Take a Shot at Diagnosing My Charging Problem with Photos?

[saha6818]

A few weeks ago, I was riding my now-heavily customized K100LT (no new electrical load) when the engine started misfiring and required a push start at the gas station. I recall hearing a loud rattling noise a couple of times that day too, but that could have been related to the new exhaust, seat, etc. I don't have a battery idiot light anymore, so I don't have information from that. I walked home, came back with a battery jump starter, and hooked it up until there was sufficient voltage to start it and ride home with the headlight disconnected.

Using a volt meter, I saw that the voltage was no higher with the engine running than off. That was enough for me to pull the alternator and attempt to get it bench tested. But nobody would bench test it for me, and my local bike dealer (not BMW) wouldn't even look at it. I tried testing on my own bench, but couldn't see any voltage coming out of it - likely because I couldn't get a good enough attachment to get it turning very fast (and I didn't know what I was doing). And the battery, now disconnected, is holding a charge just fine.

So now the bike has been languishing in pieces in the garage. I can't decide whether to get new brushes, voltage regulator, or whole alternator (talk about expensive).

So I took a bunch of photos in hopes that you who had more experience with these bikes (and alternators in general) might be able to spot the problem easily. For what it's worth, I did a search here and saw that I need ~5mm of the brushes left, and I've got almost 8. Oh, and the monkey nuts, not pictured, look fine to me - they're not obliterated, at least, like I've read about on here. That's about where my usefulness stops, so I am humbly appreciative in advance for your help.

[saha6818]

By the way, the bike has ~69k miles on it, and I have no reason to think the alternator has ever been replaced.
 :dunno2:

[TimTyler]

AFAIK The alternator voltage only overrides the battery >2000rpm. Did you test it at idle or higher?

Any chance you had bad fuel?

[Inge K.]

The alternator charge warning lamp also is in the circuit for the excitation of the rotor.
If you don't wan't the lamp, you can use a 68 ohm 3W resistor instead.

[Scott_]

Looks like your slip rings(where the brushes ride) could use a good polish/clean. I've had that issue with mine in the past.
As others have posted, the exciter circuit has to be energised, either by the ind lamp or a resistor. It might self excite if you rev it over 2500 rpm, but I can't guarantee it, some do some don't.

As long as you have it apart, you could replace the voltage regulator assy(included new brushes). When I got mine a few years ago, it wasn't terribly expensive(less than $40 us.) When I get home later I can look up the vendor I got it from.

[saha6818]

Thanks, everyone! You're awesome.
 :2thumbup:

No, I'm pretty sure it wasn't bad fuel. But I'll follow up on the other leads, and I'm hopeful. Maybe I'll replace the regulator while I'm at it as you recommend - it was a bit of a hassle pulling it in the first place.

Thanks again, and I'll try to remember to post an update after I resolve the issue.

[Scott_]

This is the unit I bought.
http://store.alternatorparts.com/ib385-359108-voltage-regulator-brush-holder-assembly-for-bosch-alternators.aspx

Jan of 2010 it was only 29.95, but now it's $39.95

[enb54]

Looks like your slip rings(where the brushes ride) could use a good polish/clean. I've had that issue with mine in the past.
As others have posted, the exciter circuit has to be energised, either by the ind lamp or a resistor. It might self excite if you rev it over 2500 rpm, but I can't guarantee it, some do some don't.

As long as you have it apart, you could replace the voltage regulator assy(included new brushes). When I got mine a few years ago, it wasn't terribly expensive(less than $40 us.) When I get home later I can look up the vendor I got it from.

Yes to the part about the charge light having to be in the circuit or as Inge suggested, about a resistor in its (the charge light) place. If you don't supply any exciter voltage, there is no magnetic field, hence no alternator output... Hope you get it fixed...

[saha6818]

I tried cleaning up the various working parts, further inspecting for problems, and following the advice provided in this thread (thanks!). I also found a good deal on a new voltage regulator and installed that. Unfortunately, the bike still produces a lower voltage while running (and revved). Next, I'll buy a new alternator and see if that helps. It'll probably be a while, though, because I've lost my job and am turning off all non-essential spending for a while.

[enb54]

Try googling "how to test motorcycle alternator" and read some of those articles regarding testing. These devices are relatively reliable and other than open/shorted windings, defective rectifier/regulator and/or poor electrical connections, there is not really a lot to go wrong. If you are uncomfortable doing this testing, you should try to get a friend who is to help out, usually electrical issues on older machines can be diagnosed and resolved after some careful thought about exactly how the system works. Don't just throw parts at it, you will spend a lot of money (which most of us here don't have) and may not fix the problem. Key to problem solving is understanding first, then resolution... Good Luck!

[saha6818]

Thanks, enb, for the motivation to think about this more carefully. I've had the nagging suspicion that I'm doing something stupid - the alternator just looks too simple to ever break.

I've done a last couple of tests that I think rule out anything but an alternator problem. My wiring diagram shows two wires coming from the alternator - one to the positive battery connection and one to the warning lamp. To rule out any bad connections, I connected the alternator directly to the battery with a test wire and put a resistor between the other connection and the negative terminal. No dice. I have 13V at rest, 12.2V at idle, and 12.1V at 3K+ RPM (and the voltage gets lower as the RPMs get higher). I conclude that there must be a short in the copper windings somewhere or else another fault that I just can't see. If/when I get a new alternator, maybe I'll compare them and find something stupid that I'm not noticing with this one.

[enb54]

Hi saha6818, family emergency right now, will try to get back with more suggestions in the next few days...

[Inge K.]

  and put a resistor between the other connection and the negative terminal.

It should be connected to switched power......

[saha6818]

This would make sense. There is what could be another plug on the alternator that I can't find a connection for, and since it's not shown on my wiring diagram, I ignored it. I'll remove the tank and poke around some more. If I can't find anything, I'll try switching it manually and see what happens. This wouldn't explain the original problem, but if the regulator was the problem, this could be causing the current issue. Thanks!

[enb54]

Youtube has got 2 videos which I think may help out, have a look at and  . These people are talking about car alternators, but motorcycle alternators on our K bikes work exactly the same way. You should be able to perform some continuity tests to check your stator and rotor coils, plus check out your diode bridge arrangement. As you have already replaced the brush/regulator assembly, am assuming that part is OK. If all else tests OK, then you likely have some kind of electrical wiring issue? Maybe some others on this forum can offer some better suggestions, but hope this helps you out a little bit more...