12 BMW 1983 R100RS Replace Alternator Brushes

I noticed that the the bike wasn’t charging very well  when the electrical load was increased, such as when the headlight was 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:

So the next step is to replace the alternator brushes. As they wear down, the pressure from the spring decreases and this can reduce the output from the alternator.

Removing the Stator

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

Protecting Battery Ground From Grounding

Protecting Battery Ground From Grounding

Alternator with Wiring

Alternator with Wiring

Then I remove the two screw holding 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 phases are lettered “U”, “V” and “W”, so the correspondence of colored wires to phases is Red-(W), Black/White (V) and Green (U).

Alternator Output Wires (U, V, W)

Alternator Output Wires (U, V, W)

Alternator with Wiring Connectors

Alternator with Wiring Connectors

I remove these 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 Brown wire goes to the D- terminal on the left and the black wire goes to the D+ terminal on the right. Then I use 600 grit wet/dry paper and clean up the metal spades to remove the oxidation. There is quite a bit of it on the alternator phase terminals. I spray them with contact cleaner and inside the plugs that fit over the spade terminals and insert the plugs on the terminals several times to help clean any oxidation off the inside of the plugs.

Then I remove the nuts securing the metal terminals to the brush holder housing.

Removing Brush Terminal Lugs

Removing Brush Terminal Lugs

Removing (3) Stator Housing Bolts

Removing (3) Stator Housing Bolts

I remove the three Allan head bolts securing the stator to the timing chest cover.

I carefully remove the stator housing and the stator wiring being careful not to nick the stator wiring as that can damage the varnish insulation on the wires leading to shorts.

Alternator Stator Assembly

Alternator Stator Assembly

Alternator Information

Alternator Information

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

Alternator Rotor Assembly

Alternator Rotor Assembly

Removing the Brush Holder

Brush Holder Mounting Nuts

Brush Holder Mounting Nuts

I can get to the nuts on the inside of the stator cover that hold the brush holder to the cover. I find that by carefully pulling the stator wiring coils out of the housing, 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.

Brush Terminal Mounting Hardware

Brush Terminal Mounting Hardware

(DF) Brush Terminal is Insulated

(DF) Brush Terminal is Insulated

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.

Removing End of Brush Retaining Coil Spring

Removing End of Brush Retaining Coil Spring

New Brush (Top) and Shorter Old Brush

New Brush (Top) and Shorter Old Brush

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

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

New Brush with Old Insulator

New Brush with Old Insulator

New Brushes Soldered To Metal Tabs of Brush Holder

New Brushes Soldered To Metal Tabs of Brush Holder

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.

Brush Holder Groove for Wire Pigtails

Brush Holder Groove for Wire Pigtails

Brush Wire Orientation in Holder

Brush Wire Orientation in Holder

This is where the wires go so that the brushes can smoothly move up and down inside the sleeve.

Reinstall the Brush Holder

I insert the insulating spacer on the (DF) 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.

(DF) Grounded Alternator Connection With Insulating Sleeve and Washer

(DF) Grounded Alternator Connection With Insulating Sleeve and Washer

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

(DF) Grounded Alternator Connection with Second Insulating Washer Inside Stator Housing

(DF) Grounded Alternator Connection with Second Insulating Washer Inside Stator Housing

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

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

Reinstall the Stator and Housing

Then 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 as I fit the stator housing over the rotor. I tighten the three Allan bolts.

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.

Attaching Brush Terminals-Brown Wire Goes on (D-) Terminal

(DF) Grounded Alternator Connection with Second Insulating Washer Inside Stator Housing

I attach the brush terminals and then the wires being sure the brown wire goes on the (D-) terminal and the black wire goes on the (DF) terminal.

Brush Terminal Wires Connected

Brush Terminal Wires Connected

I attach the alternator phase output wires at the bottom 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 “Y” terminal on the left side of the stator housing.

Alternator with Wiring Reattached

Alternator with Wiring Reattached

Test Alternator Output

Before I put the front cover on, I attached the battery ground to the engine with the speedometer cable bolt and then started the bike. I verified that the generator light went out when the RPM increased. Then I attached a volt meter to the battery (+) or red terminal and the black lead of the meter to the frame. I verified the voltage increased as the RPM increased with a maximum of 15 volts and then as RPM continued to increase, it dropped back to 14.5 volts. This shows that the alternator is charging the battery and the voltage regulator is working to limit the charging voltage to 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 worn brushes and a lot of oxidation on the alternator terminals.

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.

9 thoughts on “12 BMW 1983 R100RS Replace Alternator Brushes

    • Dale,

      Well, compared to the previous work of replacing the diode board rubber mounts with metal ones, this was a treat :-). I’m always a bit worried I’ll ding up the stator windings when I remove it, but on the two bikes I’ve put new brushes in, I haven’t damaged the windings, so so far, so good.

      Best.
      Brook.

  1. Brook – thank you for this wonderful technical resource! I’ve just cleaned up my 1995 R100RT diode board connections, in pursuit of a gremlin. If that does not work, then new brushes will be next. Your photos and explanations are very assuring for me, when I’m trying to remember how the darned thing looked like before I took it off.

    • Hi Peter,

      I’m pleased this helped remove the mysteries of what-goes-where and in what order. I’ve been there and done that and lost hair where I kept scratching my head 🙂

      If your brushes are anywhere near as short as I show, for $4.00 in round numbers, its a cheap way to remove one more variable. That said, it’s a bit fiddly to remove the old and solder in the new ones.

      I hope you exorcise that electrical gremlin. They are pesky little buggers.

      Best.
      Brook.

  2. If installing new brushes that do not need to be soldered. They have a washer already attached to the brush wire. Do you put the washer onto the short or the long side of the threaded post??

    • Hi John,

      I’ve not used that type of brush before. Note that one of the brush terminals is insulated from the housing with a washer and sleeve. The brush holder is also an insulator. So, it seems to me you would insert the brush washer so that it is sandwiched between the plastic brush holder and the insulating washer. In theory, the brush washer will contact the threaded metal post but not short against the alternator housing. When I solder the brush lead on that side, the insulating washer and sleeve keep that brush from grounding to the alternator cover. That’s important for obvious reasons.

      I hope this helps.

      Best.
      Brook.

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