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Author Topic:   Alternator Not Charging
FISHNFF posted 04-29-2014 09:27 PM ET (US)   Profile for FISHNFF   Send Email to FISHNFF  
Getting to enjoy the nostalgia and positive attributes of the classic 2 stroke outboard--raw power, unbridled growl, simple, light--and there is something pleasant about the smell of two-stroke oil. But, in the Garden of Eden lies a snake. On 1989 Mercury 50-HP three-cylinder the alternator isn't working. The tachometer works, but the voltage doesn't change whether the engine is running or not.

When I first got the motor last year (after sitting in a garage hanging off the back of a 1989 Striper for eight years), the tachometer would bounce from 0 to the proper reading occasionally. Now it has been reading steady.

What should I attempt to replace first: stator, switch box, rectifier, or coils?

Thanks to all you out there!--FISHNFF

[Nota Bene: re-start your discussion of engine running and starting problems in REPAIRS/MODS.]

saumon posted 04-29-2014 11:23 PM ET (US)     Profile for saumon  Send Email to saumon     
Here's the service manual for your particular engine:

You'll find in the section 2 the troubleshooting procedure.

jimh posted 04-30-2014 09:43 AM ET (US)     Profile for jimh  Send Email to jimh     
Trying to remedy an electrical problem by just replacing components at random can be expensive. It is better to use some basic electrical diagnostic technique to establish which component in the circuit is likely to have caused the problem.

I don't believe the component you call the switch box is involved with the alternator circuit.

The alternator circuit is probably a classic permanent magnet alternator. For background on permanent magnet alternators see

Permanent Magnet Alternators

The components are:

--magnets embedded in flywheel, rotated by engine crankshaft, producing moving magnetic field

--a coil with multiple windings or poles, excited by the magnetic field from rotating magnets, producing alternating current

--rectifier converting alternating current to pulsating direct current

--attached lead-acid battery to receive pulsating direct current and limit voltage fluctuations.

Of these components, the rectifier is most likely to fail. The rectifier typically contains silicon semi-conductor diodes, which are subject to failure if either the voltage, the current, or the temperature of the semi-conductor junction inside exceeds the ratings.

The rectifier can typically be temporarily disconnected from the circuit and tested with simple resistance measurements.

The coil assembly can become damaged, usually from being overheated, but also from mechanical damage. Again, temporarily disconnect the coil from the rest of the circuit, and test it with simple resistance measurements.

If no sign of a failure is seen in the rectifier or coil assembly, remove the flywheel and inspect the magnets. The strength of the magnetic field is difficult to test unless you happen to have a magnetometer.

If no diagnostic inquiry can be made into an electrical circuit, and the only possible technique to proceed toward a remedy is to replace components at random, the order in which components should be replaced is then:

--by their price; replace the least expensive component first, and

--by the difficulty in performing the replacement; replace the easiest to replace component first.

In this instance, the rectifier will be the least expensive and easiest to replace.

jimh posted 04-30-2014 09:58 AM ET (US)     Profile for jimh  Send Email to jimh     
Also--and important--check the battery. If the battery has a shorted cell, it could affect the alternator output voltage.

A good rule to apply in any electrical circuit which operates from battery power: the first component to check is the battery.

FISHNFF posted 04-30-2014 01:41 PM ET (US)     Profile for FISHNFF  Send Email to FISHNFF     
Thanks for the info. I'll be doing some research.

Battery is good. Brand new marine starting.
I swapped it out with another when I drained it from excessive cranking without recharging. Same deal.


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