Alternator woes

[KevinD]

Hi All,
I am concerned that my alternator might not be changing my batteries. I have an Atomic 4 with a Motorola MR12N4510 alternator. With the engine in Neutral at high idle I measured about 12.6 volts. If the alternator is working to charge the batteries I would expect over 14 volts.

I have a simple battery switch (1-2-both-off) and a battery isolator (Sure Power Model 702 multi battery isolator).

My wife and I thought we smelled burning rubber so I inspected the V-belt and it seemed loose. I tightened it and I have ordered a new belt which should be here in the morning.

I appreciate any advice you can offer.

Kevin
SV Solvento
C&C 30 Mk1
Kingston, Canada

[ndutton]

Where are you measuring the voltage, the alternator output post or somewhere else?

[blind navigator]

Alternator woes

With the belt tightened, put your red VOM probe on alternator output (red or orange wire to ammeter/battery) and black one to ground. Then check voltage at each battery with engine running-it should be more than 12.6. If it doesn't improve, it may be time for an alternator rebuild or replacement.

(PM me if you need a suggestion for a new 65 amp alternator that bolts right in, and is only a few $$ more than a rebuild.)

But consider these before alternator shopping:

I've gained voltage by cleaning up block and other connections (motor and battery off) that have corroded over time.

One last gremlin that plagued me for over a week-a new alternator was intermittently charging, sometimes not. The three wires from the loom that go to B+ on the coil looked ok, but a stout tug on the one from ignition (the one that tells the alternator to get to work) pulled the wire out from the ring terminal, and the end of the wire insulation was blackened-it had been arcing on and off-45 years of heat and vibration can do that. Cleaned them all up, crimped on a new ring terminal, and problem solved-14.2 v into the batteries that had dipped down to 12.4v.

[Al Schober]

Keep in mind that the alternator and the regulator are separate beasts. Problems with either one can prevent useful alternator output.
As I see it, you have two choices. First is to find a good electrical shop and let them get the alternator/regulator working. Second is for you to do some homework and become smarter about your charging system. This raises the question of what to do when the shop blesses your alternator/regulator - then it still doesn't work in the boat. This leaves you in the position of finding out what's wrong.
The alternator itself is pretty simple, rotor with slip rings and brushes. As the rotor spins, the brushes pass field current to the rotor. This rotating magnetic field develops voltage in the stator windings, which gets turned into DC by the diodes and then charges batteries. The only control is how much field current - and this is controlled by the regulator.
Regulators vary from an On/Off switch (you're the regulator) to the latest & greatest 4 stage charge controllers from numerous vendors (expensive). The simple ones I understand, the fancy ones you accept because they work. You'll see alternators for sale on eBay that say 'single wire, no regulator'. Guess what - the regulator is an IC chip built into the alternator, likely meant for automotive use (different voltage than we want), and that you'll have to remove the make the alternator do what you want.

[joe_db]

Make sure the wire that goes to the coil is feeding voltage to the alternator. That is what tells it to turn on.
If that alternator is beat there are a number of good replacements available