I noticed that the the bike isn’t charging very well when the electrical load was increased, such as when the headlight is on. I checked the diodes on the diode board and replaced the original rubber mounts with metal ones. This avoids having the board short out when the rubber mounts break and improves the grounding of the board so output from the alternator should be steady. But, I did not find any open or shorted diodes on the board which can cause a significant drop in charging current from the alternator as it passes through the diode board.

You can read about how I did that work here:

• 12 BMW R100RS Check Diode Board, Replace Rubber Mounts

So the next step is to replace the alternator brushes. As they wear down, the brushes recede in the holders, the pressure from the spring decreases and the short pigtail prevents the brush from contacting the slip ring consistently, This reduces the output from the alternator and eventually, there is no output from the alternator.

Removing the Stator

The brushes are attached to the outside of the stator housing. Before removing the front engine cover, I remove the battery ground and insert it into a plastic tube so there is no chance it can accidentally touch the frame while I’m working.

I remove the two screws securing the front cover and carefully remove it. I had to rotate the oil cooler to get a bit more clearance between the oil cooler bracket and the cover.

The stator housing is at the bottom of the timing chest. There are three wires connected that are from the three electrical phases generated by the alternator. Mine are colored, Red, Black/White and Green from the upper left to lower right.

In the close-up you can see that the terminal the phase wires plug into are lettered “U”, “V” and “W”, so the correspondence of colored wires to phases is Red-(W), Black/White (V) and Green (U).

I remove the three wires. I also remove the brown and black wires that connect to the terminals on the white plastic brush holder at the top of the alternator. The cylindrical drum the brushes inside the holder rub on is the nose of the rotor that spins inside the stationary stator coils of wire the surround the rotor. The Brown wire goes to the D- terminal on the left and the black wire goes to the D+ terminal on the right.

I use 600 grit wet/dry paper and clean up all the spade terminals to remove the oxidation. There is quite a bit of it on the alternator phase terminals. I spray contact cleaner on them and inside the plugs that fit over the spade terminals and insert the plugs on the terminals several times to help clean off any oxidation on the inside of the plugs.

I remove the nuts securing the metal terminals to the brush holder housing. Then I remove the three Allan head bolts securing the stator cover.

I carefully remove the stator housing with the stator wiring being careful not to nick the stator wiring as that can damage the varnish insulation on the wires leading to shorts. On the outside of the stator cover is a sticker showing the Bosch model number of the alternator, 0 120 340 005. The last three digits, “005” indicate the outside diameter of the stator housing sleeve that fits into the sleeve in the timing cover is 107 mm and the output of the alternator is 280 watts.

This exposes the rotor that spins inside the stator coils. The copper slip rings are tarnished, so I use 600 grit wet/dry paper to polish them so they are bright and shiny.

Removing the Brush Holder

I remove  the nuts on the inside of the stator cover that hold the brush holder to the cover. By carefully pulling the stator wiring coils out of the stator cover, I can get a 8 mm socket on the nuts and loosen them.

Facing the inside of the stator cover, the left nut (DF terminal) includes two insulating washers and the bolt passes through an insulating spacer. The right nut (D- terminal) and bolt are not insulated from the stator cover. It’s important to be sure the insulating spacer and washers go back on the correct side when attaching the brush holder to the stator housing, or you will short out the alternator and it will not charge the battery.

Remove and Replace Brushes

I use a small screw driver to push up the end of the coil spring that pushes the brush down inside the sleeve of the brush holder so I can pull the brush out the sleeve. The coil spring is to the right of the white brush holder in the picture below.

The old brushes are worn quite a bit when compared to the new ones.

I use a 100 watt soldering gun to remove the soldered pig tails of the old brushes from the the holes at the top of the metal brush holder bracket. I pull off the insulating tubes from the old brush pig tails and insert them onto the pig tails of the new brushes.

I insert the pig tails into the holes in the metal tab of the brush holder and solder them.

The sleeves that the brushes slide into have a ridge on the inside edge.

This is where the wire goes so that the brush can smoothly move up and down inside the sleeve and not bind against the pigtail.

Reinstall the Brush Holder

I insert the insulating spacer in the (DF) bolt hole and one of the insulating washers on the back side of the brush holder so it sits between the stator cover and the holder.

I put the second insulating washer on the inside of the stator housing and then the metal washer and nut and tighten the nut.

I connect the other stud of the brush holder with a nut and washer.

The coil springs often unwind a turn when the end is removed from the top of the brush. I wind them one more turn and then insert the end of the spring on top of the brush. I check that the brushes slide smoothly inside the sleeve and that each extends the same distance.

Reinstall the Stator Housing and Stator

I carefully insert the stator housing and stator wiring coil over the rotor again being careful not to nick the stator wiring on the rotor or the housing. Since the brushes have the springs pushing down on them, I carefully lift them up so I can slide the stator housing over the rotor. I tighten the three Allan bolts that secure the stator housing to the timing cover.

I check to be sure that the brushes are centered on the slip rings and check again that they can slide up and down freely inside the sleeves.

I reattach the metal brush terminals the brush holder mounting bracket. Then I attach the brush wires to the terminals being sure the brown wire goes on the (D-) terminal and the black wire goes on the (DF) terminal. The lengths of the wires are different which is a clue about which one should go where. 🙂

I attach the alternator phase output wires to the terminals at the bottom right of the stator cover so the red wire is on the left, the black/white in the middle and the green on the right terminal. Then I attached the black wire to the “Y” terminal on the left side of the stator housing.

NOTE:
It will not make any difference which color wire goes on which stator terminal as the alternator and diode board will work correctly no matter which colored wire is on which stator terminal. But, I like to put them back the way I found them.

Test Alternator Output

Before I put the front cover on, I attach the battery ground to the engine with the speedometer cable bolt and then start the bike. I verify that the generator light goes out when the RPM increases to about 1200 RPM. Then I attach a volt meter to the battery with the red lead to the (+) or red terminal and the black lead of the meter to the frame. I verify the voltage increases as the RPM increases and eventually as the RPM continues to increase, there is no more than 14.3-14.5 volts at the battery terminals. This shows that the alternator is charging the battery and the voltage regulator is working to limit the charging voltage to no more than 14.5 volts.

It looks like the cause of the low charging voltage was not a large diode going bad in the diode board, but likely it was due to worn brushes and a lot of oxidation on the alternator terminals.

WARNING:
>>> REMEMBER TO REMOVE THE BATTERY (-) GROUND TERMINAL <<<

This is where you can get yourself in trouble. Since you reattached the battery ground to test the new brushes, you can forget to remove it before you install the front engine cover. That will almost certainly short out the diode board.

I remove the battery ground cable and attach the front cover and then reattach the battery ground. Once more, I verify that the alternator is producing a steady 14.5 volts when the RPM is increased using my volt meter.

Revisions

2019-11-22  Edits and typos.