Bad Alternator?

[Captain Morgan I]

I got stranded in the middle of the lake this summer with a dead battery. After floating for an hour or more, I got someone to jump start the boat. We immediately got off the lake. The next trip out I brought a spare fully charged battery just in case. I measured 13.8 volts at the battery terminals with my volt meter before I started it and measured the same voltage with the engine running. I revved the engine to about 2K and still no change. Does this mean my alternator is bad? Someone suggested it may be just the inverter or rectifiier or something. It looks like new alternators are pretty expensive. Would I be better off to buy a rebuilt one or bite the bullet and buy a new one?

 

[arks]

Re: Bad Alternator?

Take it down to your local auto parts store. They should be able to test the alternators output. If somethings wrong, then compare the cost of rebuilding your alternator versus a new unit. I did this a couple years ago and discovered the cost to rebuild was a little higher than a brand new, higher output alternator. My decision was easy.
Whatever you do, MAKE SURE the alternator and its parts are rated for marine use.

 

[wncrjb]

Re: Bad Alternator?

13.8 volts with the engine off is a problem all it's own.
unless you had a very high surface charge on it.
most 12 volt batteries should be 12.6-12.8 at 100% charge
I myself have never been able to hold over a 12.70 charge without a surface charge on it.
After a good charge on a battery charger, you should always "drain" the surface charge off it.... for example, in a car, after a charge, turn lights on for 20-30 seconds, then check with volt meter.

a good charging system will check around 13.8 - 14.2 with engine running.

wncrjb

 

[wire2]

Re: Bad Alternator?

...... It looks like new alternators are pretty expensive. Would I be better off to buy a rebuilt one or bite the bullet and buy a new one?

Search "marine alternator mercruiser" on eBay, new ones starting at $65.

 

[Chris1956]

Re: Bad Alternator?

CPT Morgan, My local autoparts store will order a rebuilt Marine alternator for me. They get it in one day, and I save $50-75 over a new one.

BTW - I have not found any difference in quality or lifespan between new and rebuilt alternators. Rebuild starters tend not to be as good as new one, due to wear on the bendix drive of the rebuilt ones.

 

[BAYLINER185]

Re: Bad Alternator?

All you have to do to see if your altnator is working is pull the + batt. wire after the motor is running and see if the motor continues to run.

Also your volt meter on / in your dash should read a bit higher when the motor is running and right on the 12 v mark with motor off. Thats an easy way to see if it works.

I had a BAD alt on ym webbcraft 10 years ago. I found out the hard way I was running on my battery 100% of the time. One day I was way out deep inthe lake and when I wanted to head in the motor wouldnt start since the battery was drained and was not recharging. Not only was may alt bad but the wiring was jacked around with by previous owner I guess. well I needed a re wire and a new alt and after that I never had a problem with power on that boat.

I now have 2 boats with dual battery systems and just recently added solar chargers to each bank on my cruser so when the boat sits the batt's get a trickle to maintain them when just sitting.

 

[wire2]

Re: Bad Alternator?

All you have to do to see if your altnator is working is pull the + batt. wire after the motor is running and see if the motor continues to run.
......

That's not a good idea, it went out with '60's, '70's cars.

A modern alternator is capable of producing up to 35 volts AC. With the battery disconnected, the regulator has no reference and may allow the output voltage to spike up. When it does, all 6 rectifier diodes will blow. Possibly along with any electronics in the car that were on at the time.

 

[bruceb58]

Re: Bad Alternator?

I agree withe wire2. This will blow out your alternator.