp alternator upgrade to a 50amp

I'm running a 140 amp marine alternator without any issues
 

mstrud

Seaman
Joined
Dec 5, 2007
Messages
72
Re: Merc 5.7 30amp alternator upgrade to a 50amp

So does anyone have any thoughts on what is causing power to surge? The breaker hasn't popped when the motor was revving. It has only popped when the motor was at idle. Could this simply be a grounding issue?
 
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