• Welcome, Guest! We hope you enjoy the excellent technical knowledge, event information and discussions that the BMW MOA forum provides. Some forum content will be hidden from you if you remain logged out. If you want to view all content, please click the 'Log in' button above and enter your BMW MOA username and password.

    If you are not an MOA member, why not take the time to join the club, so you can enjoy posting on the forum, the BMW Owners News magazine, and all of the discounts and benefits the BMW MOA offers?

Alternator Question . . .

CTellman

New member
I searched the forum and have not found the thread. My 1993 R100RT has a new rotor and new brushes. Rotor rebuilt and checked out with multimeter. After putting everything back together I get a red discharge light and a voltmeter reading about 11.8 v when running. What order should the three heavier wires mounted close and parallel to each other. Green, b/w, red? Top to bottom?
Any other ideas? Brushes are riding on the copper part of the rotor.
Thanks,
Campbell J Tellman II
:scratch
 
The three wires from the alternator carry each one of the three phases to the diode board for rectification. Their order doesn't make any difference.

What resistance did you get when you checked out the rotor?
 
People often mix up the insulating washers for the positive brush. They also occasionally mix the wires on the brushes. Or, if they solder in the brushes, they occasioanally extend the wire into the stator frame. Any of these errors cause the generator light to stay lit.
 
I'm assuming that you did not have to pull the diode board - that you just disconnected all of the wires at the stator housing. If that was the case, another sort-of easy mistake is to reverse the wires from the voltage regulator to the brushes. If you get those reversed, the D+ current that is supposed to flow through the rotor just dumps straight to ground and the alternator doesn't.

Once, I was messing about under the front cover and somehow loosened or failed to properly reconnect the big red plus output wire from the diode board to the starter relay. The connector popped off the diode board lug and I got the same symptoms that you just described - plus a very sheepish look when I pulled the front cover and saw the end of the positive output write dangling near the diode board.
 
has a new rotor and new brushes

Why was this done? Because of any charging problems?

If so it could be the small diodes on the diode board are bad. Thes diodes "feed" the voltage regulator AND the charging light bulb.

Or, the D+ wire (from the small diodes) does not connect correctly to the voltage regulator (hence the 11.8V).

The D+ is the thin blue wire plugged into the back of the diode board (leaving one spade unconnected between the thick wires and the D+ spade).

/Guenther
 
Back
Top