3.0LX 135 Merc. Alternator problem?

[MH9371B]

I recently replaced the alternator in my 3.0LX mercruiser I/0. I replaced it because even though I had 2 brand new batteries installed (on a 2-battery switch) after taking it out for a while I heard my depth finder's battery alarm going off and noticed I was only running 10-11 volts. So, after replacing the alternator, I took it down to the boat ramp and was pleased to see I was running a more normal 13 volts. So, off I went into the bay only to have the same problem occur again about 15-20 minutes into my trip. <br /><br />This happens no matter which battery I run on. I have heard to not switch to "off" while running engine, which I have never done. <br /><br />The only two things I can think of are A) my "new" alternator got zapped by some loose wire possibly? Or maybe something else? or B) there is some corroded cable somewhere, but I have no idea where to look for it. The alternator is a 3-wire system with a 4th wire for ground.<br /><br />I'm new to the board. Be gentle!

 

[Boatist]

Re: 3.0LX 135 Merc. Alternator problem?

MH9371B<br />Welcome to the forum and a good job on you first post.<br />I belive battery switches the number one killer of alternators. To start with a charged battery should be 12.6 volts. With the engine running should get 13.8 to 14.8 volts.<br /><br />Alternator will blow the output diodes anytime they loose the load of a battery. The field winding will say charge and with no load the voltage will go so high it will blow the output diodes in a second or less.<br /><br />Many types of battery switches. Best type has a alternator field disconnect circuit and also will make the second battery before it breaks the connection to the first battery. This type if wired correctly will protect the alternator if you turn the swtich with the engine running even if you turned thru off. Other things that will kill the alternator would be if battery the switched is hooked to had a loose cable while the engine is running.<br /><br />I guess the first place to start is charge your batteries. Turn on the key and note voltage reading closely. Start the engine and again note the voltage. If it does not jump up to at least 13.8 volts you have a problem.<br /><br />Never run your motor with out water going to the waterpump.<br /><br />If you have a good volt meter check the battery voltage the switch is turned to first. Also make sure battery cables clean and tight. Then start engine and check battery voltage again. The voltage should Jump form 12.6 volts to at least 13.8 volts. If it does not check alternator belt tension and all alternator connections. If still the same time to get a shop manual. Alternator will likely need one of three thing. A output diode block, a internal voltage regular or exteral voltage regulator and last a diode triode. These parts are pretty cheap in the $10 to $15 doller range if you buy form a alternator shop not a marine dealer.<br /><br />This is one reason I like a Battery isolator even with a switch. Weather I switched to battery one or two the alternator still see the load of both batteries and both would have to become disconnected at the same time to hurt my alternator.<br /><br />Good luck and let us know what you find.

 

[MH9371B]

Re: 3.0LX 135 Merc. Alternator problem?

Boatist,<br /><br />Thank you for your insight. I do recall having a small problem with my battery switch when I first installed it, being that the #1 position was not working at all. After I tightened a few connections, it seemed to work fine. Possibly getting on the water and hitting a few bumps might have momentarily triggered a loss of battery load to the alternator?<br /><br />I'm thinking that any auto parts store would test that alternator for me? If my new alternator is already zapped then I would have to agree with your diagnosis.

 

[MH9371B]

Re: 3.0LX 135 Merc. Alternator problem?

I just got back from having this alternator checked out, and it too has been zapped which means something is definitely wrong. <br /><br />FYI, if anyone ever needs alternators checked out, I just had PepBoys do it for free.

 

[Boatist]

Re: 3.0LX 135 Merc. Alternator problem?

Sorry to here your alternator is bad. You can fix it or replace it. Before you put another good alternator on better find out what kind of switch you have. Make sure both batteries have good clean tight connections. Also make sure all wires to the switch clean and tight. If you do not know if your switch is a make before break and if it has a alternator field disconnect then I would never turn it with the motor running.<br /><br />If your have a volt meater and a shop manual for your motor You can probably fix it yourself. Internal componets are the same as for a car, but the case has special screens to prevent fire or explosions. Be careful not to damage any screens.<br /><br />Most likely the output diode block.<br /><br />Good luck.

 

[MH9371B]

Re: 3.0LX 135 Merc. Alternator problem?

For those of you who might have been interested..<br /><br />I ended up having a bad thermostat gasket and when the engine would rev up to say around 3000 rpm, it would shoot water out directly at my alternator. Thus, the alternator problem.<br /><br />Replaced the gasket and everything is working fine now.

 

[Bondo]

Re: 3.0LX 135 Merc. Alternator problem?

Never would have Thought of That..............