Alternator Not Putting Out?

Alternator Not Putting Out?

Checking Output Power

In some cases, you may be getting a decent no-load output voltage from your alternator, but one which will drop radically when a load is present. To test for this, try hooking up an old headlight sealed-beam (or your present one, if you don't mind going to the trouble of messing with disconnection/reconnection - I always use another one.)

The light runs as well on AC, and you should find that connecting your alternator output wires directly to the high beam of the sealed beam should give a good, bright light at 1200 RPM or so. If not, check to ensure that the bulb is not burnt out, and that you have the connections correct by verifying with a battery or battery charger. (In all cases, the sealed-beam is connected to only one of these sources at a time.) If you get a poor, yellow light, a dim red glow, or no light at all, check the voltage simultaneously, by connecting your voltmeter across the output as well. If you see less than 18VAC at 1200, it's time to look physically at the alternator, so it's off with the chaincase cover.

The permanent magnet alternator has only two parts: the rotor, a multi-pole rotary permanent magnet, and the stator, a collection of wire coils into which electricity is induced by the rotating action of the engine-driven magnet. Their output is AC current which is fed to the rectifier.

Inadequate output voltage can be due to one or both of two things:

Testing is obvious. If you have a reliable ohmeter and a manual outlining the range of resistances you should find between connections coming from your stator, that's how to test the stator. If not, you can remove the stator and check the magnet for strength. The magnet(s) in a rotor are quite powerful. Strong enough to hold a decent sized spanner/wrench against the force of gravity, for instance - with all four or six "poles" - the nexuses of magnetism on the outer surface of the rotor "drum." If the magnet isn't this strong, try replacing it first.

As for the stator, first inspect it visually. If the coils aren't sealed in epoxy, you can sometimes see physical damage. In these cases, the insulation of the coil wires - usually a clear lacquer - may appear to have dissolved in places. If you're not capable of rewinding the coils, of able to find a shop that can do it for a reasonable price, just bite the bullet and order up another stator. Be sure to check the prices - a complete alternator may be not much more than one or the other of the component pieces, so why not replace it all?

Once you've got it all back together, (be sure to check for proper stator-rotor running clearance - a match folder thickness all the way around is about right.) check for reasonable AC output, and if it's improved (it should have!), then you can move on to the investigation of the DC part of your charging system.

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