[Captain D]

Hi everyone,

I changed my alternator last year to a new Mando 65A. Worked great last year and a few runs this year as well. But NOW I get the usual 14.5V for the first 10 minutes or so. after that she drops to around 12V (this being seen at the gauge on the dash. THEN, after about 1 min. It climbs back up to it's 14.5V. After another minute of cruising she dies back to her 12V and remains there. Funny thing is that if I shut her off for 15 minutes or so, she starts off at 14.5V and does the whole thing over again. I had an extra Regulator on hand and switched it on the spot but same problem.

Can anyone please help. I can't sleep because I can't stop thinking about this.

P.S. I have a 1997 4.3LX Gen+ with Alpha 1

Thank you everyone!

[Hudson River Steve]

Sounds like you have an intermittent open circuit wire in either your sese wire (red/purple) or your excitation wire (purple) going to the alternator. Try wiggling the wire harness to determine which wire has the open circuit so you can repair or clean the connection.

[Captain D]

I tried what you suggested. Bare in mind, you could eat in this engine room. Everything is meticulous with the wiring. BUT, it is a boat, so I checked everything again. Nothing is loose and no corrosion.

Any other ideas?

Thanks!!!

[Hudson River Steve]

Have you checked the output of the alternator with a separate voltmeter rather than rely on the gage in the dash panel? It's possible that your alternator and regulator are working just fine but you have a voltage drop up at the dash that is causing your problem.

[Captain D]

Most definitely. I was running some tests last night with a separate voltmeter last night again. When checking voltage of the output terminal from the alternator, the voltage seems to be erratic. From 12.3V to sometimes 18V and even higher. But when checking straight at the battery only 12.3-12.7V. I'v GOT to figure this one out before next weekend. We're taking the boat out for 10 days and I don't want any problems.

Thanks again!!

[Hudson River Steve]

Where were you referencing the ground of your voltmeter when you were taking the readings on the back of the alternator? Did you have the ground of the voltmeter on the case of the alternator, the engine block or did you go all the way back to the negative terminal of the battery? Have you measured the voltage drop between the alternator case and the battery negative terminal. You may have a bad ground.

Have you measured the voltage drop across the output wire of the alternator back to the battery positive terminal when the alternator is charging? If you were seeing 18V out of the alternator and only 12.7V at the battery, there is a tremendous voltage drop in that wire or in the ground back to the alternator.

Do you have multiple batteries and a battery isolator installed? Is the isolator faulty or the connections to the isolator corroded?

[Captain D]

After testing and testing and more testing. I decided my new alternator was faulty. So I swaped it for a new one. Went out yesterday and it works perfectly.

Thanks again for all your help!