Alternator advice

[divert00]

If the alternator will deliver 15 volts at 1500 RPM, this, in my mind, tends to point to the battery, not the alternator. I realize you checked the batteries but this symptom, is typical of a battery that will not hold/deliver it's rated output.

Normally, the battery is supplying the power, NOT the alternator. The alternator simply charges and maintains the battery so IT can provide the power necessary. However, if the battery is not able to deliver it's rated power for some reason, then the alternator will try to pick up the load. The typical symptom for that is lights brightening or voltage rising considerably as the RPM rises. It doesn't happen if the battery (and charging system) are working properly.

As Jim R. and Genesis mentioned, you can load test the battery or the alternator but that requires equipment which is not normally in the DIY toolkit. A multimeter can measure the voltage which is very easy but it can't measure sufficient amperage to be any use. You need a load tester which can measure hundreds of amps. Any mechanic shop would have one of these if you have a friend in that business...

If it isn't too difficult, I'd try a known good battery - preferably a new and freshly charged one and see what happens before I'd mess with removing/disassembling the alternator. I'm not saying it can't be the alternator but I am saying that it is not the first place I would look. Batteries can test fine with a hydrometer and a voltmeter and not deliver their rated power. Load testing is one way to be sure but substitution is a pretty good and time-honored method as well.

In any case, let us know what the problem was when it was corrected and good luck for an easy and cheap repair!

Here are the results of this weekend's testing.
- All 4 batteries show 12.6-12.7V (disconnected from boat). Batteries ar 62 months old.
-Readings at post of alternators
Engines and b/c off P 12.04v S 13.4v
Engines running P 11.9v S -14.04v (yes, minus)
Engines off b/c on P 12.22v

-Voltmeters on bridge
Engines off P 13v S 13v
Engines running P 12v S 15v

I am going to change batteries, since they ar +5 years old, but suspect Port alternator? Thoughts?

My batteries are Deka model 908DMF. I assume this is what is termed a standard 8D? What is a fair price for these? Brands/source?

[Genesis]

Batteries sound fine. If they're reading 12.6-12.7V at rest and start up fine don't replace them. I doubt they're the problem. You can remove them and take them somewhere that has a load tester to confim but 8Ds are damn heavy. Get some help - your back will thank you.

It sounds like you have alternator/regulator or wiring problems. Check the wiring first before dismounting the alternators.

You've got a problem somewhere - there's no way you can have more voltage at the alternator terminal than at the battery with the charger and engines off. That's impossible. Figure out what you did wrong or what's wrong with the wiring.

The port engine while running wasn't charging. This is not unusual with these alternators on a cold start until the alternators run for a bit or until you bump the RPM up a bit - although it shouldn't happen, its common.

Charger on returning 12.22V is not normal either; you should either see the battery voltage (if the charger has shut down) OR a higher voltage (in the 13.5-8V range) if the batteries are on absorbtion or float charge.

Your running numbers are bizarre.

The bridge indications are within the expected margin of error for electrical gauges.

I suspect a wiring problem given that you're seeing odd numbers with the engines and charger OFF.

[first230sl]

There is an outstanding (IMHO) book that I have by Calder. Forgive me if it has been discussed before. But it has, in addition to many other excellent chapters, one or two very good chapters on diagnosing and repairing battery and alternator problems. It leads you through a series of tests to help diagnose problems. The descriptions are clear and I have found them very helpful. You've heard some of the same kind of advice here on the group, but the book will also take you through step by step.

I *really* like having this book in my library. I've read it cover to cover.

Perhaps others will recommend another book (which I would like to hear about as well).

The book I am speaking of is:

Boatowner's Mechanical and Electrical Manual: How To Maintain, Repair, and Improve Your Boat's Essential Systems — 2nd Edition by Nigel Calder

Publisher: International Marine
Author: Nigel Calder
Binding: Hardcover
Pages: 592 — Hundreds of b&w illustrations
ISBN: 007009618X

I have attached an image of the cover.

Best regards - Murray

Here are the results of this weekend's testing.
- All 4 batteries show 12.6-12.7V (disconnected from boat). Batteries ar 62 months old.
-Readings at post of alternators
Engines and b/c off P 12.04v S 13.4v
Engines running P 11.9v S -14.04v (yes, minus)
Engines off b/c on P 12.22v

-Voltmeters on bridge
Engines off P 13v S 13v
Engines running P 12v S 15v

I am going to change batteries, since they ar +5 years old, but suspect Port alternator? Thoughts?

My batteries are Deka model 908DMF. I assume this is what is termed a standard 8D? What is a fair price for these? Brands/source?

[Traveler 45C]

Very good book!

As an electronics instructor, I can say that the AC and DC sections of the book are good enough to be used in the classroom. The info is accurate and clearly explained.

Lots of good DC system setup info.

[Genesis]

Calder's book is very good - another recommendation from here.

[Trojan]

Sounds like batteries . Radio scrap has a real nice led indicator that plugs into a lighter socket. Works great for this type of stuff. Under $8. bucks

[divert00]

I have pulled the alternator and taken it for a rebuild. After seeing the oil seal, I am having my trusted independent mechanic re-install. BTW, DD wanted $1800 for a new alternator. HA HA!

My batteries are Deka model 908DMF. I assume this is what is termed a standard 8D? What is a fair price for these? Brands/source?

[Genesis]

They're not a big deal to re-install. The seal in the hub MUST BE REPLACED however, or I guarantee you it will leak (assuming you removed the coupler before taking it in; sound like it.) You can also buy a hub kit for about $100 that is a new hub and seal; you remove the coupler bolt, put the hub through the new kit (DO NOT remove the cardboard tube - press it through with the hub), place on alternator, re-torque the coupler bolt. Sounds expensive for a seal, BUT if you have the old one pressed out and a new one pressed in, it really isn't unless you happen to have an arbor press to do it yourself (if you pay a shop to do it you may as well buy the new kit - its cheaper and you know the alignment on that seal is correct.)

This also gives you the opportunity to inspect that coupler. If the sides of it have any damage at all, replace it or the seal will leak. Crocus cloth can be used to smooth VERY minor imperfections, but if your fingernail catches at all on the side anywhere around it, don't bother wasting your time trying.

WARNING: The coupler bolt MUST BE RETORQUED and SHOULD be replaced. If you're not going to replace it I'd loctite it. If it falls off during operation it will go into the gear train and do an unbelieveable amount of damage.

$1800 for an alternator eh? Someone's been smoking something funny at your Detroit dealer....

[Capt K]

I'm buying 8d's for about $165 each here in Ct. These are Interstate truck batteries that last 4-6 years.

K

[Passages]

I'm buying 8d's for about $165 each here in Ct. These are Interstate truck batteries that last 4-6 years.

K

Capt K,

Where did you buy your 8d's?