71 R75/5 Charging problem

My 1971 BMW R75/5 stopped charging.

At first, I thought the alternator light was working perfectly, even during this problem, as in the day time it's full rightness with ignition on with engine not running, and goes completely out when engine is running. But my battery voltage is 11.5 VDC or so either way, engine running or not. No charging at all.

However, I discovered at night when totally dark, I can see a very faint red on this light with the engine running, even at 3,000 RPM's. It has to be very dark to notice.

I left all my BMW books at my SSF home so I have no info. here. But I did check the AC output at the 3 phase alternator. I am getting 10-10-10 VAC (any one output measured to either of the other two)at 3,000 RPMs. What should this 3-phase AC output be?

-Don- Reno, NV

Since I have my bike apart for unrelated reasons, I did some more cold checking on the charging system. Seems everything I check is okay. All 11 diodes are fine in the diode board. The stator measures .6 .6 and .6 ohm. The rotor measures 7.0 ohms. All this seems to be within spec.

However, the stator looks very burnt up. Well cooked. I expect that is the problem, even though the ohms is with-in spec. This stator has been in the bike since new.

My rotor looks like new. It has been replaced one time since the bike was new. But more than ten years ago. But since they are cheap, I will put another new rotor in there too. I am going to rebuild the entire charging system. New solid state VR too.

I rarely have much faith in cold testing. I think measuring the AC voltage right out of the alternator is a much better test, but it seems nobody here knows what that voltage should be.

I will have my 180 watt stator redone at Motorrad Elektrik for their 280 watt conversion. But they never answered their phone, so I sent them an e-mail.

Has anybody here done this $200.00 conversion?

Anyway, when I get the stuff installed, and have it all working normally, I will mention here what the 3-phase alternator output voltages are. I expect mine now is just low enough output to not charge at all and that's why the gen light seems normal, until it's very dark and only then can see the slightest faint red.

Even though I will soon have a non-stock stator, I assume the output voltage will be exactly the same as stock. Only the current availability should be higher with the extra 100 watts.

My other new unrelated problem I will mention in another thread.

-Don- Reno, NV

.6 ohms from between any of the three connectors on the Y terminal on the right side of the alternator. In Rick Jones' Classic Boxer Charging book he recommends checking the resistance values on a hot engine. Always disconnect battery ground when removing or replacing front engine cover or you risk frying the diode board. DAMHIK.

Checking whether the problem lies on the alternator diode board side or if the regulator is at fault can be done by pulling the connector off the regulator and placing a jumper across the two bottom pins on the triangle shaped connector (DF and D-). This bypasses the regulator so when you start the bike and measure the voltage at the battery you should see full alternator/diode board output. I think it should be around 17+ volts at 2000 RPM. Since both the diode board and regulator are known to cause dim charge bulbs it might be worth trying. In very rare cases the stator can also show a dim charging bulb but Rick states in the book that the stator is an extremely rare point of failure.

I had a very similar issue on a R75 /5 I used to own that only happened when it was hot. I finally tracked it down to the diode board. One of the large diodes had come un-soldered in such a way that it was almost impossible to see. When the board was hot it became a problem due to expansion. Then it would cool off and test fine. Testing diodes with a multi meter is not 100% accurate because there is little or no current flowing and they fail under load.

happy wanderer said:

.6 ohms from between any of the three connectors on the Y terminal on the right side of the alternator. In Rick Jones' Classic Boxer Charging book he recommends checking the resistance values on a hot engine.

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I lost all charging even from a cold start. But I think even a much better test would be to check the AC voltage output when the engine is nice and warm (and running!). But it seems my charging problem is obvious. I can even see where windings could short in the stator just by looking. It is really well cooked. It's been in there since 1971. An ohmmeter won't tell us everything, but I agree it best to check when hot. My bike was cold when I checked the resistance as it was apart for other reasons.

happy wanderer said:

Checking whether the problem lies on the alternator diode board side or if the regulator is at fault can be done by pulling the connector off the regulator and placing a jumper across the two bottom pins on the triangle shaped connector (DF and D-). This bypasses the regulator so when you start the bike and measure the voltage at the battery you should see full alternator/diode board output. I think it should be around 17+ volts at 2000 RPM.

Click to expand...

When I checked the DC output from the diode board with no load, it was only 14 volts DC. Bypassing the regulator made it fall down to the battery voltage. No increase at all.

happy wanderer said:

Since both the diode board and regulator are known to cause dim charge bulbs it might be worth trying. In very rare cases the stator can also show a dim charging bulb but Rick states in the book that the stator is an extremely rare point of failure.

Click to expand...

I saw that on his website. But there is an exception to every rule! Anyway, I am rebuilding the entire charging system. Rick is going to rewind my stator for 280 watts. That comes with a new diode board. I am also putting in a new rotor as well as new brushes and springs and a new solid state VR. I still have the old mechanical VR in there.

happy wanderer said:

I had a very similar issue on a R75 /5 I used to own that only happened when it was hot. I finally tracked it down to the diode board. One of the large diodes had come un-soldered in such a way that it was almost impossible to see.

Click to expand...

I have had the small diodes unsolder themselves in the past, but not the large ones. Not really unsolder, but break loose from the soldered connection where one of the diode leads doesn't make a good connection. I resoldered them myself and all was fine. But since then, I replaced that diode board, many years ago. The new one has not given me any problems at all.

This problem I have now is a little unusual because the alternator light seems to show all is working perfectly (at least if it's not very dark) but it not charging at all. In fact, the voltage drops a bit when the engine is running. I am sure it's the stator, but even if not, everything in the charging system will soon be new.

