I was out riding, and my alternator light started coming on. It's dimmer above 1500 rpm, but it's always on now. By the time I got home the voltmeter said 10.5 volts and the bike would barely start.

My guesses are, in this order:
(1.) the starter relay connections, as well as all connections
(2.)the ignition switch is dirty
(3.)the alternator brushes are worn out
(4.)the diode board needs replacing

How can I test where the problem is for sure?

I am wagering that your alternator rotor just went "open".

Easy, fast test. Turn on ignition key. Look at dash. Is GEN light on? If not, remove front engine cover, pull blue wire off alternator at 12 o'clock position on the brush holder. Peel back the rubber boot on the wire's terminal abt, and touch the terminal to the alternator's aluminum housing. If the light goes on, the rotor is bad.

In that case, either find a good used rotor, buy a new BMW one, or get one rewound from Rick at Motorrad Elektrik. Get the puller bolt, too, or go to the tech files and see how to make one. (I just buy them, they're cheap.)

2) Put on new rotor, Check battery fluid level and adjust with distilled water to BETWEEN the lines only.) Recharge battery. Ride.

Nope, The Alternator light is on fully with the ignition switch on. It stays on at half strength with the RPMs above 1500. What now?

First, to make sure that the lamp circuit was OK, I'd pull the blue wire and note that the lamp went out.

Assuming that's OK, I'd pull the regulator connector plug (a big black three-lead plug on the bottom of the regulator) and I'd put a jumper across the two female terminals of the plug (not the regulator) that are side-by-side and parallel with each other. (You'll see what I mean when you pull the plug) A robust paper clip makes a fine jumper, as long as you put some tape on te exposed part to prevent any untoward loss of smoke.

With the harness jumped, which bypasses the regulator and sends the alternator nto full-time charge mode, i'd start the engine and raise the RPM SLOWLY, observing the voltmeter. The meter should rise steadily, as RPM rises. Stop when you hit about 15 volts, or you could start blowing little bulbs and stuff.

If this test shows a charge, replace the regulator.

If not, the diode board or its' connections are bad. Disconnect the battery, remove and inspect the diode board and its' mounts and ground wires.

Be careful removing the three large terminals on the back of the board. you want to pull them off in such a way as to avoid flexing the fiber board, which can cause the circuits to break. If you have a pair of angled-nose needlenose pliers (sold at Home Depot and Lowe's), use them to hold the diode
board rubber mount right behind the board as you remove the 8mm nuts. otherwise the rubers will twist and break, and you'll have to replace them. Don't be ashamed if they break, it happens a lot when they dry out.

-Tom Cutter

(originally posted to the Airheads list)