71 R50/5 Well this just sucks! Charging

[jdvorchak]

Got the R50 mechanical part done so now onto other clean up. First thing I did while running today is measure charging voltage at the battery. Or should I say lack of charging. Only about 13 volts at 3K rpm. So that tells me it's not charging at all. I still have the front points cover off and see the alternator. The brushes have spring tension, not a lot but I don't know how much is too little. The armature is very dark and I'm temped to clean that back up to bright and shiny with mild abrasives. But honestly I doubt that is the problem. I don't see any arcing while running which I would expect to see if the brushes and armature were dirty. I'm about to look in the Clymer manual for trouble shooting technique. But any help would be appreciated. The red indicator light is on at idle but anything above idle it goes out. I'm hoping for a corroded connection somewhere but have no idea where to start.

[jdvorchak]

I should have mentioned it before as I know better. The battery was so weak this morning it wouldn't start. I put the battery tender on it and got it up to about 13 volts resting. It's only about a month old AGM battery. It's charging now and had enough power to start the engine a few times. I know the old alternator on this thing is only a few amps so maybe I don't have a problem. I'll fully charge the battery and test it again. Sorry for the confusion.

[Bamboo812]

Don't use any abrasives on the slip rings; you will scratch the surface and the resulting coarseness will eat your brushes in a short time. Instead, use a green Scotchbrite pad while the engine is running to burnish the rings. Another thing to check is the wear on the brushes. Anything more than about 1/2 worn is reason for swapping in new ones. That said 13 volts doesn't sound that bad to me, and the original /5 charging system isn't all that powerful. I'd suspect the battery before anything else... Just my 2 cents

[Kurt in S.A.]

Does it get any better at say 4K RPM? 3K is still a bit low and 13v is not out of the question.

Look at all the connectors under the front cover and remove each one a couple of times. There's another connector that goes from the alternator and plugs into the backside of the diode board. Can't get to them without removing the board. This is a common place for corrosion to start. If you're going to remove the board, be sure and reinstall with solid mounts if not already. Also, there's a "spider" grounding cable that's good to install as well.

Rick Jones at Motorrad Elektrik has a booklet on charging that is helpful to have and use. Snowbum also has a discussion topic on the alternator. There are other places to measure voltage to see where the problem is. One of those is the connector at the starter...also corrosion can set up there.

Kurt in S.A.

[Duane Ausherman]

when you say "at the battery" are you putting the probes on the terminals, or the battery posts? I know that they should be the same reading, but they may be different. Check the first couple of inches of the battery wires. If they are stiffer, or fatter near the terminal, then they are corroded inside.

The BMW contractor in this era did a very poor job of attaching terminals. We started finding troubles in the first couple of years.

[jdvorchak]

Thank you all for your suggestions. I'll have time in a couple of days to look into it further. Have two Vintage bike shows tomorrow so I'm taking my 83 KZ1100. I wanted to take the BMW but the fuel tank sprung a leak. It sat with gas in it for about two hours and no problem. About an hour later I looked and it was dripping gas from what looks like up between the two tank halves on the right side or the weld seam. I only had about 1/2 gallon in it so I doubt it's some kind of "overflow". That took the wind out of my sales so I drained the tank and started working on the KZ carbs. Got them fixed and synced and life is good....

This BMW is fighting me every step of the way but I'll get her on the road!

[jdvorchak]

Sorry about not answering. I measure the battery voltage on the battery terminals with my Fluke DVM. I will pull the tank back off and start tracing there. Also it looks like someone added an inline automotive 3AG style fuse holder connected directly to the battery positive. I can't get to it until I pull the tank. I have seen those type connectors develop corrosion in the past and even seen those glass fuses weaken.

[jimmyg]

jd, what's the voltage at the battery terminals with the engine not running compared to when it is running?

[jdvorchak]

I have the bike down now for a few days sealing the gas tank.. As I remember It was 12.9 or 13 at the battery terminal. Started it a few times and at about 3k rpm I saw 13.02, idle was about 12.5. That is my recollection and not to be trusted. I may have seen 13.2 at 3k (guess as tach is inop). So when the tank is ready to re-install I'll do the tests again and make note.

I have since learned that the red light is a primitive voltage differential indicator. Since the light went out the alternator has to be adding voltage to the battery or the red light would have stayed on.

[jimmyg]

I would agree with your assessment. If the light goes out, generally its charging.

My next test, FWIW, would be to bypass the voltage regulator (you know how to short out the two blue wires at the voltage regulator, right?)

then start and see what the voltage is at various rpm's. If significantly higher, I would suspect the V.R. Are you running the original (non-electronic) V.R. ?

jimmyg