I should get my stator back by the end of the week. I sent it out to Rick on Saturday, the 22nd. I expect to have the bike back on the road within a week from now at the latest.

-Don- Reno, NV

Since you are repairing or changing out all the charging components you should have a solution. The only other thing that came to mind is Rick Jones' comments that the most common problems in the charge circuit are the connections. They need to be inspected, cleaned and tightened up as needed from time to time and then coated with dilectric grease or some such anti oxidant coating to inhibit corrosion.

That charge bulb circuit is really interesting. The bulb appears to have no ground. Instead it works on the differential voltage between the battery and the output of the regulator. A low voltage condition on one side or the other makes it glow. (I think...)

happy wanderer said:

That charge bulb circuit is really interesting. The bulb appears to have no ground. Instead it works on the differential voltage between the battery and the output of the regulator. A low voltage condition on one side or the other makes it glow. (I think...)

Click to expand...

Yes, sort of, but:

With the key on the current flows from the battery through the switch, kill switch, regulator, hot brush, rotor, ground brush, ground. The bulb is grounded and glows.

When the engine is running at high enough RPM, alternator (actually diode board) voltage is also applied at the regulator and at the battery. Absent abnormally high resistance in wires or connections the bulb sees identical diode board dc voltage from both directions and thus the ground is lost and there is no current flow. The bulb doesn't glow.

Low RPM. high resistance, a damaged battery, and some other causes will make the two voltages unequal so current will flow one way or the other, causing a glow - usually a dim glow rather than full brightness.

happy wanderer said:

That charge bulb circuit is really interesting. The bulb appears to have no ground.

Click to expand...

Technically, anything can be designed to not use a ground anywhere at all. All that is required is a completed path to make a circuit work. No ground ever necessary if designed that way from the start.

It's just usually very convenient to use a ground because it saves half the connections and a lot of wire, so it's rare to not see a ground used with anything that has a lot of circuits.

There are also such things as positive grounds so the negative is hot in respect to ground. And then there are floating grounds that use a common wire but no frame or chassis connection.

IOW, the lamp needs a completed path to light up, with or without using a ground.

I will probably have the bike back on the road running and charging fine by the coming weekend or so.

Then I will measure the AC 3-phase output so we know what it should be.

As for connections and such, all are fine. I am sure I have a bad stator regardless of how rare such failures are. I was surprised it had as much output as it did when I looked at it.

-Don- Reno, NV

Another point I missed when looking at the circuit is that the bulb provides the excitation required to get the alternator magnetic field going and charging. Thanks GSAddict for pointing this out.

Now that I have completely rebuilt the charging system (new rotor, new brushes, new 280 watt stator, new solid state VR, upgraded diode board), here are the new facts:

3-phase AC ouput is now 11.8-11.8-11.8 VAC ( It used to be 10.0-10.0-10.0 VAC when I was having charging problems).

My DC charging is now 0.85 volts above the battery voltage. Above 2,000 RPMS, it makes no difference if lights are on or off. Below that, voltage drops a bit with the lights on, as expected. I have 14.0 Volts DC on the battery while charging. The well charged battery measured 13.15 VDC when the ignition was off for a half hour.

While I didn't take the time to prove it, I believe the problem was my stator. It looked all burnt up. I now have a rebuilt 280 watt stator, which is more than I will ever need.

BTW, I didn't think the instructions from Mottorad were all that clear for a /5. I didn't notice any "blue wire" in my bike, for an example.The top cover has to be removed to use their diode board as they need to make a connection to the starter solenoid to get back to the battery.

On E-bay, I ordered a /6 front cover for my /5 so some air can get in there to help keep my new charging stuff a bit cooler.

I was expecting I had a charging problem that was very slowly getting worse, which is now well proven true.

Now, I am ready for a test ride after some minor finishing up.

-Don- Reno, NV

Glad you got to the bottom of it.

20774 said:

Glad you got to the bottom of it.

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I hope so, I rebuilt the entire charging system! I figured my 44 year old bike could use such.

The charging on this bike was deteriorating so slowly that it took a while for me to be sure there was really a problem. The generator indicator acted perfectly normal until the end, and even then I could barely see a faint red light only when it was very dark.

Now, I know what it is really supposed to charge like.

The test ride went well, at night with lights on nice and bright with no trace of red from the generator indicator. Lesson here is to look at it at night!

I will go on a longer ride later today. I will try to ride it a lot before it cools off in this area. I have no wind protection on this bike, my bike of choice for hot days.

As you know, I have recently done a lot of other work on this bike. Perhaps I will get enough miles on it in the next few weeks to report back how much improvement with oil consumption with the deep oil pan. Seems I had a lot of problems all come up at once right after the pan change, including the drive shaft to flange bolts, so I could not yet report back with the new oil consumption -- but that will be in another thread when I get the info. I now have a new calibrated adjustable dipstick to keep track.

-Don- Reno, NV

Today, I received my /6 front cover (vented, unlike the /5) and put it on and it fits /5 just fine. So with that, the charging system is totally done.

Yesterday, I went on a ~250 mile test ride. Went from here in Reno to Bucks Lake, CA and back.

I kept my headlight on all the time. I only would occasionally have the headlight on during the day when I had a 180 watt stator. I would then only leave it on when on the more dangerous roads. Now, I have better charging with the headlight on than I ever had even with the headlight off in the past.

I am going to do another ~250 mile ride in the morning. Going from Reno, NV to Bridgeport, CA and back.

-Don- Reno, NV