Mercruiser alternator--bad?

[jdubslopro]

A week ago took the boat out had it WOT for about 15 seconds and the warning buzzer came on steady. Checked water temp and oil pressure gauges, normal. Killed throttle immediately shut it down. popped the hatch and checked everything I thought it could be-drive lube, oil level etc. didn't find anything. Started back up kept rpm low and was fine for about half hour then when I pulled onto a sand bar the tone came on steady again, and then I noticed the voltage below 12v. I put the jump pack on and it was fine. So I pulled the batteries got them tested, one was reading 11.6v and putting out good amperage, the other was putting out 4 amps. So I replaced the bad battery, fully trickle-charged the low battery. Next time down to the boat I checked voltage, with it running at 1800rpm it's only reading 13.36V at both batteries and the alternator charging posts. Under load, I see the volt gauge drop to about 12, and drops even lower with the engine running and lifting the electric hatch. What say ye? Replace the alternator even if it's putting out 13.36 which is about 1v higher than battery voltage albeit about a volt less than it should be? Engine is a 2003 mercruiser mx6.2L serial #0M30xxxxxx

 

[Mischief Managed]

Pull the brushes out of the alternator and see if they are worn out. They are the number one reason why alternators stop working correctly and they are very easy and cheap to change.

 

[jdubslopro]

Any special tools required to get at the brushes? Pulley have to come off? I work in a woodshop for a living so I'm very familiar with replacing brushes in power tools. Also--where to source replacement brushes?

 

[alldodge]

Pull your ALT and take it to Autozone, NAPA or other ALT repair shop and have it tested. You may have another issue going on other then the ALT. The ALT, Gen starter repair shop is a real cheep option. I'm thinking you may have another issue going on.

 

[Bondo]

Replace the alternator even if it's putting out 13.36

Ayuh,.... Nothin' wrong with that,....

Cross check the gauge in the dash,.... I'd sooner think it's lyin',....

 

[jdubslopro]

I had my multimeter on the leads and it was 13.36. Took alt out had it tested at autozone they had nothing to compare it to closest thing was a 100 amp Chevy alt and it failed to meet those specs, test said it only put out 11.49v. While I have it out on going to pull the brushes and inspect as well as test diodes

 

[alldodge]

I had my multimeter on the leads and it was 13.36. Took alt out had it tested at autozone they had nothing to compare it to closest thing was a 100 amp Chevy alt and it failed to meet those specs, test said it only put out 11.49v. While I have it out on going to pull the brushes and inspect as well as test diodes

A 65Amp ALT being tested to 110Amp and puts out 11.5V (Sure messed up with my lame brain autozone comment :facepalm, I don't see a problem. When your done, lets us know how it goes

 

[NHGuy]

I looked at those brushes they are common, you need to order em on ebay by alternator type. Less than $20 online.

 

[jdubslopro]

Pulled the brushes out last night and they have plenty of serviceable life left. Accidentally pulled one of the brush leads off the tab so I had to compress the spring while I soldered it back on. I ran my own bench test, I have a 12v power supply that I connected to ground and powered both contact pins (one is +12v constant, the other +12v switched for "ignition") hooked up my test leads to power and ground outputs, and spun the alternator with a drill and a 15/16" socket. With a fresh battery on the drill I was able to crank out 12.54 volts. After 5 seconds or so of spinning it dropped to 11.9 I attribute this to loss of speed bc the battery was being drained in my cordless drill, I think max free speed is around 1500 rpm in my drill. So I put the alternator back in the boat for now...I'm tempted to just throw an $80 DB marine alternator in it. I would have to paint the housing mercruiser black though.

 

[jdubslopro]

Ok guys, just an update. I haven't had much time for boating this year with wedding/honeymoon blah blah. Haven't done anything with the boat in a while.
But from some recent research, it appears that the brush springs could possibly be worn out in the alt. At 1500-2000 rpm it charges around 13v according to the gauge on the dash. Under throttle/high rpm it drops in voltage to as low as 10-11 volts. It actually almost stalled out the other day. I know the belt isn't slipping, so I can really only assume the brushes are spreading too far apart when loaded up. So I think it comes down to replacing the alt.

 

[Bt Doctur]

If its the Korean Mando alt, the winding is going open at high rpm, replace it with the GM version and youll be fine.