Boxerworks.com

I'm trying to help a buddy troubleshoot his '75 R90s.

We put in new alternator brushes as well as a new rotor a couple of weeks ago. When turning on the ignition, the red Gen light doesn't illuminate which I know means the bike won't charge.

Sure enough, we fire it up and the battery voltage is 12.3vdc, and 12.7vdc when not running.

With engine not running, I check continuity between both brush contacts (solder points) and I have a circuit (can't remember the exact resistance), so this confirms that the brushes are making contact with the rotor and the rotor is good.

I had to leave but asked him to check the bulb and sure enough it's blown. He installs a new bulb but still no illumination.

I then ask him to check voltage between pins 4, 12 on the black connector that plugs into the instrument cluster.

With key on, he has 0vdc.

The next place to look is the connectors (the female pins in the rubber connector), OR, tracing the blue wire all the way back to the voltage regulator.

From instrument cluster to starter relay (D+ terminal)
From starter relay D+ terminal to Diode board D+ terminal
From diode board D+ terminal to voltage regulator D+ terminal

Am I missing something?

thanks,

jimmyg

I think your method for testing the rotor is flawed. If you complete the circuit by connecting the two brushes the way you did, does the light come on? That will tell you if there is a fault further up the line. To check the rotor, you need to put something like a piece of cardboard between one of the brushes and a slip ring on the rotor. Then check the continuity between the two slip rings...

hi Bamboo,

The rotor was measured before installation. Slip ring to slip ring showed a resistance of about 3 ohms if I recall.

No, when everything is connected, (brushes contacting front and rear slip rings), the GEN light does not come on.

Bulb is good (ohmed out)

thanks,

jimmyg

IIRC there's a specific washer orientation with the brushes that must be followed otherwise there will be no charging.

Kurt in S.A.

Kurt,

Can you elaborate please? I assume you are referring to the insulating washers for the brush holders?

If so, I made sure that there was no continuity between the brushes and the alternator (stator holder case).

thanks,

jimmyg

Search Snowbum's page for the word "insulating"

http://bmwmotorcycletech.info/altbrushrotor.htm

Kurt in S.A.