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Old 02-02-2010, 21:23   #1
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Alternator Maintenance

Is there anything I should be doing to maintain my alternator?

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Old 02-02-2010, 22:42   #2
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Every couple of years you can check the brushes and bearings and replace as needed. Keeping the batteries up to par (not too old) keeps from working the alt. windings too much.

Too much vibration can cause an alt. to brake wires. Tightening the belts too much can wear out the bearings, too loose can cause the sheave to wear and loose it's grip forcing one to tighten the belt too much.

Keep the saltwater away. I paint my alt. inside and out. Keep the connecting wires corrosion free.

And don't forget your starter!
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Old 02-02-2010, 23:01   #3
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The enemies of an alternator are salt and dirt.
Things to check.
Periodically check for any blockage of airflow through the alternator. Of special concern is the diodes. The diodes are typically mounted on a ribbed aluminum block just inside the back of the housing. Check for corrosion or dirt preventing passage of air. Overheating reduces output and can/will cause them to burn out.

If you are mechanically inclined.......
If the output slowly declines check all internal connections for corossion especially the diodes on AC Delco alternators.
The brushes that conduct current to the armature slowly wear down. Any overheating of the brushes due to arching caused for whatever reason will slowly burn the brushes, reducing contact area and reducing the current passed. The heat also takes the temper out of the springs which results in poor or no contact of the brush to the commutator as the brush wears. The result of either is reduced output or none at all. Should the output of the alternator decline with time or fail this is the place to look. Easy to replace.

Bearings require no servicing and rarely fail. They can be easily replaced.

The stator can burn out due to overheating, usually where the connectors are crimped onto the wire.

Other than the usual concerns of clean electrical connections there is not much else to do.

In Latin America the brushes (springs and housing) sell for less than 5 bucks, new diodes for under $10 and bearings they practically give away. A stator
is relatively inexpensive.
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Old 02-02-2010, 23:47   #4
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Thanks both

Delmarrey - I'm more than impressed by the cleanliness!!
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Old 03-02-2010, 03:00   #5
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Basic Alternator Testing


A loose drive belt can cause slipping and undercharging, noise and vibration, and premature belt wear. An over-tight belt may cause bearing wear.

Look at and feel the drive-belt, examining for cracking, fraying, glazing, and/or separation. Replace damaged belt(s).

Test the belt tension by firmly depressing midway between the sheaves. The belt should deflect between 1/4" (for sheaves with 7" - 10" centerline separations) and " (for 12" - 16"separations). Adjust tension as necessary.

Inspect all wiring and terminations. Wire should be adequately sized for the application (see “Ohm’s Law & Boats”), secured at least every 18", and run as high and dry as possible. Terminations must be clean (no visible corrosion), and tight. Examine the wire for thermal (melting or charred insulation) or mechanical damage (frayed, cracked, or missing insulation).
Replace damaged or undersized wire, and replace or repair inadequate terminations.


1. Set Voltmeter to DC Volts (20V scale), and connect the DC Voltmeter test leads to the battery post (Red lead to Positive Post, and Black(or Yellow) to Negative). With engine “off”, read and note battery voltage.

2. Start the engine, and at “Idle” RPM, check the voltmeter reading. At very low speeds the alternator output may be lower than the regulator setting; hence you will expect to read the battery voltage.

3. Increase engine RPM, and note increasing voltage reading to a maximum of about 14.2 volts (12V Nominal System - expect 29.5V in 24V system).

If no voltage increase noted, you likely have a bad voltage regulator, or poor wiring
(Alt’ to Batt.). Go on to test 3B.

If Voltage increases go on to test 4.

3B. With engine “off”, connect Positive test lead to Alternator Output (“B+”, or “A”), and Negative Lead to the Negative Ground Post (“Neg”, or “E”) of Alternator. Start engine. If Voltage increases with increased RPM, your regulator is OK, and the problem is in the wiring. Go on to test 3C.

3C. Discharge the Battery to less than 12.5 Volts. Check for Voltage drop between the Alternator and Battery, as follows ...

Connect the Positive Test Lead to the Alternator Positive Output Terminal (B+) and the Negative Lead to the Positive Battery Post. Crank the engine, and increase RPM to a fast idle. Observe Voltage. If the voltage climbs to as much as 0.02 Volts (or more), the positive cable has a high resistance. Repair or replace cable.

Check for voltage drop in the negative cable, as above - but: connect the Positive Lead to the Alternator’s Negative Output Terminal, and the Negative Lead to the Negative Battery Post.
Analyze as above.
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 03-02-2010, 09:23   #6
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Good info, have printed out thread, so I don't forget to check when commissioning the boat. Thanks gentlemen.
s/v Little Jumps
Lancer 30
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Old 03-02-2010, 12:39   #7

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"I paint my alt. inside and out."
Hopefully, very carefully. All it takes is one layer of paint on the heat sink areas, or sprayed into the slots outside of the diode frame, to cause an overheat and failure. A lot of alternator shops sell cheap rebuilds that are finished by a quick coat of silver paint all over everything--and those alternators tend to fail very shortly because of the extra paint in the wrong places.

Belt deflection is nice, but if you troll eBay and other places, sometimes you can pick up a proper belt tension gauge for $25-50, and make life simpler. With ribbed belts using automatic tensioning devices, testing belt tension is something only us fossils with v-belts still do.

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