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:
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.
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.
>>> 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.
2019-11-22 Edits and typos.
Very nice tutorial on a job I hate to do
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.
You are the best, I am following and doing all your tutorials.
Thank you for your great job¡¡¡¡
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.
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.
Very nicely done. Will be a big help when I tackle this in the next day or two. Thanks!
Thanks Scott. Best of success when you do yours.
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??
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.
MY BIKE R100 RS MONOLEVER 1992
BEFORE I HAD USE MOTOBATT 12V 14AH 250CCA BUT I RIDE 10 DAYS BATTERY EMPTY (VERY LOW NOT START)
AND AFTER I REPLACEMENT TENDER 12V 26-35AH 480 CCA BUT 20 DAYS BATTERY EMPTY (VERY LOW NOT START)
HOW CAN I DO IT? PLEASE
There is a short that is drawing power from the battery. These can be difficult to location. Best of success.
WHAT I NEED REPLACEMENT?
I do not know. Somewhere electricity is being used all the time. There are many ways that can happen. I can not solve this for you. I’m sorry. A mechanic can help find where the electricity is being used all the time.
IF ALTERNATOR ERROR?
I SHOULD REPLACEMENT BORSCH 14V 10/17A OR ALTERNATOR ROTOR?
It the alternator is not working, you would see the alternator light glowing in the instrument cluster. You should also test the output to be sure it is broken. If it is working, the output at 3500 RPM should be about 14.5 volts. Much lower indicates a problem.
Please appreciate I can not diagnose this problem by guessing at what is wrong over the internet. It requires testing to verify what is broken.
3500 RPM ONLY 13.2 VOLT HOW CAN I FIX ?
A bit low, but that should charge the battery.
You could try installing new brushes in the alternator. If they are worn and getting too short, they don’t make good contact with the slip ring on the rotor and may decrease the voltage.
Mycket bra arbetsbeskrivning!
Meticulous attention to detail assisted me greatly in my own alternator overhaul. Great photography and so well written. Thank you Brook
You’re welcome. I’m pleased this material helped you with you project. 🙂