Alternator/Battery Question

Engine is a 1995 Merc 3.0LX

Had my boat in for some service last week. The mechanic stated to me that my alternator was bad and needed to be replaced. He stated that when he revved up the motor, the voltmeter on the dash would drop down, which indicates a bad alternator. Is this true?

I brought it home and started it up. At idle speed it reads just over 13v, when you rev it up, it stays put at 12v no matter what the RPM.

Re: Alternator/Battery Question

It's gone, have it rebuilt.

Re: Alternator/Battery Question

Another thing you could try is running the engine and pulling the battery cables off as it idles, if your alternator is dead the engine should die with out the battery as a power source since the alternator isnt producing any voltage.

Re: Alternator/Battery Question

freelunch said:

Another thing you could try is running the engine and pulling the battery cables off as it idles, if your alternator is dead the engine should die with out the battery as a power source since the alternator isnt producing any voltage.

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Do not do that! This will fry your regulator. Your alternator may be rebuildable right now. Do this and you will have even more to rebuild!

Re: Alternator/Battery Question

crap sorry about that, ive done it with vehicles of mine several times, didnt mean to steer you wrong

Re: Alternator/Battery Question

I agree DO NOT pull the cable off for even a part of a second it will blow your alternator diodes and regulator.

You coud do that back in the old days with generators but not with alternators. This is also why they tell you not to turn your battery switch thru off with the motor running.

Your alternator should raise up in voltage as you rev the engine until it reaches the voltage regulator voltage then remain steady.

Make sure the belt is snug and not riding all the way in the bottom of the pulley.

Likely just worn slip ring brushes or a bad spring that pushes the brush out to the slip ring. If you can do it yourself cost is under $2 Possible it could be a bad regulator. If you can do it yourself about $15.

I agree your alternator needs work. If you have the OEM manual it will have step by step checks to test the regulator, diodes, diode triode if it has one. Brushes and bearings very easy to see if worn or bad.

Re: Alternator/Battery Question

Thanks for the help, everyone. I took the alternator out and had it tested at an auto parts store and indeed the diodes were bad. A local alternator shop fixed it right up for $25 and it's back in the boat and running great!

Thankfully, I did not pull the cable off. I have an A/B battery switch in the boat and I remembered reading somewhere to never turn it to the "Off" position while the boat was running, so even though I appreciate the advice, freelunch, I'm glad I didn't follow it!