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  #1   IP: 71.233.251.176
Old 09-25-2008, 12:10 PM
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knitchie knitchie is offline
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Cool Failed/failing Alternator: How do I know?

I have noticed recently that after running my A4 for more than 30 minutes, my battery voltage does not change, so I'm suspecting an alternator or wiring problem. Last night while at the boat, before going home to read Nigel Calder's book, I took a few voltage readings.

Battery with engine off: 12.1v
Battery with engine on about 5 minutes: 12.1v
Alternator with engine on about 5 minutes: 12.7-12.8v range, depending on rpm

After reading Calder, I have the idea that I need to verify the wiring and connections by taking measurements between the battery and alternator, but I honestly only understand about half of his text. The single thing that was not apparent to me is how I know whether my alternator is bad or going bad. Can I do this, or should I just pull the alternator and have it checked at a shop?

BTW, I have the original 30 year old Motorola 35 amp alternator.

Thanks,
Ken - Obsession
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Lake Champlain, VT
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  #2   IP: 69.118.143.123
Old 09-25-2008, 12:43 PM
tenders tenders is offline
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Because there's a difference in voltage across the batteries vs. across the alternator, you appear to have a loose, broken, or corroded connection between the alternator and the batteries. Check every single one of them. I'd suggest removing every one and sanding both sides of the terminals down with a bit of 200 grit paper or so.
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  #3   IP: 142.68.119.22
Old 09-25-2008, 03:56 PM
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rigspelt rigspelt is offline
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I agree - The first step is to verify that all the wiring and connections are clean and solid, and then check again. Look for signs of corrosion in the wires, and looseness in the terminals. Consider replacing old cables. Check the alternator belt too.

If, after that, the voltage at the battery and alternator output are equal, then cleaning the connections fixed the unequal voltage issue. If the voltage steadily increases as the engine runs, then the alternator is working and the battery is accepting a charge. If voltage gets to whatever is normal for that alternator (say 13.4 volts for an old original Motorola, or up to 14.6 volts for a newer one) then the alternator should be fine. If the voltage does not come up, then either the alternator is not fine, or the battery may be bad and not accepting a charge, or there is a wire problem. If you have a shore power charger and its output is OK but the battery voltage remains low and the connections look good, then be suspicious of a bad battery.

Simplest way to check an alternator is take it to an alternator shop to get it checked on the bench -- only takes a couple of minutes.
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  #4   IP: 71.233.251.176
Old 09-25-2008, 04:22 PM
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knitchie knitchie is offline
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Failed/failing Alternator

Thanks guys, that makes sense to me.

The batteries are new this year and I'm on a mooring with a broken solar panel. Hence, how I noticed the alternator wasn't charging the batteries. I recently purchased a NavPod instrument pod for the boat and plan to rewire the engine electrical with Don's kit when that is installed. A few boat bucks for all of Don's support and advise.

I'll cleanup what I can reach and see when I end up. If the alternator is toast, Don will get a few more boat bucks.

Fair winds,
Ken - Obsession
C30 TRBS - #0973
Lake Champlain, VT
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  #5   IP: 70.90.87.69
Old 09-25-2008, 04:37 PM
Jesse Delanoy Jesse Delanoy is offline
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The 55 amp Mando alternator is a nice upgrade without major hassles. Also check out the alternator belt tensioning device on the Indigo website - on the page advertising their fresh water cooling system.
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  #6   IP: 142.68.119.22
Old 09-25-2008, 06:55 PM
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rigspelt rigspelt is offline
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One more thought: make sure the multimeter battery is new. I got fooled on that last spring. Took a battery to the recycler that in retrospect may not have been so bad. When I compared my readings with a mechanic's, my multimeter was giving flakey readings until I put in a new 9 volt.
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  #7   IP: 206.181.246.34
Old 09-26-2008, 08:23 AM
SEMIJim SEMIJim is offline
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Quote:
Originally Posted by Jesse Delanoy View Post
The 55 amp Mando alternator is a nice upgrade without major hassles.
Please define "without major hassles" .

Back in the day, when I used to wrench on engines and other car stuff all the time: Whenever I replaced an alternator on one of my cars I always upgraded to the largest alternator that would just drop in. But the space that little 35A Motorola alternator's in on the A-4 doesn't leave much room. I recall having to coax the belt off the bottom pulley so I could raise the alternator up all the way up so I could barely sneak it past the distributer to get it off.

Thanks,
Jim
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  #8   IP: 70.90.87.69
Old 09-26-2008, 09:33 AM
Jesse Delanoy Jesse Delanoy is offline
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The Mando is slightly larger than the OEM Motorola, but I've found that they swap back and forth interchangeably. I have a Catalina 30, with very little room above the top of the engine (my engine access is mostly from the top down, through a lift-off wooden panel in a settee), and I haven't had any trouble installing the Mando.
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